Brain size
was previously considered a major indicator of the intelligence of an
animal. However, many other factors also affect intelligence, and recent
discoveries concerning bird intelligence have called into question the influence of brain size.
Since most of the brain is used for maintaining bodily functions,
greater ratios of brain to body mass may increase the amount of brain
mass available for more complex cognitive tasks. Allometric analysis indicates that in general, mammalian brain size scales at approximately the 2⁄3 or 3⁄4 exponent of body mass. Comparison of actual brain size with the size expected from allometry provides an encephalization quotient (EQ) that can be used as a more accurate indicator of an animal's intelligence.
Brain of the sperm whale, considered the largest brain in the animal kingdom
Sperm whales (Physeter macrocephalus) have the largest known brain mass of any extant animal, averaging 7.8 kg in mature males.
Orcas (Orcinus orca) have the second largest known brain mass of any extant animal. (5.4-6.8 kg)
Bottlenose dolphins (Tursiops truncatus)
have an absolute brain mass of 1,500–1,700 grams. This is slightly
greater than that of humans (1,300–1,400 grams) and about four times
that of chimpanzees (400 grams).
The brain to body mass ratio
(not the encephalization quotient) in some members of the odontocete
superfamily Delphinoidea (dolphins, porpoises, belugas, and narwhals) is
greater than modern humans, and greater than all other mammals (there
is debate whether that of the treeshrew might be second in place of humans). In some dolphins, it is less than half that of humans: 0.9% versus 2.1%. However, this comparison is complicated by the large amount of insulating blubber Delphinoidea brains have (15-20% of mass).
The majority of mammals are born with a brain close to 90% of the adult brain weight. Humans are born with 28% of the adult brain weight, chimpanzees with 54%, bottlenose dolphins with 42.5%, and elephants with 35%.
Spindle cells (neurons without extensive branching) have been discovered in the brains of the humpback whale, fin whale, sperm whale, orca. bottlenose dolphins, Risso's dolphins, and beluga whales.
Humans, great apes, and elephants, species all well known for their
high intelligence, are the only others known to have spindle cells. Spindle neurons appear to play a central role in the development of intelligent behavior. Such a discovery may suggest a convergent evolution of these species.
Elephant brains also show a complexity similar to dolphin brains, and are also more convoluted than that of humans, and with a cortex thicker than that of cetaceans. It is generally agreed that the growth of the neocortex,
both absolutely and relative to the rest of the brain, during human
evolution, has been responsible for the evolution of human intelligence,
however defined. While a complex neocortex usually indicates high
intelligence, there are exceptions. For example, the echidna has a highly developed brain, yet is not widely considered very intelligent,
though preliminary investigations into their intelligence suggest that
echidnas are capable of more advanced cognitive tasks than were
previously assumed.
In 2014, it was shown for the first time that a species of dolphin, the long-finned pilot whale, has more neocortical neurons than any mammal studied to date including humans.
Unlike terrestrial mammals, dolphin brains contain a paralimbic lobe,
which may possibly be used for sensory processing. It has also been
suggested that similar to humans, the paralimbic region of the brain is
responsible for a dolphin's self-control, motivation, and emotions. The dolphin is a voluntary breather, even during sleep, with the result that veterinary anaesthesia of dolphins would result in asphyxiation.
Ridgway reports that EEGs show alternating hemispheric asymmetry in
slow waves during sleep, with occasional sleep-like waves from both
hemispheres.
This result has been interpreted to mean that dolphins sleep only one
hemisphere of their brain at a time, possibly to control their voluntary
respiration system or to be vigilant for predators.
The dolphin's greater dependence on sound processing is evident
in the structure of its brain: its neural area devoted to visual imaging
is only about one-tenth that of the human brain, while the area devoted
to acoustical imaging is about 10 times as large.
Sensory experiments suggest a great degree of cross-modal integration
in the processing of shapes between echolocative and visual areas of the
brain.
Brain evolution
The evolution of encephalization in cetaceans is similar to that in primates.
Though the general trend in their evolutionary history increased brain
mass, body mass, and encephalization quotient, a few lineages actually
underwent decephalization, although the selective pressures that caused
this are still under debate.
