Citation signal

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Citation_signal 

In law, a citation or introductory signal is a set of phrases or words used to clarify the authority (or significance) of a legal citation as it relates to a proposition. It is used in citations to present authorities and indicate how those authorities relate to propositions in statements. Legal writers use citation signals to tell readers how the citations support (or do not support) their propositions, organizing citations in a hierarchy of importance so the reader can quickly determine the relative weight of a citation. Citation signals help a reader to discern meaning or usefulness of a reference when the reference itself provides inadequate information.

Citation signals have different meanings in different U.S. citation-style systems. The two most prominent citation manuals are The Bluebook: A Uniform System of Citation and the ALWD Citation Manual. Some state-specific style manuals also provide guidance on legal citation. The Bluebook citation system is the most comprehensive and the most widely used system by courts, law firms and law reviews.

Use

Most citation signals are placed in front of the citation to which they apply. In the paragraph:

When writing a legal argument, it is important to refer to primary sources. To assist readers in locating these sources, it is desirable to use a standardized citation format. See generally Harvard Law Review Association, The Bluebook: A Uniform System of Citation (18th ed. 2005). Note, however, that some courts may require any legal papers that are submitted to them to conform to a different citation format.

the signal is "see generally", which indicates that The Bluebook: A Uniform System of Citation (18th ed. 2005) provides background information on the topic.

Signals indicating support

No signal

When writers do not signal a citation, the cited authority states the proposition, is the source of the cited quotation or identifies an authority referred to in the text; for example, a court points out that "the proper role of the trial and appellate courts in the federal system in reviewing the size of jury verdicts is a matter of federal law" or "Bilida was prosecuted in state court for the misdemeanor offense of possessing the raccoon without a permit".

e.g.

This signal, an abbreviation of the Latin exempli gratia, means "for example". It tells the reader that the citation supports the proposition; although other authorities also support the proposition, their citation(s) may not be useful or necessary. This signal may be used in combination with other signals, preceded by an italicized comma. The comma after e.g., is not italicized when attached to another signal at the end (whether supportive or not), but is italicized when e.g. appears alone. Examples: Parties challenging state abortion laws have sharply disputed in some courts the contention that a purpose of these laws, when enacted, was to protect prenatal life. See, e.g., Abele v. Markle, 342 F. Supp. 800 (D. Conn.1972), appeal docketed, No. 72-56. Unfortunately, hiring undocumented laborers is a widespread industry practice. E.g., Transamerica Ins. Co. v. Bellefonte Ins. Co., 548 F. Supp. 1329, 1331 (E.D. Pa. 1982).

Accord

"Accord" is used when two or more sources state or support the proposition, but the text quotes (or refers to) only one; the other sources are then introduced by "accord". Legal writers often use accord to indicate that the law of one jurisdiction is in accord with that of another jurisdiction. Examples: "[N]ervousness alone does not justify extended detention and questioning about matters not related to the stop." United States v. Chavez-Valenzuela, 268 F.3d 719,725 (9th Cir. 2001); accord United States v. Beck, 140 F.3d 1129, 1139 (8th Cir. 1998); United States v. Wood, 106 F.3d 942, 248 (10th Cir. 1997); United States v. Tapia, 912 F.2d 1367, 1370 (11th Cir. 1990). "... The term 'Fifth Amendment' in the context of our time is commonly regarded as being synonymous with the privilege against self-incrimination". Quinn v. United States, 349 U.S. 155, 163, 75 S. Ct. 668, 99 L. Ed. 964 (1955); accord In re Johnny V., 85 Cal. App. 3d 120, 149 Cal.Rptr. 180, 184, 188 (Cal. Ct. App. 1978) (holding that the statement "I'll take the fifth" was an assertion of the Fifth Amendment privilege).

See

"See" indicates that the cited authority supports, but does not directly state, the proposition given. Used similarly to no signal, to indicate that the proposition follows from the cited authority. It may also be used to refer to a cited authority which supports the proposition. For example, before 1997 the IDEA was silent on the subject of private school reimbursement, but courts had granted such reimbursement as "appropriate" relief under principles of equity pursuant to 20 U.S.C. § 1415(i)(2)(C ). See Burlington, 471 U.S. at 370, 105 S.Ct. 1996 ("[W]e are confident that by empowering the court to grant 'appropriate' relief Congress meant to include retroactive reimbursement to parents as an available remedy in a proper case."); 20 U.S.C. § 1415(i)(2)(C ) ("In any action brought under this paragraph, the court ... shall grant such relief as the court determines is appropriate.").

See also

This indicates that the cited authority constitutes additional material which supports the proposition less directly than that indicated by "see" or "accord". "See also" may be used to introduce a case supporting the stated proposition which is distinguishable from previously-cited cases. It is sometimes used to refer readers to authorities supporting a proposition when other supporting authorities have already been cited or discussed. A parenthetical explanation of the source's relevance, after a citation introduced by "see also", is encouraged. For example, " ... Omitting the same mental element in a similar weapons possession statute, such as RCW 9.41.040, strongly indicates that the omission was purposeful and that strict liability was intended. See generally State v. Alvarez, 74 Wash. App. 250, 260, 872 P.2d 1123 (1994) (omission of "course of conduct" language in criminal counterpart to civil antiharassment act indicated "Legislature consciously chose to criminalize a single act rather than a course of conduct.") aff'd, 128 Wash.2d 1, 904 P.2d 754 (1995); see also State v. Roberts, 117 Wash.2d 576, 586, 817 P.2d 855 (1991) (use of certain statutory language in one instance, and different language in another, evinces different legislative intent) (citing cases)." Source: State v. Anderson, 141 Wash.2d 357, 5 P.3d 1247, 1253 (2000).

Cf.

Main article: Cf.

From the Latin confer ("compare"), this signals that a cited proposition differs from the main proposition but is sufficiently analogous to lend support. An explanatory parenthetical note is recommended to clarify the citation's relevance. For example, it is precisely this kind of conjecture and hair-splitting that the Supreme Court wanted to avoid when it fashioned the bright-line rule in Miranda. Cf. Davis, 512 U.S. at 461 (noting that where the suspect asks for counsel, the benefit of the bright-line rule is the "clarity and ease of application" that "can be applied by officers in the real world without unduly hampering the gathering of information" by forcing them "to make difficult judgment calls" with a "threat of suppression if they guess wrong").

Signal indicating background material

See generally

This signal indicates that the cited authority presents background material relevant to the proposition. Legal scholars generally encourage the use of parenthetical explanations of the source material's relevance following each authority using "see generally", and this signal can be used with primary and secondary sources. For example, it is a form of "discrimination" because the complainant is being subjected to differential treatment. See generally Olmstead v. L. C., 527 U.S. 581, 614, 144 L. Ed. 2d 540, 119 S. Ct. 2176 (1999) (Kennedy, J., concurring in judgment) (finding that the "normal definition of discrimination" is "differential treatment").

Signals indicating contradiction

Contra

This signals that the cited authority directly contradicts a given point. Contra is used where no signal would be used for support. For example: "Before Blakely, courts around the country had found that 'statutory minimum' was the maximum sentence allowed by law for the crime, rather than the maximum standard range sentence. See, e.g., State v. Gore, 143 Wash. 2d 288, 313-14, 21 P.3d 262 (2001), overruled by State v. Hughes, 154 Wash. 2d 118, 110 P.3d 192 (2005). Contra Blakely, 124 S. Ct. at 2536-37."

But see

The cited authority contradicts the stated proposition, directly or implicitly. "But see" is used in opposition where "see" is used for support. For example: "Specifically, under Roberts, there may have been cases in which courts erroneously determined that testimonial statements were reliable. But see Bockting v. Bayer, 418 F.3d at 1058 (O'Scannlain, J., dissenting from denial of rehearing en banc)."

But cf.

The cited authority contradicts the stated proposition by analogy; a parenthetical explanation of the source's relevance is recommended. For example: But cf. 995 F.2d, at 1137 (observing that "[i]n the ordinary tort claim arising when a government driver negligently runs into another car, jury trial is precisely what is lost to a plaintiff when the government is substituted for the employee").

"But" should be omitted from "but see" and "but cf." when the signal follows another negative signal: Contra Blake v. Kiline, 612 F.2d 718, 723-24 (3d Cir. 1979); see CHARLES ALAN WRIGHT, LAW OF FEDERAL COURTS 48 (4th ed. 1983).

