Examples of patterns that have been postulated to undergo variation and selection, and thus adaptation, are genes, ideas (memes), theories, technologies, neurons and their connections, words, computer programs, firms, antibodies, institutions, law and judicial systems, quantum states and even whole universes.
History and development
Conceptually, "evolutionary theorizing about cultural, social, and economic phenomena" preceded Darwin, but was still lacking the concept of natural selection.
Starting in the 1950s, Donald T. Campbell was one of the first and most influential authors to revive the tradition, and to formulate a generalized Darwinian algorithm directly applicable to phenomena outside of biology. In this, he was inspired by William Ross Ashby's view of self-organization and intelligence as fundamental processes of selection. His aim was to explain the development of science and other forms of knowledge by focusing on the variation and selection of ideas and theories, thus laying the basis for the domain of evolutionary epistemology.
In the 1990s, Campbell's formulation of the mechanism of
"blind-variation-and-selective-retention" (BVSR) was further developed
and extended to other domains under the labels of "universal selection
theory" or "universal selectionism" by his disciples Gary Cziko, Mark Bickhard, and Francis Heylighen.
Richard Dawkins
may have first coined the term "universal Darwinism" in 1983 to
describe his conjecture that any possible life forms existing outside
the Solar System would evolve by natural selection just as they do on Earth. This conjecture was also presented in 1983 in a paper entitled “The
Darwinian Dynamic” that dealt with the evolution of order in living
systems and certain nonliving physical systems. It was suggested “that ‘life’, wherever it might exist in the
universe, evolves according to the same dynamical law” termed the
Darwinian dynamic. Henry Plotkin in his 1997 book on Darwin machines makes the link between universal Darwinism and Campbell's evolutionary epistemology.
The philosopher of mind Daniel Dennett, in his 1995 book Darwin's Dangerous Idea,
developed the idea of a Darwinian process, involving variation,
selection and retention, as a generic algorithm that is
substrate-neutral and could be applied to many fields of knowledge
outside of biology. He described the idea of natural selection as a
"universal acid" that cannot be contained in any vessel, as it seeps
through the walls and spreads ever further, touching and transforming
ever more domains. He notes in particular the field of memetics in the social sciences.
Author D. B. Kelley has formulated one of the most all-encompassing approaches to universal Darwinism. In his 2013 book The Origin of Everything, he holds that natural selection involves not the preservation of favored races in the struggle for life, as shown by Darwin,
but the preservation of favored systems in contention for existence.
The fundamental mechanism behind all such stability and evolution is
therefore what Kelley calls "survival of the fittest systems." Because all systems are cyclical, the Darwinian processes of iteration, variation and selection
are operative not only among species but among all natural phenomena
both large-scale and small. Kelley thus maintains that, since the Big Bang especially, the universe has evolved from a highly chaotic state to one that is now highly ordered with many stable phenomena, naturally selected.
Gene-based Darwinian extensions
Evolutionary psychology assumes that our emotions, preferences and cognitive mechanisms are the product of natural selection
Cultural materialism is an anthropological approach that contends that the physical world impacts and sets constraints on human behavior.
Environmental determinism is a social science theory that proposes that it is the environment that ultimately determines human culture.
Evolutionary economics studies the variation and selection of economic phenomena, such as commodities, technologies, institutions and organizations.
Evolutionary ethics investigates the origin of morality, and uses Darwinian foundations to formulate ethical values
Big History
is the science-based narrative integrating the history of the universe,
earth, life, and humanity. Scholars consider Universal Darwinism to be a
possible unifying theme for the discipline.
Democracy and economic growth
and development have had a strong correlative and interactive
relationship throughout history. Effects of democracy on economic growth
and effect of economic growth on democracy can be distinguished. While
evidence of a relationship is irrefutable, economists' and historians' opinions of its exact nature have been
sharply split, hence the latter has been the subject of many debates and
studies.
History
The period of Ancient Greece 4th century B.C. and later of the Roman Empire marks the beginning not only of democracy, but as well as its connection to economic growth. The first showings in Ancient Greece in the city of Athens show a highly positive correlation with respect to economic growth and democracy. With the introduction of markets, specialization and reforms like having trial by jury, civil liberties as well as free speech,
they were able to sustain a self-sufficient city at the public expense.
The first document describing such a structure was written by Xenophon.
Romans enjoyed an even greater boom to their economy. Granted a
lot of their success was due to their unbeatable production of iron as
well as the development of trade routes i.e. Pax Romana. They ruled with a mixture of kingship, aristocracy and democracy. Despite their accomplishments from the reformed political structure, the need to invest in the military
to keep their growing competition at bay, by producing less and less
valuable coins, ultimately led to their collapse, recessing back to the
country side and barter system.
Industrial Revolution and Great Divergence have been connected to changes in political institutions related to democratization. The period of the transition from mercantilism to liberalism in England with expansion of international trade
shows the requirement of the needed change in political institutions
and policies for further development. Individuals which enjoyed more
political power due to their increased profits in international trade
influenced the political institutions to grant them the tools to further
their own goals, creating different policies, by which the economy grew
as a whole.