Among cetaceans, Odontoceti tend to have higher encephalization
quotients than Mysticeti, which is at least partially due to the fact
that Mysticeti have much larger body masses without a compensating
increase in brain mass.
As far as which selective pressures drove the encephalization (or
decephalization) of cetacean brains, current research espouses a few
main theories. The most promising suggests that cetacean brain size and
complexity increased to support complex social relations. It could also have been driven by changes in diet, the emergence of echolocation, or an increase in territorial range.
Problem-solving ability
Some
research shows that dolphins, among other animals, understand concepts
such as numerical continuity, though not necessarily counting. Dolphins may be able to discriminate between numbers.
Several researchers observing animals' ability to learn set formation tend to rank dolphins at about the level of elephants in intelligence, and show that dolphins do not surpass other highly intelligent animals in problem solving.
A 1982 survey of other studies showed that in the learning of "set
formation", dolphins rank highly, but not as high as some other animals.
Dolphin group sizes vary quite dramatically. River dolphins
usually congregate in fairly small groups from 6 to 12 in number or, in
some species, singly or in pairs. The individuals in these small groups
know and recognize one another. Other species such as the oceanic pantropical spotted dolphin, common dolphin and spinner dolphin
travel in large groups of hundreds of individuals. It is unknown
whether every member of the group is acquainted with every other.
However, large packs can act as a single cohesive unit – observations
show that if an unexpected disturbance, such as a shark approach, occurs
from the flank or from beneath the group, the group moves in
near-unison to avoid the threat. This means that the dolphins must be
aware not only of their near neighbors but also of other individuals
nearby – in a similar manner to which humans perform "audience waves".
This is achieved by sight, and possibly also echolocation. One
hypothesis proposed by Jerison (1986) is that members of a pod of
dolphins are able to share echolocation results with each other to
create a better understanding of their surroundings.
Southern resident orcas in British Columbia, Canada, and
Washington, United States, live in extended family groups. The basis of
the southern resident orca social structure
is the matriline, consisting of a matriarch and her descendants of all
generations. A number of matrilines form a southern resident orca pod,
which is ongoing and extremely stable in membership, and has its own dialect which is stable over time. A southern resident calf is born into the pod of their mother and remains in it for life.
Members of a southern resident orca family unit travelling in formation with the mother and youngest offspring in the centre
A cetacean dialect is a socially–determined vocal tradition. The
complex vocal communication systems of orcas correspond with their large
brains and complex social structure. The three southern resident orca pods share some calls with one another, and also have unique calls.
Discussing the function of resident orca dialects, researchers John
Ford, Graeme Ellis and Ken Balcomb wrote, "It may well be that dialects
are used by the whales as acoustic indicators of group identity and
membership, which might serve to preserve the integrity and cohesiveness
of the social unit."
Resident orcas form closed societies with no emigration or dispersal of
individuals, and no gene flow with other orca populations. There is evidence that other species of dolphins may also have dialects.
In bottlenose dolphin studies by Wells in Sarasota, Florida, and Smolker in Shark Bay, Australia, females of a community are all linked either directly or through a mutual association in an overall social structure known as fission-fusion.
Groups of the strongest association are known as "bands", and their
composition can remain stable over years. There is some genetic evidence
that band members may be related, but these bands are not necessarily
limited to a single matrilineal line. There is no evidence that bands
compete with each other. In the same research areas, as well as in Moray Firth, Scotland,
males form strong associations of two to three individuals, with a
coefficient of association between 70 and 100. These groups of males are
known as "alliances", and members often display synchronous behaviors
such as respiration, jumping, and breaching. Alliance composition is
stable on the order of tens of years, and may provide a benefit for the
acquisition of females for mating.
The complex social strategies of marine mammals such as bottlenose
dolphins, "provide interesting parallels" with the social strategies of
elephants and chimpanzees.
Complex play
Dolphins are known to engage in complex play behavior, which includes such things as producing stable underwater toroidal air-core vortex rings or "bubble rings".
There are two main methods of bubble ring production: rapid puffing of a
burst of air into the water and allowing it to rise to the surface,
forming a ring; or swimming repeatedly in a circle and then stopping to
inject air into the helical
vortex currents thus formed. The dolphin will often then examine its
creation visually and with sonar. They also appear to enjoy biting the
vortex-rings they have created, so that they burst into many separate
normal bubbles and then rise quickly to the surface.