Signals indicating comparison

Compare

This signal compares two or more authorities who reach different outcomes for a stated proposition. Because the relevance of the comparison may not be readily apparent to the reader, The Bluebook recommends adding a parenthetical explanation after each authority. Either "compare" or "with" may be followed by more than one source, using "and" between each. Legal writers italicize "compare", "with" and "and". "Compare" is used with "with", with the "with" preceded by a comma. If "and" is used, it is also preceded by a comma. For example: To characterize the first element as a "distortion", however, requires the concurrence to second-guess the way in which the state court resolved a plain conflict in the language of different statutes. Compare Fla. Stat. 102.166 (2001) (foreseeing manual recounts during the protest period), with 102.111 (setting what is arguably too short a deadline for manual recounts to be conducted); compare 102.112(1) (stating that the Secretary "may" ignore late returns), with 102.111(1) (stating that the Secretary "shall" ignore late returns).

Signals as verbs

In footnotes, signals may function as verbs in sentences; this allows material which would otherwise be included in a parenthetical explanation to be integrated. When used in this manner, signals should not be italicized. See Christina L. Anderson, Comment, Double Jeopardy: The Modern Dilemma for Juvenile Justice, 152 U. Pa. L. Rev. 1181, 1204-07 (2004) (discussing four main types of restorative justice programs) becomes: See Christina L. Anderson, Comment, Double Jeopardy: The Modern Dilemma for Juvenile Justice, 152 U. Pa. L. Rev. 1181, 1204-07 (2004), for a discussion of restorative justice as a reasonable replacement for retributive sanctions. "Cf." becomes "compare" and "e.g." becomes "for example" when the signals are used as verbs.

Formatting

Capitalization

The first letter of a signal should be capitalized when it begins a citation sentence. If it is in a citation clause or sentence, it should not be capitalized.

Placement and typeface

One space should separate an introductory signal from the rest of the citation, with no punctuation between. For example, See American Trucking Associations v. United States EPA, 195 F.3d 4 (D.C. Cir. 1999).

Do not italicize a signal used as a verb; for example, for a discussion of the Environmental Protection Agency's failure to interpret a statute to provide intelligible principles, see American Trucking Associations v. United States EPA, 195 F.3d 4 (D.C. Cir. 1999).

Order

When one or more signals are used, the signals should appear in the following order:

• Introductory signals
  • No signal
  • e.g.,
  • Accord
  • See
  • See also
  • Cf.
• Signals indicating comparison
  • Compare
• Signals indicating contradiction
  • Contra
  • But see
  • But cf.
• Signal indicating background material
  • See generally

When multiple signals are used, they must be consistent with this order. Signals of the same basic type - supportive, comparative, contradictory or background - are strung together in a single citation sentence, separated by semicolons. Signals of different types should be grouped in different citation sentences. For example:

"See Mass. Bd. of Ret. v. Murgia, 427 U.S. 307 (1976) (per curiam); cf. Palmer v. Ticcione, 433 F.Supp. 653 (E.D.N.Y 1977) (upholding a mandatory retirement age for kindergarten teachers). But see Gault v. Garrison, 569 F.2d 993 (7th Cir. 1977) (holding that a classification of public school teachers based on age violated equal protection absent a showing of justifiable and rational state purpose). See generally Comment, O'Neill v. Baine: Application of Middle-Level Scrutiny to Old-Age Classifications, 127 U. Pa. L. Rev. 798 (1979) (advocating a new constitutional approach to old-age classifications)."

When e.g. is combined with another signal, the placement of the combined signal is determined by the non-e.g. signal; the combined signal "see, e.g." should be placed where the "see" signal would normally be. In a citation clause, citation strings may contain different types of signals; these signals are separated by semicolons.

Order of authorities

Authorities in a signal are separated by semicolons. If an authority is more helpful or authoritative than others cited in a signal, it should precede them. Otherwise, authorities are cited in the following order:

• Constitutions and other foundation documents

1. Federal
2. State (alphabetically by name)
3. Foreign (alphabetically by jurisdiction)
4. Foundation documents of the United Nations, the League of Nations and the European Union (in that order). Constitutions of the same jurisdiction are cited in reverse chronological order.

• Statutes

1. Federal: Statutes in U.S.C., U.S.C.A., or U.S.C.S.; other statutes currently in force, by reverse chronological order of enactment; rules of evidence and procedure and repealed statutes (by reverse chronological order of enactment)
2. State (alphabetically by state): Statutes in the current code, by order in the code; statutes in force but not in the current code, by order in the code; rules of evidence and procedure, and repealed statutes (by reverse chronological order of enactment)
3. Foreign (alphabetically by jurisdiction): Codes or statutes in the current code, by order in the code; statutes in force but not in the current code, by reverse chronological order of enactment, and repealed statutes (by reverse chronological order of enactment)
4. Treaties and other international agreements, other than the foundation documents of the UN, League of Nations, and the EU: cite in reverse chronological order.

• Cases: Cases decided by the same court are arranged in reverse chronological order; all U.S. circuit courts of appeals are treated as one court, and all federal district courts are treated as one court. Cite cases in the following order, in the order of federal, state, foreign, and international:

1. Federal:
   1. Supreme Court
   2. Court of Appeals, Emergency Court of Appeals and Temporary Emergency Court of Appeals
   3. Court of Claims, Court of Customs and Patent Appeals and bankruptcy appeals panels
   4. District courts, Judicial Panel on Multidistrict Litigation and Court of International Trade
   5. District bankruptcy courts and the Railroad Reorganization Court
   6. Court of Federal Claims, Court of Appeals for the Armed Forces, and Tax Court
   7. Administrative agencies, alphabetically by agency
2. State
   1. Courts, alphabetically by state and then by rank within each state
   2. Agencies, alphabetically by state and then alphabetically by agency within states
3. Foreign
   1. Courts, alphabetically by jurisdiction and then by rank within each jurisdiction
   2. Agencies, alphabetically by jurisdiction and then alphabetically by agency within each jurisdiction
4. International
   1. International Court of Justice, Permanent Court of International Justice
   2. Other international tribunals and arbitration panels, alphabetically by name

• Legislative materials: Cite in the following order: bills and resolutions, committee hearings, reports, documents, and committee prints, floor debates (in reverse chronological order)
• Administrative and executive materials: Cite in the following order: federal (Executive Orders, current Treasury Regulations, other regulations in force, proposed rules not in force, repealed materials), state (alphabetically by state) and foreign (alphabetically by jurisdiction)
• Resolutions, decisions, and regulations of intergovernmental organizations: Cite in the following order: UN and League of Nations (General Assembly, then Security Council, then other organs in alphabetical order) and other organizations (alphabetically by name of organization)
• Records, briefs, and petitions, cited in that order
• Secondary materials: Cite in the following order: uniform codes, model codes and restatements (in reverse chronological order by category); books and pamphlets, works in journals, book reviews not written by students, student-written law-review materials, annotations, magazine and newspaper articles, working papers, unpublished materials and electronic sources (including Internet sources). For all secondary sources except codes and restatements, cite alphabetically by last name of author; if none, by first word of title.
• Cross-references to the author's own material in text or footnotes; for example: See Arnold v. Runnels, 421 F.3d 859, 866 n. 8 (9th Cir.2005); United States v. Soliz, 129 F.3d 499, 504 n. 3 (1997), overruled on other grounds by United States v. Johnson, 256 F.3d 895 (9th Cir.2001) (en banc) (per curiam); Evans v. Demosthenes, 98 F.3d 1174, 1176 (9th Cir.1996).

Parenthetical information

Parentheticals, as needed, explain the relevance of an authority to the proposition in the text. Parenthetical information is recommended when the relevance of a cited authority might not otherwise be clear to the reader. Explanatory information takes the form of a present-participle phrase, a quoted sentence or a short statement appropriate in context. Unlike the other signals, it immediately follows the full citation. Usually brief (about one sentence), it quickly explains how the citation supports or disagrees with the proposition. For example: Brown v. Board of Education, 347 U.S. 483 (1954) (overruling Plessy v. Ferguson, 163 U.S. 537 (1896)).

Phrases not quoting an authority

Explanatory parenthetical phrases not directly quoting the authority usually begin with a present participle and should not begin with a capital letter: See generally John Copeland Nagle & J.B. Ruhl, The Law of Biodiversity and Ecosystem Management 227-45 (2002) (detailing the ESA's prohibition on the possession of protected species). When a complete participial phrase is unnecessary in context, a shorter parenthetical may be substituted: Such standards have been adopted to address a variety of environmental problems. See, e.g., H.B. Jacobini, The New International Sanitary Regulations, 46 Am. J. INT'L L. 727, 727-28(1952) (health-related water quality); Robert L. Meyer, Travaux Preparatoires for the UNESCO World Heritage Convention, 2 EARTH L.J. 45, 45-81 (1976)(conservation of protected areas).