Further ahead, after World War II
over 100 nations undergone the transition of political and economic
development. In the past 2 decades democratic revolution has been
sweeping the whole world. There are 117 out of 191 independent states
that declare themselves as democratic. Even so, while cases like Brazil, India and Mauritius
have had several important economic achievements in their
late-democratic period, it is not safe to imply that these countries are
exemplary. Although they have performed better than expected, many more
changes lie in the future, while cases like Tunisia and Libya have had a much better period before their transition to a democratic regime. Reasons being their culture, history and many others.
Effects of democracy on economic growth
Democratization of a country from a non-democratic regime is usually preceded by a fall in GDP,
and a volatile but expected growth in the long run. On the other hand,
authoritarian regimes experience significant growth at the beginning and
decline in the long run. The cause of such behavior is that non-democratic regimes, mainly
authoritarian ones, are more effective at implementing decisive policies
and choices as well as solving ethnic and sub-national conflicts, but
are unsustainable in the long run as there is more incentive to extract
money from society which in turn leads to less prosperity. Democratic regimes revolve around institutions and policies which lay the foundations, through which principles of liberty
and equality are designed and followed, thus directly or indirectly
affecting firms or individuals who benefit from the directives and
increase their growth, which in turn has a positive impact on economy.
The positive changes of democracy to economic growth such as
delegation of authority and regulations of social conflicts heavily
outweigh the negative and restrictive effects, especially when compared
to autocracy.
One of the main reasons for this is that society, i.e. voters are able
to support difficult trade offs and changes when there is no perceived
alternative. This is primarily true in countries with a higher level of
education. So it ties the development level of a country as one of the
decisive factors to undergo positive democratic changes and reforms.
Thus, countries that embark in democratization at higher levels of
education are more likely than not to continue their development under
democracy.
As mentioned before, all of these factors do not guarantee
success. As for each such case, there is a failure. There is never a
single formula for democracy. The processes in associations with peace,
social stability and rapid socioeconomic development are not yet fully
understood, which may be the reason for a widespread opinion and many
hypotheses.
A 2008 meta-analysis found that democracy has no direct effect on
economic growth. However, it has strong and significant indirect
effects which contribute to growth. Democracy is associated with higher
human capital accumulation, lower inflation, lower political instability, and higher economic freedom. Democracy is closely tied with economic sources of growth, like
education levels and lifespan through improvement of educative
institutions as well as healthcare. "As democracy expands in developing
countries, newly empowered workers are likely to demand better living
conditions, health care, access to clean water, and so on—all conditions
that contribute to increased life expectancy and, in turn, to increased
productivity". There is also some evidence that it is associated with larger governments and more restrictions on international trade.
If leaving out East Asia,
then during the last forty-five years poor democracies have grown their
economies 50% more rapidly than nondemocracies. Poor democracies such
as the Baltic countries, Botswana, Costa Rica, Ghana, and Senegal have
grown more rapidly than nondemocracies such as Angola, Syria,
Uzbekistan, and Zimbabwe.
Democratizing African countries can prefer the economically
larger autocratic China over democratic Taiwan in search of economic
advantages (aid, trade and FDI). This correlation is true in all 7
African who became new democracies after 2000 or were approaching an
election following previous electoral performance combined with an
economic recession.
Survival of autocracies
Some autocracies disappear in the midst of an economic crises, while
other after a long period of prosperity, some after the founding
dictator dies or some as a result of defeat in foreign wars. However,
observing the conditions and predicting a transition to democracy
is so difficult, because the conditions only lay the ground-works for
the possibility that it may occur. But it is actions of people under
these conditions that shape the outcome. Many dissertations have been
written on the history of different transitions, and the opinions are
divided into two main categories.
One party proclaims that it boils down to the creation of civil society,
which comes to fruition almost of itself. A process fostered by
transformations of the social structure. However, others proclaim that
it is those that start with play the "strategic game" and reach a
bargain under conditions taken as a datum. The literature pits
"sociological" against "strategic" perspectives, yet we can say that
both of them are needed for a transition, and they are not mutually
exclusive.
In other words, a country that undergoes democratization does not have to necessarily experience economic growth, most often measured in income per capita,
or vice versa. For every such case there exists a counter example. What
this means is that there are multiple factors, such as political stability and political institutions, social insurance, government capacity, religion
and many other which influence the outcome. In two similar countries,
almost identical democratic regimes can yield completely different
results. However, the concepts highly complement each other, and in cases
through history where they were separated there has been great
difficulty.
Executive constraint by legislatures affects the size of economic downturns during leadership turnover.
Survival of democracies
The conditions for their origins may be hard to determine, but the
factors on which its survival depends are easily identifiable, and are
tightly connected to economic growth, that is the level of development
measured as per capita income. Another factor would be the education of
the labor force. Specifically the years of schooling of an average
citizen. This greatly elevates the probability that a democracy will
survive. However, even though income and education are highly
correlated, their impact seems to be to some extent independent, with
the impact of per capita income being much stronger. Empirical patterns
show that a democracy is more fragile in countries where per capita
income stagnates or declines, but the causality is not clear. The fact
that economic growth is tightly connected to democracies does not come
as a surprise, since democracies are more frequent among the
economically developed countries, and are rarer among poor ones.