Certain whales are also known to produce bubble rings or bubble nets
for the purpose of foraging. Many dolphin species also play by riding in
waves, whether natural waves near the shoreline in a method akin to
human "body-surfing", or within the waves induced by the bow of a moving
boat in a behavior known as bow riding.
Cross-species cooperation
There
have been instances in captivity of various species of dolphin and
porpoise helping and interacting across species, including helping
beached whales. Dolphins have also been known to aid human swimmers in need, and in at least one instance a distressed dolphin approached human divers seeking assistance.
Creative behavior
A pair of bottlenose dolphins responding back with squawking behavior
Aside from having exhibited the ability to learn complex tricks,
dolphins have also demonstrated the ability to produce creative
responses. This was studied by Karen Pryor during the mid-1960s at Sea Life Park in Hawaii, and was published as The Creative Porpoise: Training for Novel Behavior in 1969. The two test subjects were two rough-toothed dolphins (Steno bredanensis),
named Malia (a regular show performer at Sea Life Park) and Hou (a
research subject at adjacent Oceanic Institute). The experiment tested
when and whether the dolphins would identify that they were being
rewarded (with fish) for originality in behavior and was very
successful. However, since only two dolphins were involved in the
experiment, the study is difficult to generalize.
Starting with the dolphin named Malia, the method of the
experiment was to choose a particular behavior exhibited by her each day
and reward each display of that behavior throughout the day's session.
At the start of each new day Malia would present the prior day's
behavior, but only when a new behavior was exhibited was a reward given.
All behaviors exhibited were, at least for a time, known behaviors of
dolphins. After approximately two weeks Malia apparently exhausted
"normal" behaviors and began to repeat performances. This was not
rewarded.
According to Pryor, the dolphin became almost despondent.
However, at the sixteenth session without novel behavior, the
researchers were presented with a flip they had never seen before. This
was reinforced.
As related by Pryor, after the new display: "instead of offering that
again she offered a tail swipe we'd never seen; we reinforced that. She
began offering us all kinds of behavior that we hadn't seen in such a
mad flurry that finally we could hardly choose what to throw fish at".
The second test subject, Hou, took thirty-three sessions to reach
the same stage. On each occasion the experiment was stopped when the
variability of dolphin behavior became too complex to make further
positive reinforcement meaningful.
The same experiment was repeated with humans, and it took the
volunteers about the same length of time to figure out what was being
asked of them. After an initial period of frustration or anger, the
humans realised they were being rewarded for novel behavior. In dolphins
this realisation produced excitement and more and more novel
behaviors – in humans it mostly just produced relief.
Captive orcas have displayed responses indicating they get bored with activities. For instance, when Paul Spong
worked with the orca Skana, he researched her visual skills. However,
after performing favorably in the 72 trials per day, Skana suddenly
began consistently getting every answer wrong. Spong concluded that a
few fish were not enough motivation. He began playing music, which
seemed to provide Skana with much more motivation.
At the Institute for Marine Mammal Studies in Mississippi, it has
also been observed that the resident dolphins seem to show an awareness
of the future. The dolphins are trained to keep their own tank clean by
retrieving rubbish and bringing it to a keeper, to be rewarded with a
fish. However, one dolphin, named Kelly, has apparently learned a way to
get more fish, by hoarding the rubbish under a rock at the bottom of
the pool and bringing it up one small piece at a time.
As of 1984, scientists have observed wild bottlenose dolphins in Shark Bay,
Western Australia using a basic tool. When searching for food on the
sea floor, many of these dolphins were seen tearing off pieces of sponge and wrapping them around their rostra, presumably to prevent abrasions and facilitate digging.
Whales use a variety of sounds for their communication and sensation. Odontocete (toothed whale) vocal production is classified in three categories: clicks, whistles, and pulsed calls:
Clicks are very brief vocal sounds produced in rapid series for echolocation.
Echoes of the clicks contain sound data about the surroundings
transmitted through the ears to the brain, which is able to resolve
echoes into information.