Phrases quoting an authority

If the parenthetical quotes one or more full sentences, it begins with a capital letter and ends with punctuation: See Committee Note to Interim Rule 8001(f) ("Given the short time limit to file the petition with the circuit clerk, subdivision (f)(1) provides that entry of a certification on the docket does not occur until an effective appeal is taken under Rule 8003(a) or (b)."). Insert a space before the opening parenthesis of the explanatory parenthetical. If the parenthetical does not contain a complete sentence, the writer should not place final punctuation (such as a period) inside it.

Place a parenthetical included as part of a citation before an explanatory parenthetical: Fed. R. Civ. P. 30(1) (emphasis added) (also indicating that "[a] party may instruct a deponent not to answer ... when necessary to preserve a privilege"). Shorter parenthetical phrases may be used if a complete participial phrase is unnecessary in the context of the citation: The Florida Supreme court recently declared that "where the seller of a home knows facts materially affecting the value of the property which are not readily observable and are not known to the buyer, the seller is under a duty to disclose them to the buyer." Johnson v. Davis, 480 So. 2d 625, 629 (Fla. 1985) (defective roof in three-year-old home). If a source directly quotes or supports an argument (no signal or "see" before a citation), no parenthetical is necessary.

Order in a citation

If a cited case has subsequent history or other relevant authority, it follows the parenthetical: Anderson v. Terhune, 467 F.3d 1208 (9th Cir.2006) (claiming that a police officer's continued questioning violated due process rights), rehearing en banc granted, 486 F.3d 1115 (9th Cir.2007).

Internal cross-references

Portions of text, footnotes, and groups of authorities within the piece are cited with supra or infra. Supra refers to material already in the piece, and infra to material appearing later in the piece. "Note" and "Part" refer to footnotes and parts (when parts are specifically designed) in the same piece; "p." and "pp." are used to refer to other pages in the same piece. These abbreviations should be used sparingly to avoid repeating a lengthy footnote or to cross-reference a nearby footnote.

Sexual conflict

From Wikipedia, the free encyclopedia

 

Drosophila melanogaster (shown mating) is an important model organism in sexual conflict research.

Sexual conflict or sexual antagonism occurs when the two sexes have conflicting optimal fitness strategies concerning reproduction, particularly over the mode and frequency of mating, potentially leading to an evolutionary arms race between males and females. In one example, males may benefit from multiple matings, while multiple matings may harm or endanger females, due to the anatomical differences of that species. Sexual conflict underlies the evolutionary distinction between male and female.

The development of an evolutionary arms race can also be seen in the chase-away sexual selection model, which places inter-sexual conflicts in the context of secondary sexual characteristic evolution, sensory exploitation, and female resistance. According to chase-away selection, continuous sexual conflict creates an environment in which mating frequency and male secondary sexual trait development are somewhat in step with the female's degree of resistance. It has primarily been studied in animals, though it can in principle apply to any sexually reproducing organism, such as plants and fungi. There is some evidence for sexual conflict in plants.

Sexual conflict takes two major forms:

1. Interlocus sexual conflict is the interaction of a set of antagonistic alleles at one or more loci in males and females. An example is conflict over mating rates. Males frequently have a higher optimal mating rate than females because in most animal species, they invest fewer resources in offspring than their female counterparts. Therefore, males have numerous adaptations to induce females to mate with them. Another well-documented example of inter-locus sexual conflict are the seminal fluid proteins of Drosophila melanogaster, which up-regulate females' egg-laying rate and reduces her desire to re-mate with another male (serving the male's interests), but also shorten the female's lifespan, reducing her fitness.
2. Intralocus sexual conflict – This kind of conflict represents a tug of war between natural selection on both sexes and sexual selection on one sex. An example would be the bill color in zebra finches. Ornamentation could be costly to produce, but it is important in mate choice. However, it also makes an individual more vulnerable to predators. As a result, the alleles for such phenotypic traits exist under antagonistic selection. This conflict is resolved via elaborate sexual dimorphism thus maintaining sexually antagonistic alleles in the population. Evidence indicates that intralocus conflict may be an important constraint in the evolution of many traits.

Sexual conflict may lead to antagonistic co-evolution, in which one sex (usually male) evolves a favorable trait that is offset by a countering trait in the other sex. Similarly, interlocus sexual conflict can be the result of what is called a perpetual cycle. The perpetual cycle begins with the traits that favor male reproductive competition, which eventually manifests into male persistence. These favorable traits will cause a reduction in the fitness of females due to their persistence. Following this event, females may develop a counter-adaptation, that is, a favorable trait that reduces the direct costs implemented by males. This is known as female resistance. After this event, females' fitness depression decreases, and the cycle starts again. Interlocus sexual conflict reflects interactions among mates to achieve their optimal fitness strategies and can be explained through evolutionary concepts.

Sensory exploitation by males is one mechanism that involves males attempting to overcome female reluctance. It can result in chase-away selection, which then leads to a co-evolutionary arms race. There are also other mechanisms involved in sexual conflict such as traumatic insemination, forced copulation, penis fencing, love darts and others.

Female resistance traditionally includes reducing negative effects to mechanisms implemented by males, but outside the norm may include sexual cannibalism, increased fitness in females on offspring and increased aggression to males.

Some regard sexual conflict as a subset of sexual selection (which was traditionally regarded as mutualistic), while others suggest it is a separate evolutionary phenomenon.

Conflicts of interests between sexes

Various factors that affect sexual conflict between a male and female. Only the relative positions of the optimal trait values are important as the comparative positions of the male and female provide information regarding their sexual conflict. The trait value bar at the bottom of this figure indicates the relative intensity of each trait.

The differences between male and female general evolutionary interests can be better understood through the analysis of the various factors that affect sexual conflict. In situations involving a male and female, only the relative positions of the optimal trait values are important as it is their comparative positions that provide insight into the resulting conflict. The trait value bar at the bottom of the accompanying figure indicates the relative intensity of each trait. The left side represents the poorly developed end of intensity range, while the right side represents the strongly developed end of the range.

Males and females differ in the following general components of fitness, thus leading to sexual conflict. Refer to the accompanying figure in this section.

Mating rate: Males generally increase their fitness by mating with multiple mates, while females are on the middle section of the range because they do not favor a particular side of the spectrum. For instance, females tend to be the choosier sex, but the presence of female sexual promiscuity in Soay sheep show that females might not have an established mating preference. However, Soay sheep are a breed of domestic sheep, ergo might not be a subject to traditional evolutionary mechanisms due to human interference.

Female stimulation threshold: Generally, females benefit from being more selective than males would like them to be. For example, the Neotropical spider, Paratrechalea ornata, displays nuptial gift-giving behaviors during courtship as a part of their male mating efforts. These nuptials gifts allow the male to control copulation duration and to increase the speed of female oviposition.

Degree of female fidelity: Because female fidelity depends on the species' particular mating system, therefore they are in the middle section of the spectrum. However, males seeking mates have different preferences depending on whether they are unpaired or paired. Paired males benefit from high female fidelity, while unpaired males benefit from low female fidelity in order to increase their mating frequencies.

Toxicity of seminal fluid: Females benefit from low seminal fluid toxicity, while males benefit from a high toxicity level as it increases their competitive edge.

Female fecundity: Males benefit from a high female fecundity as it means that females can produce more offspring and have a higher potential for reproduction. It is important to note that females also benefit from high fecundity, and thus this trait is probably more affected by classical natural selection.

Maternal investment: In many species, males benefit from high maternal investment as it allows them to preserve more energy and time for additional matings rather than investing their resources on one offspring. Females are expected to invest a certain amount of time and resources, but it can also be detrimental to the female if too much maternal investment is expected.

Sex-biased gene expression

Natural and/or sexual selection on traits that influence the fitness of either male or female give rise to fundamental phenotypic and behavioral differences between them referred to as sexual dimorphism. Selective pressures on such traits give rise to differences in expression of these genes either at transcriptional or translational level. In certain cases these differences are as dramatic as genes not being expressed at all in either of the sexes. These differences in gene expression are the result of either natural selection on reproductive potential and survival traits of either sex or sexual selection on traits relevant to intra-sexual competition and inter-sexual mate choice.

Sex-biased genes could either be male- or female-biased and sequence analysis of these protein coding genes have revealed their faster rate of evolution which has been attributed to their positive selection vs. reduced selective constraint. Apart from sex specific natural selection and sexual selection that includes both intersexual and intrasexual selection, a third phenomenon also explains the differences in gene expressions between two sexes – sexual antagonism. Sexual antagonism represents an evolutionary conflict at a single or multiple locus that contribute differentially to the male and female fitness. The conflict occurs as the spread of an allele at one locus in either male or female that lowers the fitness of the other sex. This gives rise to different selection pressure on males and females. Since the allele is beneficial for one sex and detrimental to the other, counter adaptations in the form of suppressor alleles at different genetic loci can develop that reduce the effects of deleterious allele, giving rise to differences in gene expression. Selection on such traits in males would select for suppressor alleles in females thus increasing the chances of retaining the deleterious allele in the population in interlocus sexual conflict.