The notion of economic growth having a greater influence on democracy
was a very popular opinion in the 1950s. The most important work on the
subject has been done by Lipset 1959, where he states that economic development is one of the prerequisites
for democracy. However, this is true. Both concepts are of equal
importance and there are many cases where one acts as a prerequisite for
the other, i.e. highly influencing the outcome.
Economic development may influence democracy in many ways. By tightening the revolution constraint, creating rising inequality or simply increasing the level of income in the society. And while the increase in GDP may be the primary method of measurement,
there is much more, such as forming or greatly changing productive
relationships, migrating firms and workers to cities up to affecting human capital
and technology.
This means that as an economic structure transforms, and since it is
related to capital intensity, capital itself becomes more important than
land, which is one of the reasons that states with a higher income per
capita would generally perform better.
As mentioned, the causality of economic development and democracy
is inconclusive. However, if we consider that democracy should be
supported by some preconditions, it is economic growth that creates
these conditions for democracy: industrialization, urbanization,
widespread of education and literacy, wealth, and a strong middle class
which are involved with the protection of their right and issues of
public affairs. Work done by Lipset
is best well known on this topic. By his comparative studies Lipset
shows a strong statistical association between GNP per capita and the
level of democracy, to finally conclude that "the more well-to-do a
nation, the greater the chance that it will sustain democracy". It is
especially relevant in just shaping democracies, even though they may
survive in poorer conditions.
As democracies require certain political institutions, it is
quite interesting that they do not have a high impact on economic
growth. What matters for economic development is, in fact political
stability, rather than a particular political institution. As it is safe
to assume that any political institution will promote development as
long as it is stable, which means that the danger lies in political
instability. And as measured in the past by the frequency of strikes,
demonstrations, riots, it is much greater in democracies, and a lot less
likely in e.g. dictatorships. Yet, political instability
does not affect economic growth in democracies, only in dictatorships.
The reasons for this are not entirely clear, whether it may be due to
institutional constraints or of motivations of those who govern
democracies. Under dictatorships, it slows down significantly when the
tenure of rulers is threatened. Similar outcomes emerge under various
forms of "socio-political unrest" such as strikes, anti-government demonstrations and riots.
Under different regimes, political phenomena have a different meaning,
and as such, it is not surprising that economic actors react
differently. Under dictatorships, whenever the regime is threatened, or
there are expected changes, workers or masses of people assemble to
strike and protest against their opposition, that is the government, and
the economy suffers. Under democracies, this is rarer, since everyone
knows that the government will change from time to time, and while they
know that they are able to protest in the same manner, most often than
not they do not.
For instance, it would be enriching to see Gerald Scully for some strong
arguments on political instability and growth. Studies actually observe
that democracies can somewhat affect growth. Studies have also shown
that the low economic growth may increase the probability of political
instability. In fact democracies have a negative but weak impact on
growth. but we can't miss addressing that major instability on involving
dramatical political changes can be harmful for economic growth.
Direction of effect
Empirical data tends to consistently suggest a causal relationship
between democracy and economic development. The causal direction does
appear to change, however. In some nations, economic growth has been
observed to promote democracy, while in others the opposite is true. Research done in post-socialist nations has shown increases in
political freedom to have little to no effect on economic growth,
however changes in political freedom have been influenced by the
aforementioned growth. Meanwhile, wider studies have found democracy to improve economic
growth when investigating the effects of democratic variables such as
increased government spending, increased private investment due to
higher economic freedom, and even social unrest.
There are several once very impoverished countries that have
experienced significant and rapid economic growth without democratic
institutions, for example Chile, Hong Kong, Taiwan, Singapore and South Korea. To the extent that political democracy exists in these countries today,
it has only recently emerged. What they have in common, being backward
countries in the past, is that they all have relatively free markets. We could say that they are economically free, meaning they have little to no protectionism, i.e. tariff and quotas on imports,
except for South Korea. This allows them to relieve the citizens of the
burden in the form of taxation and economic regulation, and this is one
of the reasons of their growth. Another characteristic common between
them and vital is that they have secure property rights and the rule of
law. A different case can be shown with India, where economic prosperity
was jeopardized due to people forming interest groups and losing their
political freedom. This compromises the free market institutions which
are essential to economic growth. A similar case can be said about Africa north of the Sahara.
Evolution of cells refers to the evolutionary origin and subsequent evolutionary development of cells. Cells first emerged at least 3.8 billion years ago[1][2][3] approximately 750 million years after Earth was formed.
The initial development of the cell marked the passage from prebiotic
chemistry to partitioned units resembling modern cells. The final
transition to living entities that fulfill all the definitions of modern
cells depended on the ability to evolve effectively by natural
selection. This transition has been called the Darwinian transition.
If life is viewed from the point of view of replicator
molecules, cells satisfy two fundamental conditions: protection from
the outside environment and confinement of biochemical activity. The
former condition is needed to keep complex molecules stable in a varying
and sometimes aggressive environment; the latter is fundamental for the
evolution of biocomplexity. If freely floating molecules that code for enzymes
are not enclosed in cells, the enzymes will automatically benefit
neighboring replicator molecules as well. Thus, the consequences of
diffusion in non-partitioned lifeforms would result in "parasitism by default." Therefore, the selection pressure
on replicator molecules will be lower, as the 'lucky' molecule that
produces the better enzyme does not fully leverage its advantage over
its close neighbors. In contrast, if the molecule is enclosed in a cell
membrane, the enzymes coded will be available only to itself. That
molecule will uniquely benefit from the enzymes it codes for, increasing
individuality and thus accelerating natural selection.