Whistles – narrow-band frequency modulated (FM) signals – are used for communicative purposes, such as contact calls, or the signature whistle of bottlenose dolphins. Whistles are the primary social vocalization among the majority of Delphinidae species.
Pulsed calls are significant for a few cetacean species, such as the narwhal,
and the orca. These calls have distinct tonal qualities and a complex
harmonic structure. Typically 0.5–1.5 s in duration, they are the
primary social vocalization of orcas.
Researchers John Ford, Graeme Ellis, and Ken Balcomb wrote, "By varying
the timbre and frequency structure of the calls, the whales can
generate a variety of signals…Most calls contain sudden shifts or rapid
sweeps in pitch, which give them distinctive qualities recognizable over
distance and background noise."
There is strong evidence that some specific whistles, called signature whistles,
are used by dolphins to identify and/or call each other; dolphins have
been observed emitting both other specimens' signature whistles, and
their own. A unique signature whistle develops quite early in a
dolphin's life, and it appears to be created in imitation of the
signature whistle of the dolphin's mother. Imitation of the signature whistle seems to occur only among the mother and its young, and among befriended adult males.
Xitco reported the ability of dolphins to eavesdrop passively on
the active echolocative inspection of an object by another dolphin. Herman
calls this effect the "acoustic flashlight" hypothesis, and may be
related to findings by both Herman and Xitco on the comprehension of
variations on the pointing gesture, including human pointing, dolphin
postural pointing, and human gaze, in the sense of a redirection of
another individual's attention, an ability which may require theory of mind.
The environment where dolphins live makes experiments much more
expensive and complicated than for many other species; additionally, the
fact that cetaceans can emit and hear sounds
(which are believed to be their main means of communication) in a range
of frequencies much wider than humans can means that sophisticated
equipment, which was scarcely available in the past, is needed to record
and analyse them. For example, clicks can contain significant energy in
frequencies greater than 110 kHz (for comparison, it is unusual for a human to be able to hear sounds above 20 kHz), requiring that equipment have a sampling rates of at least 220 kHz; MHz-capable hardware is often used.
In addition to the acoustic communication channel, the visual modality is also significant. The contrasting pigmentation
of the body may be used, for example with "flashes" of the
hypopigmented ventral area of some species, as can the production of
bubble streams during signature whistling. Also, much of the synchronous
and cooperative behaviors, as described in the Behavior section of this entry, as well as cooperative foraging methods, likely are managed at least partly by visual means.
Experiments have shown that they can learn human sign language and can use whistles for 2-way human–animal communication. Phoenix and Akeakamai,
bottlenose dolphins, understood individual words and basic sentences
like "touch the frisbee with your tail and then jump over it".
Phoenix learned whistles, and Akeakamai learned sign language. Both
dolphins understood the significance of the ordering of tasks in a
sentence.
A study conducted by Jason Bruck of the University of Chicago showed that bottlenose dolphins can remember whistles
of other dolphins they had lived with after 20 years of separation.
Each dolphin has a unique whistle that functions like a name, allowing
the marine mammals to keep close social bonds.
The new research shows that dolphins have the longest memory yet known
in any species other than humans.
Self-awareness
Self-awareness, though not well defined scientifically, is believed to be the precursor to more advanced processes like meta-cognitive reasoning (thinking about thinking) that are typical of humans. Scientific research in this field has suggested that bottlenose dolphins, alongside elephants and great apes, possess self-awareness.
The most widely used test for self-awareness in animals is the mirror test, developed by Gordon Gallup in the 1970s, in which a temporary dye is placed on an animal's body, and the animal is then presented with a mirror.
In 1995, Marten and Psarakos used television to test dolphin self-awareness.
They showed dolphins real-time footage of themselves, recorded footage,
and another dolphin. They concluded that their evidence suggested
self-awareness rather than social behavior. While this particular study
has not been repeated since then, dolphins have since passed the mirror
test. However, some researchers have argued that evidence for self-awareness has not been convincingly demonstrated.
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 generallyState 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 alsoState 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).
From the Latinconfer
("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: ContraBlake 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, SeeAmerican 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:
"SeeMass. 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)."
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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:
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.
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
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.
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).
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 bugsCimex 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 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.
The penis of a Callosobruchus analisbean 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.
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 (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.
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 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.