The retention of such antagonistic alleles in a population could also be explained in terms of increase in the net fitness of the maternal line, for example, the locus for male sexual orientation in humans was identified on subtelomeric regions of X chromosomes after studies conducted on 114 families of homosexual men. Same sex orientation was found to be higher in maternal uncles and male cousins of the gay subjects. An evolutionary model explained this finding in terms of increased fertility of the females in maternal lines, hence adding to net fitness gain.

Evidence of positive selection in sexually antagonistic genes

Combined data from coding sequence studies in C. elegans, Drosophila, Humans and Chimps show a similar pattern of molecular evolution in sex-biased genes, i.e. most of the male- and female-biased genes when compared to genes equally expressed in both had higher Ka/Ks ratio. Male-biased genes show greater divergence than female-biased genes. The Ka/Ks ratio was higher for male-biased genes which are expressed exclusively in reproductive tissues e.g. testis in primate lineages. In C. elegans, which is an androdioecious species (a population consisting of only hermaphrodites and males), the rate of evolution for genes expressed during spermatogenesis was higher in males than in hermaphrodites. In Drosophila, interspecies divergence was found to be higher than intraspecific polymorphism at non synonymous sites of male-biased genes which elucidated the role of positive selection and showed that male-biased genes undergo frequent adaptive evolution. Although positive evolution is associated with most of the male and female-biased genes, it's difficult to isolate genes which shown bias solely due to sexual conflict/antagonism. Nevertheless, since sexually antagonistic genes give rise to biased expression and most biased genes are under positive selection we can argue the same in favor of sexually antagonistic genes. A similar trend as seen in coding sequence evolution was seen with gene expression levels. Interspecific expression divergence was higher than intraspecific expression polymorphism. Positive selection in Accessory gland proteins (Acps) (produced by males) and Female Reproductive Tract Proteins (Frtps) has also been reported previously.

Sexual antagonism, sex linkage and genomic location of genes under conflict

Although X chromosomes have been considered as hot spots for accumulating sexually antagonistic alleles, other autosomal locations have also been reported to harbor sexually antagonistic alleles. The XY, XX and ZW, ZZ system of sex determination allows accelerated fixation of alleles that are sex-linked recessive, male-beneficial and female-detrimental due to constant exposure to positive selection acting on heterogametic sex (XY, ZW) as compared to purifying selection removing the alleles only in homozygous state. In case of partial or completely dominant sex linked traits which are detrimental to male, the probability of selecting for the allele would be 2/3 as compared to selecting against probability of 1/3. Considering the above scenario it is likely that X and W chromosomes would harbor many sexually antagonistic alleles. However, recently Innocenti et al. identified sexually antagonistic candidate genes in Drosophila melanogaster that contributed about 8% of the total genes. These were distributed on X, second and third chromosomes. Accessory gland proteins which are male-biased and shows positive selection reside entirely on autosomes. They are partially sexually antagonistic as they are not expressed in females and dominant in nature and hence under represented on X.

Evolutionary theories

Interlocus sexual conflict involves numerous evolutionary concepts that are applied to a wide range of species in order to provide explanations for the interactions between sexes. The conflict between the interactions of male and females can be described as an ongoing evolutionary arms race.

According to Darwin (1859), sexual selection occurs when some individuals are favored over others of the same sex in the context of reproduction. Sexual selection and sexual conflict are related because males usually mate with multiple females while females typically mate with fewer males. It is hypothesized that both chase away selection and sexual conflict may be the result of males use of sensory exploitation. Males are able to exploit females' sensory biases due to the existence of female choice. For example, females may behave in ways that are considerably biased towards mating and fertilization success due to the attractiveness of males who exhibit a deceptive or exaggerated secondary sex characteristic. Since some male traits are detrimental to females, the female becomes insensitive to these traits. Sexually antagonistic co-evolution entails the cyclic process between the exaggerated (persistent) traits and the resistant traits by the sexes. If male traits that decrease female fitness spread, then female preference will change.

Female's resistance

Female resistance is an evolutionary concept where females develop traits to counter the males' influence. This concept can be supported by the examples of sexual conflict in the water strider and pygmy fish.

Male water striders exhibit forced copulation on the female. As a result, the female will struggle with the male to reduce the detrimental effects. Female struggle is a by-product of female resistance.

The population of pygmy fish Xiphophorus pygmaeus or pygmy sword-tail fish initially consisted of small males. A study tested female choice using large hetero-specific males. They found that the female pigmy swordtail fish favored larger sized males, indicating that females changed their preference from small males to large males. This pattern of female preference for larger male body size disappeared in populations consisting of smaller males. The study concluded that this behavior is caused by female resistance and not due to a general preference for larger body size males.

Sperm competition

Sperm competition is an evolutionary concept developed by Geoff Parker (1970) and describes a mechanism by which different males will compete to fertilize a female's egg. Sperm competition selects for both offensive and defensive traits. Offensive sperm competition consists of males displacing sperm from the previous male as well as the use of toxic sperm to destroy rival sperm. Conversely, defensive sperm competition consists of males preventing females from remating by prolonging the duration of their own mating or by restricting the females' interest in other males. Sperm competition can be exhibited throughout behavioral, morphological and physiological male adaptations. Some examples of behavioral adaptations are mate guarding or forced copulation. Morphological adaptations may include male claspers, altered genitalia (e.g. spiky genitals) and copulatory plugs (i.e. mating plugs). Physiological adaptations may consist of toxic sperm or other chemicals in the seminal fluid that delays a female's ability to remate.

Sexual conflict is exhibited when males target other males through sperm competition. For example, Iberian rock lizard (Lacerta monticola) males create hard mating plugs. These mating plugs are placed within the female cloaca instantly after copulation, which was hypothesized to function as a "chastity belt." However, the study found no evidence to support the hypothesis, as males were able to displace the mating plugs of other males. There is no direct conflict between males and females, but males may evolve manipulative traits to counter the removal of their mating plugs.

Males also develop different behaviors for paternity assurance. A study of sperm competition revealed that there was a positive relationship between testis size and levels of sperm competition within groups. Higher levels of sperm competition were correlated to larger accessory reproductive glands, seminal vesicles, and interior prostates. Larger mating plugs were less likely to be removed.

Advantages and disadvantages

Males

Males inflicting harm on females is a by-product of male adaptation in the context of sperm competition. The advantages to males may include: a) a decrease in the likelihood of females remating, b) the ability to produce more offspring, c) sperm maintenance, and d) sperm storage. These advantages are seen throughout all variations of mate traits such as toxic sperm, spiky genitalia, forced copulation, sexual cannibalism, penis fencing, love darts, mate guarding, harassment/aggressive behavior, and traumatic insemination.

Females

Females can experience a wide range of detrimental effects from males. This may include: a) longevity reduction, b) distortion in feeding behaviors (which could increase food intake as seen in Drosophila fruit flies) c) increased risk of infection, d) wound repair through energy consumption, e) male manipulation of female reproductive schedules, f) susceptibility to predators, and g) reduced female immune response.

Hermaphrodites

Hermaphroditic mating Cornu aspersum (garden snails)

Hermaphrodites are organisms that have both male and female reproductive organs. It is possible for there to be sexual conflict within a species that is entirely hermaphroditic. An example of such is seen in some hermaphroditic flatworms such as Pseudobiceros bedfordi. Their mating ritual involves penis fencing in which both try to stab to inseminate the other and at the same time avoid being stabbed. Being inseminated represents a cost because striking and hypodermic insemination can cause considerable injury; as a result, the conflict lies in adapting to be more adept at striking and parrying and avoiding being stabbed.

Also the earthworm Lumbricus terrestris show behavior where both parts try to make sure as much sperm as possible is absorbed by their partner. To do this they use 40 to 44 copulatory setae to pierce into the partner's skin, causing substantial damage.

There are cases where hermaphrodites can fertilize their own eggs, but this is usually rare. Most hermaphrodites take on the role of a male or female to reproduce. Sexual conflict over mating can cause hermaphrodites to either cooperate or display aggressive behavior in the context of gender choice.

Sexual conflict before and during mating

Infanticide

Further information: Infanticide (zoology)

Infanticide is a behavior that occurs in many species in which an adult kills the younger individuals, including eggs. Sexual conflict is one of the most common causes, although there are exceptions as demonstrated by the male bass eating their own juvenile descendants. Although males usually exhibit such behavior, females can also behave in the same way.