Partitioning may have begun from cell-like spheroids formed by proteinoids, which are observed by heating amino acids with phosphoric acid
as a catalyst. They bear much of the basic features provided by cell
membranes. Proteinoid-based protocells enclosing RNA molecules could
have been the first cellular life forms on Earth.
Another possibility is that the shores of the ancient coastal
waters may have been a suitable environment for the initial development
of cells. Waves breaking on the shore create a delicate foam composed of
bubbles. Shallow coastal waters also tend to be warmer, further
concentrating the molecules through evaporation. While bubbles made mostly of water tend to burst quickly, oily bubbles are much more stable. The phospholipid, the primary material of cell membranes, is an example of a common oily compound prevalent in the prebiotic seas.
Both of these options require the presence of massive amounts of
chemicals and organic material in order to form cells. A large gathering
of organic molecules most likely came from what scientists now call the
prebiotic soup. The prebiotic soup refers to the collection of every
organic compound that appeared on Earth after it was formed. This soup
would have most likely contained the compounds necessary to form early
cells.
Phospholipids are composed of a hydrophilic head on one end and a hydrophobic tail on the other. They can come together to form a bilayer
membrane. A lipid monolayer bubble can only contain oil and is not
conducive to harboring water-soluble organic molecules. On the other
hand, a lipid bilayer bubble can contain water and was a likely
precursor to the modern cell membrane.If a protein was introduced that increased the integrity of its parent bubble, then that bubble had an advantage. Primitive reproduction may have occurred when the bubbles burst,
releasing the results of the experiment into the surrounding medium.
Once enough of the right compounds were released into the medium, the
development of the first prokaryotes, eukaryotes, and multi-cellular organisms could be achieved.
However, the first cell membrane could not have been composed of
phospholipids due its low permeability, as ions would not able to pass
through the membrane. Rather it is suggested they were composed of fatty
acids, as they can freely exchange ions, allowing geochemically
sustained proton gradients at alkaline hydrothermal vents that might
lead to prebiotic chemical reactions via CO2 fixation.
Community metabolism
The common ancestor of the now existing cellular lineages
(eukaryotes, bacteria, and archaea) may have been a community of
organisms that readily exchanged components and genes. It would have
contained:
Autotrophs that produced organic compounds from CO2, either photosynthetically or by inorganic chemical reactions;
Heterotrophs that obtained organics from leakage of other organisms
Saprotrophs that absorbed nutrients from decaying organisms
Phagotrophs that were sufficiently complex to envelop and digest particulate nutrients, including other organisms.
The eukaryotic cell seems to have evolved from a symbiotic community
of prokaryotic cells. DNA-bearing organelles like mitochondria and
chloroplasts are remnants of ancient symbiotic oxygen-breathing bacteria and cyanobacteria, respectively, where at least part of the rest of the cell may have been derived from an ancestral archaean prokaryote cell. The archean prokaryote cell concept is often termed as the endosymbiotic theory. There is still debate about whether organelles like the hydrogenosome predated the origin of mitochondria, or vice versa: see the hydrogen hypothesis for the origin of eukaryotic cells.
How the current lineages of microbes evolved from this postulated
community is currently unsolved, but subject to intense research by
biologists, stimulated by the great flow of new discoveries in genome science.
Genetic code and the RNA world
Modern evidence suggests that early cellular evolution occurred in a
biological realm radically distinct from modern biology. It is thought
that in this ancient realm, the current genetic role of DNA was largely
filled by RNA, and catalysis was also largely mediated by RNA (that is,
by ribozyme counterparts of enzymes). This concept is known as the RNA world hypothesis.
According to this hypothesis, the ancient RNA world transitioned
into the modern cellular world via the evolution of protein synthesis,
followed by replacement of many cellular ribozyme catalysts by
protein-based enzymes. Proteins are much more flexible in catalysis than
RNA due to the existence of diverse amino acid side chains with
distinct chemical characteristics. The RNA record in existing cells
appears to preserve some 'molecular fossils'
from this RNA world. These RNA fossils include the ribosome itself (in
which RNA catalyzes peptide-bond formation), the modern ribozyme
catalyst RNase P, and RNAs.
The nearly universal genetic code
preserves some evidence for the RNA world. For instance, recent studies
of transfer RNAs, the enzymes that charge them with amino acids (the
first step in protein synthesis) and the way these components recognize
and exploit the genetic code, have been used to suggest that the
universal genetic code emerged before the evolution of the modern amino
acid activation method for protein synthesis. The first RNA polymers probably emerged prior to 4.17 Gya if life
originated at freshwater environments similar to Darwin's warm little
pond.
Sexual reproduction
The evolution of sexual reproduction may be a primordial and fundamental characteristic of eukaryotes, including single cell eukaryotes. Based on a phylogenetic analysis, Dacks and Roger proposed that facultative sex was present in the common ancestor of all eukaryotes. Hofstatter and Lehr reviewed evidence supporting the hypothesis that all eukaryotes can be regarded as sexual, unless proven otherwise.