Infanticide has been extensively studied in vertebrates such as hanuman langurs, big cats, house sparrows and mice. However, this behavior also occurs in the invertebrates. For example, in the spider Stegodyphus lineatus, males invade female nests and toss out their egg sacs. Females only have one clutch in their lifetime, and experience reduced reproductive success if the clutch is lost. This results in vicious battles where injury and even death can occur. Jacana jacana, a tropical wading bird, provides an example of infanticide by the female sex. Females guard a territory while males care for their young. As males are a limited resource, other females will commonly displace or kill their young. Males can then mate again and care for the young of the new female.

This behavior is costly to both sides, and counter-adaptations have evolved in the affected sex ranging from cooperative defense of their young to loss minimization strategies such as aborting existing offspring upon the arrival of a new male (the Bruce effect).

Traumatic insemination

Main article: Traumatic insemination

A male bed bug (Cimex lectularius) traumatically inseminates a female bed bug (top). The female's ventral carapace is visibly cracked around the point of insemination.

Traumatic insemination describes the male's tactics of piercing a female and depositing sperm in order to ensure paternity success. Traumatic insemination in this sense incorporates species which display extra-genitalic traumatic insemination. Males have a needle-like intromittent organ. Examples include bed bugs, bat bugs and spiders.

In bed bugs Cimex lectularius, for example, males initiate mating by climbing onto the female and piercing her abdomen. The male will then directly inject his sperm along with the accessory gland fluids into the female's blood. As a result, the female will have a distinct melanized scar in the region the male pierced. It was observed that males not only pierce females but also other males and nymphs. The females may suffer detrimental effects which can include blood leaking, wounds, the risk of infection, and the immune system having difficulty fighting off sperm in the blood.

A study focused on the mating effects of bed bugs of other species such as female Hesperocimex sonorensis and a male Hesperocimex cochimiensis. It was observed that H. sonorensis females died in a period of 24 to 48 hours after mating with H. cochimiensis males. When examining the females, it was evident that their abdomens were blackened and swollen due to an enormous number of immunoreactions. There is a direct relationship between the increase of mating and the decrease in female's lifespan.

Female bed bug mortality rate due to traumatic insemination could be related more to STDs rather than just the open wound. The same environmental microbes that were found on the male's genital were also found within the female. A study found a total of nine microbes, with five microbes actually causing mortality of females during copulation.

African bat bugs Afrocimex constrictus also perform extra-genitalic traumatic insemination. Males will puncture the female outside her genitals and ultimately inseminate them. It was observed that both males and females suffer from traumatic insemination. Males suffer from traumatic insemination because they expressed female like genitals, and were often at times mistaken for females. Females also displayed polymorphism because some females had distinct "female-like" genitals while others had a "male-like" appearance. The results showed that males along with females who had "male-like" genitals suffer less traumatic insemination compared to the distinct females. Female polymorphism could in fact be a result of evolution due to sexual conflict.

Male spiders Harpactea sadistica perform extra-genitalic traumatic insemination with their needle-like intromittent organs that puncture the female's wall, resulting in direct insemination. Males also puncture females with their cheliceral fangs during courtship. Females have atrophied spermathecae (sperm-storage organs). The sperm storage organ removes sperm from males who mate later, which reflects cryptic female choice. Cryptic female choice refers to a female's opportunity to choose with which sperm to fertilize her eggs. It has been suggested that males may have developed this aggressive mate tactic as a result of the female sperm storage organ.

Toxic semen

See also: Seminal fluid protein

Toxic semen is most associated with Drosophila melanogaster fruit flies. Drosophila fruit flies exhibit toxic semen along with intra-genitalic traumatic insemination. The male places his intromittent organ within the female genitalia, following the piercing of her inner wall, to inject toxic semen.

Frequent mating in D. melanogaster is associated with a reduction in female lifespan. This cost of mating in D. melanogaster females is not due to receipt of sperm but is instead mediated by accessory gland proteins (Acps). Acps are found in male seminal fluid. The toxic effects of Acps on females may have evolved as a side effect of the other functions of Acps (e.g. male-male competition or increased egg production). Drosophila males may benefit from transferring toxic semen but it is not likely that their main reproductive benefit is directly from reducing female lifespan.

After Acps are transferred to the female, they cause various changes in her behavior and physiology. Studies have revealed that females who received Acps from males suffered decreased lifespan and fitness. Currently it has been estimated that there are more than 100 different Acps in D. melanogaster. Acp genes have been found in a variety of species and genera. Acps have been described as displaying a conservation function because they reserve protein biochemical classes within the seminal fluid.

Drosophila hibisci use mating plugs rather than traumatic insemination. The mating plugs of Drosophila hibisci are gelatinous, hard composites that adhere to the uterus of the female in the event of copulation. A study tested two hypotheses concerning mating plugs: a) that they were nutritional gifts for females to digest to provide maintenance of the eggs during maturation, or b) that they could serve as a chastity device to prevent sperm of rivals. The study found that mating plugs had no effect on female nutrition and serve as an enforcement device against rival males. Although this species of fruit flies (Drosophila hibisci) found success in mating plugs, they are ineffective for other Drosophila species. A study found that males who insert their mating plugs within females were unable to prevent females from remating just four hours after mating. Therefore, the assumption can be made that male Drosophila melanogaster develop other male adaptations to compensate for mating plug insufficiency, including intra-genitalic traumatic insemination to directly deposit their sperm.

Spiky genitals

See also: Penile spines

The penis of a Callosobruchus analis bean weevil. Some species of insect have evolved spiny penises, which damage the female reproductive tract. This has triggered an evolutionary arms race in which females use various techniques to resist being bred.

Bruchid beetle or bean weevil Callosobruchus maculatus males are known to express extra-genitalic traumatic insemination on females. The male Bruchid beetle's intromittent organ is described as having spines that are used to pierce the reproductive tract of the female.

Males which had multiple copulations with the same female caused greater damage to her genitals. However, those same males transferred a small quantity of ejaculate compared to the virgin males. It was also observed that males that participated in copulation with females sometimes deposit no sperm through the wounds they created on the females.

Females which mated with more than one male suffered higher mortality. Females had a decrease in longevity as a result of receiving a large single ejaculate from males. However, females which received a total of two ejaculates were less likely to die compared to those that received just one ejaculate. The assumption could be made that females that mated 48 hours after the first copulation were lacking nutrition as they do not drink or eat. The ejaculate that was provided after the second copulation was nutritionally beneficial and lengthened female longevity, allowing them to produce more offspring.

Females which mated with virgin males were less likely to suffer genital damage compared to those which mated with sexually experienced males. It was suggested that factors contributing to male virgins being less harmful were ejaculate size and the amount of sperm contained.

Love darts

Main article: Love dart

SEM image of lateral view of a love dart of the land snail Monachoides vicinus. The scale bar is 500 μm (0.5 mm).

Hermaphroditic gastropod snails mate using love darts. The love darts are described as a sharp "stiletto," created by the males. The love darts are shot at the females during courtship. A single love dart is shot at a time, due to the lengthy process of regeneration. Snails of the genus Helix are model organisms for the study of love darts. It was observed that snails that rub against their mates, will forcefully place the love dart into their mate. It has been shown that though darts may aid in mating, they do not necessarily ensure mating success. However, love darts do in fact aid in mating success. Hermaphroditic snails will selectively take on a female or male role. Snails transmitted darts into these females so that they would store more sperm (about twice as much) compared to males who were not as successful. Males who successfully hit females with love darts had higher paternity assurance. Many snails inflicted with love darts suffer open wounds and sometimes death.

Forced copulation

Main article: Sexual coercion among animals

Forced copulation (sexual coercion) by males occurs in a wide range of species and may elicit behaviors such as aggression, harassment and grasping. In the time prior to or during copulation, females suffer detrimental effects due to forceful male mating tactics. Ultimately, females are forced to copulate against their will (a.k.a. "rape").

Harassment

Harassment is a behavior displayed during or prior to forced copulation. A male may follow the female at a distance in preparation to attack. In the Malabar ricefish Horaichthys setnai (Beloniformes), males harass females of interest from a distance. This behavior may consist of swimming below or behind the females, and even following them at a distance. When the male Malabar ricefish is ready to copulate, he dashes at high speed towards the female and release his club-shaped organ, the gonopodium also known as an anal fin. The purpose of the gonopodium is to deliver the spermatophore. The male takes his gonopodium and forcefully places it near the female genitalia. The sharp end of the spermatophore stabs the female's skin. As a result, the male is firmly attached to the female. Following this event, the male's spermatophore bursts, releasing sperm that travel towards the female's genital opening.