Sexual reproduction may have arisen in early protocells with RNA genomes (RNA world). Initially, each protocell
would likely have contained one RNA genome (rather than multiple) since
this maximizes the growth rate. However, the occurrence of damages to
the RNA which block RNA replication or interfere with ribozyme
function would make it advantageous to fuse periodically with another
protocell to restore reproductive ability. This early, simple form of
genetic recovery is similar to that occurring in extant segmented
single-stranded RNA viruses (see influenza A virus).
As duplex DNA
became the predominant form of the genetic material, the mechanism of
genetic recovery evolved into the more complex process of meiotic recombination, found today in most species. It thus appears likely that sexual reproduction arose early in the evolution of cells and has had a continuous evolutionary history.
Horizontal gene transfer
Horizontal gene transfer
(HGT) is the movement of genetic information between different
organisms of the same species mainly being bacteria. This is not the
movement of genetic information between a parent and their offspring but
by other factors. In contrast to how animals reproduce and evolve from
sexual reproduction, bacteria evolve by sharing DNA with other bacteria
or their environment.
There are three common mechanisms of transferring genetic material by HGT:
Transformation: The bacteria assimilates DNA from the environment into their own
Conjugation: Bacteria directly transfer genes from one cell to another
Once one of these mechanisms has occurred the bacteria will continue
to multiply and grow resistance and evolve by natural selection. HGT is
the main cause of the assimilation of certain genetic material and the
passing down of antibiotic resistance genes (ARGs).
Canonical patterns
Although the evolutionary origins of the major lineages of modern
cells are disputed, the primary distinctions between the three major
lineages of cellular life (called domains) are firmly established.
In each of these three domains, DNA replication, transcription, and translation
all display distinctive features. There are three versions of ribosomal
RNAs, and generally three versions of each ribosomal protein, one for
each domain of life. These three versions of the protein synthesis
apparatus are called the canonical patterns, and the existence of these canonical patterns provides the basis for a definition of the three domains - Bacteria, Archaea, and Eukarya (or Eukaryota) - of currently existing cells.
Using genomics to infer early lines of evolution
Instead of relying on a single gene such as the small-subunit
ribosomal RNA (SSU rRNA) gene to reconstruct early evolution, or a few
genes, scientific effort has shifted to analyzing complete genome
sequences.
Evolutionary trees based only on SSU rRNA alone do not capture
the events of early eukaryote evolution accurately, and the progenitors
of the first nucleated cells are still uncertain. For instance, analysis
of the complete genome of the eukaryote yeast shows that many of its
genes are more closely related to bacterial genes than they are to
archaea, and it is now clear that archaea were not the simple
progenitors of the eukaryotes, in contradiction to earlier findings
based on SSU rRNA and limited samples of other genes.
One hypothesis is that the first nucleated cell arose from two
distinctly different ancient prokaryotic (non-nucleated) species that
had formed a symbiotic
relationship with one another to carry out different aspects of
metabolism. One partner of this symbiosis is proposed to be a bacterial
cell, and the other an archaeal cell. It is postulated that this
symbiotic partnership progressed via the cellular fusion of the partners
to generate a chimeric
or hybrid cell with a membrane bound internal structure that was the
forerunner of the nucleus. The next stage in this scheme was transfer of
both partner genomes into the nucleus and their fusion with one
another. Several variations of this hypothesis for the origin of
nucleated cells have been suggested. Other biologists dispute this conception and emphasize the community metabolism theme, the idea that early
living communities would comprise many different entities to extant
cells, and would have shared their genetic material more extensively
than current microbes.
Quotes
"The First Cell arose in the previously prebiotic world
with the coming together of several entities that gave a single vesicle
the unique chance to carry out three essential and quite different life
processes. These were: (a) to copy informational macromolecules, (b) to
carry out specific catalytic functions, and (c) to couple energy from
the environment into usable chemical forms. These would foster
subsequent cellular evolution and metabolism. Each of these three
essential processes probably originated and was lost many times prior to
The First Cell, but only when these three occurred together was life
jump-started and Darwinian evolution of organisms began." (Koch and
Silver, 2005)
"The evolution of modern cells is arguably the most
challenging and important problem the field of Biology has ever faced.
In Darwin's day the problem could hardly be imagined. For much of the
20th century it was intractable. In any case, the problem lay buried in
the catch-all rubric "origin of life"--where, because it is a biological
not a (bio)chemical problem, it was effectively ignored. Scientific
interest in cellular evolution started to pick up once the universal
phylogenetic tree, the framework within which the problem had to be
addressed, was determined. But it was not until microbial genomics
arrived on the scene that biologists could actually do much about the
problem of cellular evolution." (Carl Woese, 2002)
Bonobos mating in a zooStags fighting while competing for females—a common sexual behaviorAnatomical structures on the head and throat of a domestic turkey.
1. Caruncles 2. Snood 3. Wattle (dewlap) 4. Major caruncle 5. Beard.
During sexual behavior, these structures enlarge or become brightly
colored.