Grasping

See also: Sexual coercion among animals § Grasping and grappling

Forced copulation can lead to aggressive behaviors such as grasping. Males express grasping behaviors during the event of copulation with a desired female. Darwin (1871) described males with grasping qualities as having "organs for prehension." His view was that males perform these aggressive behaviors in order to prevent the female from leaving or escaping. The purpose of male grasping devices is to increase the duration of copulation along with restricting females from other males. Grasping traits can also be considered as a way of males expressing mate-guarding. Examples of species with grasping traits are water striders, diving beetles, and the dung fly Sepsis cynipsea.

During forced copulation, male water striders (genus Gerris) attack females. As a result, a struggle occurs because the female is resistant. When the male water strider is successfully attached to the female, the female carries the male during and after copulation. This can be energetically costly to the female because she has to support the heavy weight of the male at the same time as she is gliding on the water surface. The speed of the female is usually reduced by 20% when the male is attached. The purpose of long copulation is for the male to achieve paternity assurance in order to restrict the female from other males. Long periods of copulation can strongly affect females because females will depart from the water surface after mating and discontinue foraging. The duration of copulation can be extremely long. For water strider Aquarius najas it was a total of 3 months. For water strider Gerris lateralis the time ranged from 4 to 7 minutes.

In water strider Gerris odontogaster, males have an abdominal clasping mechanism that grasps females in highly complex struggles before mating. Males that have clasps that are longer than those of other males were able to endure more somersaults by resistant females and achieved mating success. Males' genital structures had a particular shape to aid in female resistance.

Water striders G. gracilicornis have a behavioral mechanism and grasping structures allowing grasping. Male water striders use what is called an "intimidating courtship". This mechanism involves males using a signal vibration to attract predators in order to manipulate females to mate. Females face more risks of being captured by predators since they idle on the water's surface for long periods of time. If a male were attached to the female, it would be less likely for the male to be harmed by the predators because he would be resting on top of the female. Therefore, males will tap their legs in order to create ripples in the water to attract predators. The female become fearful, causing her to be less resistant towards the male. As a result, copulation occurs faster, during which the male stops signaling.

Male water striders Gerris odontogaster have grasping structures that can prolong copulation depending on the size of their abdominal processes. Males who had longer abdominal processes were able to restrain females longer than males who had shorter abdominal processes.

In diving beetles Dytiscidae, males approach females in the water with a grasping mechanism before copulation. When this occurs, females repeatedly resist. Males evolved an anatomical advantage towards grasping. Males have a particular structure located on their tarsae that enhances grasping of female anatomical structures, pronotum and elytra, which are located on her dorsal surface.

Sepsis cynipsea is another example of sexual conflict via grasping. Males cannot force copulation; however, while females lay eggs fertilized from a previous mating, a new male mounts the female and guards her from other males. Although the females are larger than the males, the males are still able to grasp onto a female. Females are also known to attempt to shake off the male from her back. If she does not shake him off successfully, they mate.

Sexual cannibalism

Main article: Sexual cannibalism

Sexual cannibalism in praying mantises: a female biting off the head of a male

Sexual cannibalism contradicts the traditional male-female relationship in terms of sexual conflict. Sexual cannibalism involves females slaying and consuming males during attempted courtship or copulation, as in the interaction between male and female funnel-web spider (Hololena curta).

A possible explanation for sexual cannibalism occurring across taxa is "paternal investment". This means that females kill and consume males, sometimes after sperm exchange, in order to enhance the quality and number of her offspring. Male consumption by females serves as a blood meal since they volunteer their soma. The idea of "paternal investment" supports the concept of female choice because female spiders consume males in order to receive an increase in quality of offspring. Males may tap into female sensory biases that may influence female mate selection. Male gift-giving spiders are known to provide gifts to females in order to avoid being eaten. This is a tactic that males may use in order to manipulate females to not kill them. Females may have a strong, uncontrollable appetite, which males may use to their advantage by manipulating females through edible gifts.

Antiaphrodisiac

Males of several species of Heliconius butterflies, such as Heliconius melpomene and Heliconius erato, have been found to transfer an antiaphrodisiac to the female during copulation. This compound is only produced in the male and is how males identify one another as male. Therefore, when it is transferred to the female, she then smells like a male. This prevents future males from attempting to copulate with her. This behavior both benefits the female because harassment from males post mating has been found to decrease reproductive success by disturbing the production of eggs, and increases the reproductive success of the male by ensuring that his sperm will be used to fertilize the egg.

Sexual conflict after mating

The most well known examples of sexual conflict occur before and during mating, but conflicts of interest do not end once mating has happened. Initially there may be a conflict over female reproductive patterns such as reproductive rate, remating rate, and sperm utilization. In species with parental care, there may be a conflict over which sex provides care and the amount of care given to the offspring.

Cryptic female choice

Cryptic female choice falls under the conflict in reproductive patterns. Cryptic female choice results from process that occurs after intromission which allows the female to preferentially fertilize or produce offspring with a particular male phenotype. It is thought that if the female's cryptic choice provides her with indirect genetic benefits in the form of more fit offspring, any male phenotype that limits female cryptic choice will induce a cost on the female. Often, cryptic female choice occurs in polyandrous or polygamous species.

The cricket species, Gryllus bimaculatus, is a polygamous species. Multiple matings increases the hatching success of clutch of eggs which is hypothesized to be a result of increased chances of finding compatible sperm. Therefore, it is in the female's best interest to mate with multiple males to increase the offspring genetic fitness; however, males would prefer to sire more of the females' offspring and will try to prevent the female from having multiple matings by mate guarding to exclude rival males.

Similarly, the polyandrous species of spider Pisaura mirabilis has been demonstrated to have cryptic female choice. The presence of a nuptial gift by a male increases the proportion of sperm retained by the female (With copulation duration controlled for).

Parental care

Parental investment is when either parent cares for eggs or offspring resulting in increased offspring fitness. Though intuitively one might assume that since providing care to offspring would provide indirect genetic benefits to both parents, there would not be much sexual conflict; however, since neither is interested in the other's genetic fitness, it is more beneficial to be selfish and push the costs of parental care onto the other sex. Therefore, each partner would exert selection on the other partner to provide more care, creating sexual conflict. Additionally, since it is beneficial for one partner to develop adaptations in parental care at the expense of the other, the other partner is likely to evolve counter adaptations to avoid being exploited, creating a situation to be predicted by game theory.

In the species Nicrophorus defodiens, the burying beetle, there is biparental care; however, males of the species will resume releasing pheromones after mating with the primary female in order to attract more females to increase his reproductive output. However, it is in the female's best interest if she can monopolize the male's parental care and food providence for her offspring. Therefore, the female will bite and attempt to push the male off his signaling perch and interfere with the male's secondary mating attempts in order to impose monogamy on the male.

A singing Eurasian penduline tit

In Remiz pendulinus, the Eurasian penduline tit, the male will build an elaborate nest and may or may not be joined by a female at any stage of construction. After eggs are laid, it is strictly uniparental incubation and offspring care; however, either parent may take the role of caregiver. Females will give care 50-70% of initiated breedings while males will give care 5-20% of the time, and approximately 30%-35% of the time, the eggs, which consist of four to five viable eggs, will be left to die, which is a considerable cost to both parents. However, being deserted also represents a large cost for the deserted parent. Therefore, timing of desertion becomes very important. Optimal timing for the males depends on the status of the clutch, and as a result the male frequently enters and remains near the nest during the egg-laying period. For females it is important not to desert too early so that the male does not also desert the eggs, but also not too late else the male deserts before the female does. Females adapt by being very aggressive towards males that try to approach the nest as well as hiding one or more eggs so that males do not have full information on the clutch status.

Breeding success of Eurasian penduline tits suggests conflicting interests between males and females in a wild population: by deserting the clutch each parent increases her (or his) reproductive success although desertion reduces the reproductive success of their mate. This tug-of-war between males and females over care provisioning has been suggested to drive flexible parenting strategies in this species. In the closely related Cape penduline tit Anthoscopus minutus, however, both parents incubate the eggs and rear the young. A contributing factor to parenting decision is extra-pair paternity since in Cape penduline tit less than 8% of young were extra-pair whereas in Eurasian penduline tit over 24% young resulted from extra-pair paternity.

In other species such as the Guianan cock-of-the-rock, as well as other lekking species, sexual conflict may not even manifest itself in parental care. The females of these species have the tendency to select males to mate with, become fertilized, and the females raise the offspring on their own in their nests.

Sexual cannibalism

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Sexual_cannibalism

The prevalence of sexual cannibalism gives several species of Latrodectus the common name "black widow spider".

Sexual cannibalism is when an animal, usually the female, cannibalizes its mate prior to, during, or after copulation. It is a trait observed in many arachnid orders, several insect and crustacean clades, gastropods, and some snake species. Several hypotheses to explain this seemingly paradoxical behavior have been proposed. The adaptive foraging hypothesis, aggressive spillover hypothesis and mistaken identity hypothesis are among the proposed hypotheses to explain how sexual cannibalism evolved. This behavior is believed to have evolved as a manifestation of sexual conflict, occurring when the reproductive interests of males and females differ. In many species that exhibit sexual cannibalism, the female consumes the male upon detection. Females of cannibalistic species are generally hostile and unwilling to mate; thus many males of these species have developed adaptive behaviors to counteract female aggression.

Prevalence

Sexual cannibalism is common among insects, arachnids and amphipods. There is also evidence of sexual cannibalism in gastropods and copepods. Sexual cannibalism is common among species with prominent sexual size dimorphism (SSD); extreme SSD likely drives this trait of sexual cannibalism in spiders. It also sometimes occurs in some anacondas, especially the green anaconda (Eunectes murinus), where females are larger than the males.

Proposed explanations

Female Chinese mantis eats a male copulating with her.

Different hypotheses have been proposed to explain sexual cannibalism, namely adaptive foraging, aggressive spillover, mate choice, and mistaken identity.

Adaptive foraging

The adaptive foraging hypothesis is a proposed pre-copulatory explanation in which females assess the nutritional value of a male compared to the male's value as a mate. Starving females are usually in poor physical condition and are therefore more likely to cannibalize a male than to mate with him. Among mantises, cannibalism by female Pseudomantis albofimbriata improves fecundity, overall growth, and body condition. A study on the Chinese mantis found that cannibalism occurred in up to 50% of matings. Among spiders, Dolomedes triton females in need of additional energy and nutrients for egg development choose to consume the closest nutritional source, even if this means cannibalizing a potential mate. In Agelenopsis pennsylvanica and Lycosa tarantula, a significant increase in fecundity, egg case size, hatching success, and survivor-ship of offspring has been observed when hungry females choose to cannibalize smaller males before copulating with larger, genetically superior males. This reproductive success was largely due to the increased energy uptake by cannibalizing males and investing that additional energy in the development of larger, higher-quality egg cases. In D. triton, post-copulatory sexual cannibalism was observed in the females that had a limited food source; these females copulated with the males and then cannibalized them.

The adaptive foraging hypothesis has been criticized because males are considered poor meals when compared to crickets; however, recent findings discovered Hogna helluo males have nutrients crickets lack, including various proteins and lipids. In H. helluo, females have a higher protein diet when cannibalizing males than when consuming only house crickets. Further studies show that Argiope keyserlingi females with high-protein/low-lipid diets resulting from sexual cannibalism may produce eggs of greater egg energy density (yolk investment).

Aggressive spillover

The aggressive spillover hypothesis suggests that the more aggressive a female is concerning prey, the more likely the female is to cannibalize a potential mate. The decision of a female to cannibalize a male is not defined by the nutritional value or genetic advantage (courtship dances, male aggressiveness, & large body size) of males but instead depends strictly on her aggressive state. Aggression of the female is measured by latency (speed) of attack on prey. The faster the speed of attack and consumption of prey, the higher the aggressiveness level. Females displaying aggressive characteristics tend to grow larger than other females and display continuous cannibalistic behavior. Such behavior may drive away potential mates, reducing chances of mating. Aggressive behavior is less common in an environment that is female-biased, because there is more competition to mate with a male. In these female dominated environments, such aggressive behavior comes with the risk of scaring away potential mates.

Males of the Pisaura mirabilis species feign death to avoid being cannibalized by a female prior to copulation. When males feign death, their success in reproduction depends on the level of aggressiveness the female displays. Research has shown that in the Nephilengys livida species, female aggressiveness had no effect on the likelihood of her cannibalizing a potential mate; male aggressiveness and male-male competition determined which male the female cannibalized. Males with aggressive characteristics were favored and had a higher chance of mating with a female.

Mate choice

Nephila sp. eating a conspecific

Females exercise mate choice, rejecting unwanted and unfit males by cannibalizing them. Mate choice often correlates size with fitness level; smaller males tend to be less aggressive and display a low level of fitness; smaller males are therefore eaten more often because of their undesirable traits. Males perform elaborate courtship dances to display fitness and genetic advantage. Female orb-web spiders (Nephilengys livida) tend to cannibalize males displaying less aggressive behavior and mate with males displaying more aggressive behavior, showing a preference for this trait, which, along with large body size that indicates a strong foraging ability, displays high male quality and genetic advantage. Indirect mate choice can be witnessed in fishing spiders, Dolomedes fimbriatus, where females do not discriminate against smaller body size, attacking males of all sizes. Females had lower success rates cannibalizing large males, which managed to escape where smaller males could not. It was shown that males with desirable traits (large body size, high aggression, and long courtship dances) had longer copulation duration than males with undesirable traits. In A. keyserlingi and Nephila edulis females allow longer copulation duration and a second copulation for smaller males. The gravity hypothesis suggests that some species of spiders may favor smaller body sizes because they enable them to climb up plants more efficiently and find a mate faster. Also smaller males may be favored because they hatch and mature faster, giving them a direct advantage in finding and mating with a female. In Leucauge mariana females will cannibalize males if their sexual performance was poor. They use palpal inflations to determine sperm count and if the female deems sperm count too low she will consume the male. In Latrodectus revivensis females tend to limit copulation duration for small males and deny them a second copulation, showing preference for larger body size. Another form of mate choice is the genetic bet-hedging hypothesis in which a female consumes males to prevent them from exploiting her. It is not beneficial for a female exploited by multiple males because it may result in prey theft, reduction in web, and reduced time of foraging. Sexual cannibalism might have promoted the evolution of some behavioral and morphological traits exhibited by spiders today.

Mistaken identity

The mistaken identity hypothesis suggests that sexual cannibalism occurs when females fail to identify males that try to court. This hypothesis suggests that a cannibalistic female attacks and consumes the male without the knowledge of mate quality. In pre-copulatory sexual cannibalism, mistaken identity can be seen when a female does not allow the male to perform the courtship dance and engages in attack. There is no conclusive evidence for this hypothesis because scientists struggle to distinguish between mistaken identity and the other hypotheses (aggressive spillover, adaptive foraging, and mate choice).

Male adaptive behaviours

In some cases, sexual cannibalism may characterize an extreme form of male monogamy, in which the male sacrifices itself to the female. Males may gain reproductive success from being cannibalized by either providing nutrients to the female (indirectly to the offspring), or through enhancing the probability that their sperm is used to fertilize the female's eggs. Although sexual cannibalism is fairly common in spiders, male self-sacrifice has only been reported in six genera of araneoid spiders. However, much of the evidence for male complicity in such cannibalistic behavior may be anecdotal, and has not been replicated in experimental and behavioural studies.

Male members of cannibalistic species have adapted different mating tactics as a mechanism for escaping the cannibalistic tendencies of their female counterparts. Current theory suggests antagonistic co-evolution has occurred, where adaptations seen in one sex produce adaptations in the other. Adaptations consist of courtship displays, opportunistic mating tactics, and mate binding.

Opportunistic mating

The risk of cannibalism becomes greatly reduced when opportunistic mating is practiced. Opportunistic mating has been characterized in numerous orb-weaving spider species, such as Nephila fenestrata, where the male spider waits until the female is feeding or distracted, and then proceeds with copulation; this greatly reduces the chances of cannibalization. This distraction can be facilitated by the male's presentation of nuptial gifts, where they provide a distracting meal for the female in order to prolong copulation and increase paternity.

Altered sexual approach

Multiple methods of sexual approaches have appeared in cannibalistic species as a result of sexual cannibalism. The mechanism by which the male approaches the female is imperative for his survival. If the female is unable to detect his presence, the male is less likely to face cannibalization. This is evident in the mantid species, Tenodera aridifolia, where the male alters his approach utilizing the surrounding windy conditions. The male attempts to avoid detection by approaching the female when the wind impairs her ability to hear him. In the praying mantid species Pseudomantis albofimbrata, the males approach the female either from a "slow mounting from the rear" or a "slow approach from the front" position to remain undetected. The male alters his approach through the utilization of the surrounding windy conditions, and thus the risk of facing cannibalization is reduced.

Mate guarding

Sexual cannibalism has impaired the ability of the orb-weaving spider, N. fenestrata, to perform mate guarding. If a male successfully mates with a female, he then exhibits mate guarding, inhibiting the female from re-mating, thus ensuring his paternity and eliminating sperm competition. Guarding can refer to the blockage of female genital openings to prevent further insertion of a competing male's pedipalps, or physical guarding from potential mates. Guarding can decrease female re-mating by fifty percent. Males who experience genital mutilation can sometimes exhibit the "gloves off" hypothesis which states that a male's body weight and his endurance are inversely proportional. Thus when a male's body weight decreases substantially, his endurance increases as a result, allowing him to guard his female mate with increased efficiency.