When animal sexual behaviour is reproductively motivated, it is often termed mating or copulation; for most non-human mammals, mating and copulation occur at oestrus (the most fertile period in the mammalian female's reproductive cycle), which increases the chances of successful impregnation. Some animal sexual behaviour involves competition,
sometimes fighting, between multiple males. Females often select males
for mating only if they appear strong and able to protect themselves.
The male that wins a fight may also have the chance to mate with a
larger number of females and will therefore pass on his genes to their
offspring.
Historically, it was believed that only humans and a small number
of other species performed sexual acts other than for reproduction, and
that animals' sexuality was instinctive and a simple "stimulus–response" behaviour. However, in addition to homosexual behaviours, a range of species masturbate and may use objects as tools
to help them do so. Sexual behaviour may be tied more strongly to the
establishment and maintenance of complex social bonds across a
population which support its success in non-reproductive ways. Both
reproductive and non-reproductive behaviours can be related to
expressions of dominance over another animal or survival within a
stressful situation (such as sex due to duress or coercion).
Mating systems
Greater sage-grouse at a lek, with multiple males displaying for the less conspicuous females
In sociobiology and behavioural ecology, the term "mating system"
is used to describe the ways in which animal societies are structured
in relation to sexual behaviour. The mating system specifies which males
mate with which females, and under what circumstances. There are four
basic systems:
Monogamy
occurs when one male and one female mate exclusively with each other. A
monogamous mating system is one in which individuals form long-lasting pairs and cooperate in raising offspring. These pairs may last for a lifetime, such as in pigeons, or it may occasionally change from one mating season to another, such as in emperor penguins. In contrast with tournament species, these pair-bonding species have lower levels of male aggression, competition and little sexual dimorphism. Zoologists and biologists
now have evidence that monogamous pairs of animals are not always
sexually exclusive. Many animals that form pairs to mate and raise
offspring regularly engage in sexual activities with extra-pair partners. This includes previous examples, such as swans. Sometimes, these extra-pair sexual activities
lead to offspring. Genetic tests frequently show that some of the
offspring raised by a monogamous pair come from the female mating with
an extra-pair male partner. These discoveries have led biologists to adopt new ways of talking about monogamy. According to Ulrich Reichard (2003):
Social monogamy refers to a male
and female's social living arrangement (e.g., shared use of a territory,
behaviour indicative of a social pair, and/or proximity between a male
and female) without inferring any sexual interactions or reproductive
patterns. In humans, social monogamy takes the form of monogamous
marriage. Sexual monogamy is defined as an exclusive sexual relationship
between a female and a male based on observations of sexual
interactions. Finally, the term genetic monogamy is used when DNA
analyses can confirm that a female-male pair reproduce exclusively with
each other. A combination of terms indicates examples where levels of
relationships coincide, e.g., sociosexual and sociogenetic monogamy
describe corresponding social and sexual, and social and genetic
monogamous relationships, respectively.
Whatever makes a pair of animals socially monogamous does not
necessarily make them sexually or genetically monogamous. Social
monogamy, sexual monogamy, and genetic monogamy can occur in different
combinations.
Social monogamy is relatively rare in the animal kingdom. The
actual incidence of social monogamy varies greatly across different
branches of the evolutionary tree. Over 90% of avian species are
socially monogamous. This stands in contrast to mammals. Only 3% of mammalian species are
socially monogamous, although up to 15% of primate species are. Social monogamy has also been observed in reptiles, fish, and insects.
Sexual monogamy is also rare among animals. Many socially monogamous species engage in extra-pair copulations,
making them sexually non-monogamous. For example, while over 90% of
birds are socially monogamous, "on average, 30% or more of the baby
birds in any nest [are] sired by someone other than the resident male." Patricia Adair Gowaty has estimated that, out of 180 different species of socially monogamous songbirds, only 10% are sexually monogamous.
The incidence of genetic monogamy, determined by DNA
fingerprinting, varies widely across species. For a few rare species,
the incidence of genetic monogamy is 100%, with all offspring
genetically related to the socially monogamous pair. But genetic
monogamy is strikingly low in other species. Barash and Lipton note:
The highest known frequency of extra-pair copulations are found among the fairy-wrens, lovely tropical creatures technically known as Malurus splendens and Malurus cyaneus. More than 65% of all fairy-wren chicks are fathered by males outside the supposed breeding group.
Such low levels of genetic monogamy have surprised biologists and
zoologists, forcing them to rethink the role of social monogamy in
evolution. They can no longer assume social monogamy determines how
genes are distributed in a species. The lower the rates of genetic
monogamy among socially monogamous pairs, the less of a role social
monogamy plays in determining how genes are distributed among offspring.
The term polygamy
is an umbrella term used to refer generally to non-monogamous matings.
As such, polygamous relationships can be polygynous, polyandrous or
polygynandrous. In a small number of species, individuals can display
either polygamous or monogamous behaviour depending on environmental
conditions. An example is the social wasp Apoica flavissima. In some species, polygyny and polyandry is displayed by both sexes in
the population. Polygamy in both sexes has been observed in red flour beetle (Tribolium castaneum). Polygamy is also seen in many Lepidoptera species including Mythimna unipuncta (true armyworm moth).