Mate binding

Mate binding refers to a pre-copulatory courtship behavior where the male deposits silk onto the abdomen of the female while simultaneously massaging her in order to reduce her aggressive behavior. This action allows for initial and subsequent copulatory bouts. While both chemical and tactile cues are important factors for reducing cannibalistic behaviors, the latter functions as a resource to calm the female, exhibited in the orb-weaver spider species, Nephila pilipes. Additional hypotheses suggest that male silk contains pheromones which seduce the female into submission. However, silk deposits are not necessary for successful copulation. The primary factor in successful subsequent copulation lies in the tactile communication between the male and female spider that results in female acceptance of the male. The male mounts the posterior portion of the female's abdomen, while rubbing his spinnerets on her abdomen during his attempt at copulation. Mate binding was not necessary for the initiation of copulation in the golden orb-weaving spider, except when the female was resistant to mating. Subsequent copulatory bouts are imperative for the male's ability to copulate due to prolonged sperm transfer, therefore increasing his probability of paternity.

Courtship displays

Courtship displays in sexually cannibalistic spiders are imperative in order to ensure the female is less aggressive. Additional courtship displays include pre-copulatory dances such as those observed in the redback spider, and vibrant male coloration morphologies which function as female attraction mechanisms, as seen in the peacock spider, Maratus volans. Nuptial gifts play a vital role in safe copulation for males in some species. Males present meals to the female to facilitate opportunistic mating while the female is distracted. Subsequent improvements in male adaptive mating success include web reduction, as seen in the Western black widow, Latrodectus hesperus. Once mating occurs, the males destroy a large portion of the female's web to discourage the female from future mating, thus reducing polyandry, which has been observed in the Australian redback spider, Latrodectus hasselti.

Male-induced cataleptic state

In some species of spiders, such as Agelenopsis aperta, the male induces a passive state in the female prior to copulation. It has been hypothesized that the cause of this "quiescent" state is the male's massaging of the female's abdomen, following male vibratory signals on the web. The female enters a passive state, and the male's risk of facing cannibalism is reduced. This state is most likely induced as a result of a male volatile pheromone. The chemical structure of the pheromone utilized by the male A. aperta is currently unknown; however, physical contact is not necessary for the induced passive state. Eunuch males, or males with partially or fully removed palps, are unable to induce the passive state on females from a distance, but can induce quiescence upon physical contact with the female; this suggests that the pheromone produced is potentially related to sperm production, since the male inserts sperm from his pedipalps, structures which are removed in eunuchs. This adaptation has most likely evolved in response to the overly aggressive nature of female spiders.

Copulatory silk wrapping

In order to avoid being consumed by the female, some male spiders may utilize their silk to physically bind the female spider. For example, in Pisaurina mira, also known as the nursery web spider, the male wraps the legs of the female in silk prior to and during copulation. While he holds legs III and IV of the female, he uses the silk to bind legs I and II. Because the male spider legs play a significant role in copulation, longer leg lengths in male P. mira are generally favored over shorter lengths.

Costs and benefits for males

The physiological impacts of cannibalism on male fitness include his inability to father any offspring if he is unable to mate with a female. There are males in species of arachnids, such as N. plumipes, that sire more offspring if the male is cannibalized after or during mating; copulation is prolonged and sperm transfer is increased. In the species of orb-weaving spider, Argiope arantia, males prefer short copulation duration upon the first palp insertion in order to avoid cannibalism. Upon the second insertion, however, the male remains inserted in the female. The male exhibits a "programmed death" to function as a full-body genital plug. This causes it to become increasingly difficult for the female to remove him from her genital openings, discouraging her from mating with other males. An additional benefit to cannibalization is the idea that a well-fed female is less likely to mate again. If the female has no desire to mate again, the male who has already mated with her has his paternity ensured.

Genital mutilation

Before or after copulating with females, certain males of spider species in the superfamily Araneoidea become half or full eunuchs with one or both of their pedipalps (male genitals) severed. This behavior is often seen in sexually cannibalistic spiders, causing them to exhibit the "eunuch phenomenon". Due to the chance that they may be eaten during or after copulation, male spiders use genital mutilation to increase their chances of successful mating. The male can increase his chances of paternity if the female's copulatory organs are blocked, which decreases sperm competition and her chances of mating with other males. In one study, females with mating plugs had a 75% lower chance of re-mating. Additionally, if a male successfully severs his pedipalp within the female copulatory duct the pedipalp can not only serve as a plug but can continue to release sperm to the female spermathacae, again increasing the male's chances of paternity. This is referred to as "remote copulation". Occasionally (in 12% of cases in a 2012 study on Nephilidae spiders) palp severance is only partial due to copulation interruption by sexual cannibalism. Partial palp severance can result in a successful mating plug but not to the extent of full palp severance. Some males, as in the orb-weaving spider, Argiope arantia, have been found to spontaneously die within fifteen minutes of their second copulation with a female. The male dies while his pedipalps are still intact within the female, as well as still swollen from copulation. In this "programmed death", the male is able to utilize his entire body as a genital plug for the female, causing it to be much more difficult for her to remove him from her copulatory ducts. In other species males voluntarily self-amputate a pedipalp prior to mating and thus the mutilation is not driven by sexual cannibalism. This has been hypothesized to be due to an increased fitness advantage of half or full eunuchs. Upon losing a pedipalp, males experience a significant decrease in body weight that provides them with enhanced locomotor abilities and endurance, enabling them to better search for a mate and mate-guard after mating. This is referred to as the "gloves-off" theory. Males and females have also been seen with the roles reversed in terms of genital mutilation. In Cyclosa argenteoalba, males mutilate female spider's genitals by detaching the female's scape, making it impossible for another male to mate with them.

Male self-sacrifice

Male reproductive success can be determined by their number of fathered offspring, and monogyny is seen quite often in sexually cannibalistic species. Males are willing to sacrifice themselves, or lose their reproductive organs in order to ensure their paternity from one mating instance. Whether it is by spontaneous programmed death, or the male catapulting into the mouth of the female, these self-sacrificing males die in order for prolonged copulation to occur. Males of many of these species cannot replenish sperm stores, therefore they must exhibit these extreme behaviors in order to ensure sperm transfer and fathered offspring during their one and only mating instance. An example of such behavior can be seen in the redback spider. The males of this species "somersault" into the mouths of the female after copulation has occurred, which has been shown to increase paternity by sixty-five percent when compared to males that are not cannibalized. A majority of males in this species are likely to die on the search for a mate, so the male must sacrifice himself as an offering if it means prolonged copulation and doubled paternity. In many species, cannibalized males can mate longer, thus having longer sperm transfers.

Male sexual cannibalism

Although females often instigate sexual cannibalism, reversed sexual cannibalism has been observed in the spiders Micaria sociabilis and Allocosa brasiliensis. In a laboratory experiment on M. sociabilis, males preferred to eat older females. This behavior may be interpreted as adaptive foraging, because older females have low reproductive potential and food may be limited. Reversed cannibalism in M. sociabilis may also be influenced by size dimorphism. Males and females are similar in size, and bigger males were more likely to be cannibalistic. In A. brasiliensis males tend to be cannibalistic in between mating seasons, after they have mated, gone out of their burrows to search for food, and left their mates in their burrows. Any females they cross during this period likely have little reproductive value, so this may also be interpreted as adaptive foraging. It has also been observed in the crab Ovalipes catharus. Reversed sexual cannibalism is also observed in a snake species called Malpolon monspessulanus, commonly known as Montpellier snakes. This behavior may occur due to their opportunistic feeding habits, lack of availability of prey, or competition for resources among the individuals of the species. As this species exhibits male-biased sexual dimorphism, it is easier for male Montpellier snakes to attack and cannibalize the females. Male cannibalism might also be triggered by the refusal to mate by female M. monspessulanus.

Monogamy

Males in these mating systems are generally monogamous, if not bigynous. Since males of these cannibalistic species have adapted to the extreme mating system, and usually mate only once with a polyandrous female, they are considered monogynous.

Other factors

Sexual dimorphism

Sexual dimorphism in size has been proposed as an explanation for the widespread nature of sexual cannibalism across distantly related arthropods. Typically, male birds and mammals are larger as they participate in male-male competition. However, in arthropods this size dimorphism ratio is reversed, with females commonly larger than males. Sexual cannibalism may have led to selection for larger, stronger females in invertebrates. Further research is needed to evaluate the explanation. To date, studies have been done on wolf spiders such as Zyuzicosa (Lycosidae), where the female is much larger than the male.