A tournament species is one in which "mating tends to be highly polygamous and involves high levels of male-male aggression and competition." Tournament behaviour often correlates with high levels of sexual dimorphism, examples of species including chimpanzees and baboons. Most polygamous species present high levels of tournament behaviour, with a notable exception being bonobos.
Polygyny occurs when one male gets exclusive mating rights with multiple females. In some species, notably those with harem-like structures, only one of a few males in a group of females will mate. Technically, polygyny
in sociobiology and zoology is defined as a system in which a male has a
relationship with more than one female, but the females are
predominantly bonded to a single male. Should the active male be driven
out, killed, or otherwise removed from the group, in a number of species
the new male will ensure that breeding resources are not wasted on
another male's young. The new male may achieve this in many different ways, including:
competitive infanticide: in lions, hippopotamuses, and some monkeys,
the new male will kill the offspring of the previous alpha male to
cause their mothers to become receptive to his sexual advances since
they are no longer nursing. To prevent this, many female primates exhibit ovulation cues among all males, and show situation-dependent receptivity.
in some rodents such as mice,
a new male with a different scent will cause females who are pregnant
to spontaneously fail to implant recently fertilised eggs. This does not
require contact; it is mediated by scent alone. It is known as the Bruce effect.
Von Haartman specifically described the mating behaviour of the European pied flycatcher as successive polygyny. Within this system, the males leave their home territory once their
primary female lays her first egg. Males then create a second territory,
presumably in order to attract a secondary female to breed. Even when
they succeed at acquiring a second mate, the males typically return to
the first female to exclusively provide for her and her offspring.
Polygynous mating structures are estimated to occur in up to 90% of mammal species. As polygyny is the most common form of polygamy among vertebrates
(including humans), it has been studied far more extensively than
polyandry or polygynandry.
Polyandry
The anglerfish Haplophryne mollis is polyandrous. This female is trailing the atrophied remains of males she has encountered.
Polyandry occurs when one female gets exclusive mating rights with multiple males. In some species, such as redlip blennies, both polygyny and polyandry are observed.
The males in some deep sea anglerfishes are much smaller than the females. When they find a female they bite into her skin, releasing an enzyme that digests the skin of their mouths and her body and fusing the pair down to the blood-vessel level. The male then slowly atrophies, losing first his digestive organs, then his brain, heart, and eyes, ending as nothing more than a pair of gonads, which release sperm in response to hormones in the female's bloodstream indicating egg release. This extreme sexual dimorphism ensures that, when the female is ready to spawn, she has a mate immediately available. A single anglerfish female can "mate" with many males in this manner.
Polygynandry
occurs when multiple males mate indiscriminately with multiple females.
The numbers of males and females need not be equal, and in vertebrate
species studied so far, there are usually fewer males. Two examples of
systems in primates are promiscuous mating chimpanzees and bonobos.
These species live in social groups consisting of several males and
several females. Each female copulates with many males, and vice versa.
In bonobos, the amount of promiscuity is particularly striking because
bonobos use sex to alleviate social conflict as well as to reproduce. This mutual promiscuity is the approach most commonly used by spawning
animals, and is perhaps the "original fish mating system." Common examples are forage fish, such as herrings, which form huge mating shoals in shallow water. The water becomes milky with sperm and the bottom is draped with millions of fertilised eggs.
Female and male sexual behaviour differ in many species. Often, males
are more active in initiating mating, and bear the more conspicuous
sexual ornamentation like antlers and colourful plumage. This is a
result of anisogamy, where sperm are smaller and much less costly (energetically) to produce than eggs.
This difference in physiological cost means that males are more limited
by the number of mates they can secure, while females are limited by
the quality of genes of her mates, a phenomenon known as Bateman's principle. Many females also have extra reproductive burdens in that parental care often falls mainly, or exclusively, on them. Thus, females are more limited in their potential reproductive success. In species where males take on more of the reproductive costs, such as sea horses and jacanas, the role is reversed, and the females are larger, more aggressive and more brightly coloured than the males.
In hermaphroditic animals, the costs of parental care can be evenly distributed between the sexes, e.g. earthworms. In some species of planarians, sexual behaviour takes the form of penis fencing.
In this form of copulation, the individual that first penetrates the
other with the penis, forces the other to be female, thus carrying the
majority of the cost of reproduction. Post mating, banana slugs will some times gnaw off their partners penis as an act of sperm competition called apophallation. This is costly as they must heal, and spend more energy courting
conspecifics that can act as male and female. A hypothesis suggests
these slugs may be able to compensate the loss of the male function by
directing energy that would have been put towards it to the female
function. In the grey slug,
the sharing of cost leads to a spectacular display, where the mates
suspend themselves high above the ground from a slime thread, ensuring
none of them can refrain from taking on the cost of egg-bearer.
Many animal species have specific mating (or breeding) periods e.g. (seasonal breeding) so that offspring are born or hatch at an optimal time. In marine species with limited mobility and external fertilisation like corals, sea urchins and clams, the timing of the common spawning is the only externally visible form of sexual behaviour. In areas with continuously high primary production,
some species have a series of breeding seasons throughout the year.
This is the case with most primates (who are primarily tropical and
subtropical animals). Some animals (opportunistic breeders) breed dependent upon other conditions in their environment aside from time of year.
Mammals
Mating seasons are often associated with changes to herd or group
structure, and behavioural changes, including territorialism amongst
individuals. These may be annual (e.g. wolves), biannual (e.g. dogs)
or more frequently (e.g. horses). During these periods, females of most
mammalian species are more mentally and physically receptive to sexual
advances, a period scientifically described as oestrus but commonly described as being "in season" or "in heat". Sexual behaviour may occur outside oestrus, and such acts as do occur are not necessarily harmful.
Some mammals (e.g. domestic cats, rabbits and camelids) are termed "induced ovulators".
For these species, the female ovulates due to an external stimulus
during, or just prior to, mating, rather than ovulating cyclically or
spontaneously. Stimuli causing induced ovulation include the sexual
behaviour of coitus, sperm and pheromones. Domestic cats have penile spines. Upon withdrawal of a cat's penis, the spines rake the walls of the female's vagina, which may cause ovulation.
Amphibians
For many amphibians, an annual breeding cycle applies, typically
regulated by ambient temperature, precipitation, availability of surface
water and food supply. This breeding season is accentuated in temperate
regions, in boreal climate the breeding season is typically
concentrated to a few short days in the spring. Some species, such as
the Rana clamitans (green frog), spend from June to August
defending their territory. In order to protect these territories, they
use five vocalizations.
Fish
Like many coral reef dwellers, the clownfish
spawn around the time of the full moon in the wild. In a group of
clownfish, there is a strict dominance hierarchy. The largest and most
aggressive female is found at the top. Only two clownfish, a male and a
female, in a group reproduce through external fertilisation. Clownfish
are sequential hermaphrodites, meaning that they develop into males
first, and when they mature, they become females. If the female
clownfish is removed from the group, such as by death, one of the
largest and most dominant males will become a female. The remaining
males will move up a rank in the hierarchy.
Motivation
Various neurohormones stimulate sexual wanting in animals. In general, studies have suggested that dopamine is involved in sexual incentive motivation, oxytocin and melanocortins in sexual attraction, and noradrenaline in sexual arousal. Vasopressin is also involved in the sexual behaviour of some animals.
Neurohormones in the mating systems of voles
The mating system of prairie voles is monogamous; after mating, they form a lifelong bond. In contrast, montane voles have a polygamous mating system. When montane voles mate, they form no strong attachments, and separate after copulation. Studies on the brains of these two species have found that it is two
neurohormones and their respective receptors that are responsible for
these differences in mating strategies. Male prairie voles release
vasopressin after copulation with a partner, and an attachment to their
partner then develops. Female prairie voles release oxytocin after
copulation with a partner, and similarly develop an attachment to their
partner.
Neither male nor female montane voles release high quantities of
oxytocin or vasopressin when they mate. Even when injected with these
neurohormones, their mating system does not change. In contrast, if
prairie voles are injected with the neurohormones, they may form a
lifelong attachment, even if they have not mated. The differing response
to the neurohormones between the two species is due to a difference in
the number of oxytocin and vasopressin receptors. Prairie voles have a
greater number of oxytocin and vasopressin receptors compared to montane
voles, and are therefore more sensitive to those two neurohormones.
It's believed that it's the quantity of receptors, rather than the
quantity of the hormones, that determines the mating system and
bond-formation of either species.
Oxytocin and rat sexual behaviour
Mother rats experience a postpartum estrus
which makes them highly motivated to mate. However, they also have a
strong motivation to protect their newly born pups. As a consequence,
the mother rat solicits males to the nest but simultaneously becomes
aggressive towards them to protect her young. If the mother rat is given
injections of an oxytocin receptor antagonist, they no longer experience these maternal motivations.
Prolactin influences social bonding in rats.
Oxytocin and primate sexual behaviour
Oxytocin plays a similar role in non-human primates as it does in humans.
Grooming, sex, and cuddling frequencies correlate positively with
levels of oxytocin. As the level of oxytocin increases so does sexual
motivation. While oxytocin plays a major role in parent child
relationships, it is also found to play a role in adult sexual
relationships. Its secretion affects the nature of the relationship or
if there will even be a relationship at all.
Studies have shown that oxytocin is higher in monkeys in lifelong
monogamous relationships compared to monkeys which are single.
Furthermore, the oxytocin levels of the couples correlate positively;
when the oxytocin secretion of one increases, the other one also
increases. Higher levels of oxytocin are related to monkeys expressing
more behaviours such as cuddling, grooming and sex, while lower levels
of oxytocin reduce motivation for these activities.
Research on oxytocin's role in the animal brain suggests that it
plays less of a role in behaviours of love and affection than previously
believed. "When oxytocin was first discovered in 1909, it was thought
mostly to influence a mother's labour contractions and milk let-down.
Then, in the 1990s, research with prairie voles found that giving them a
dose of oxytocin resulted in the formation of a bond with their future
mate (Azar, 40)." Oxytocin has since been treated by the media as the
sole player in the "love and mating game" in mammals. This view,
however, is proving to be false as, "most hormones don't influence
behaviour directly. Rather, they affect thinking and emotions in
variable ways (Azar, 40)." There is much more involved in sexual
behaviour in the mammalian animal than oxytocin and vasopressin can
explain.
Pleasure
It is often assumed that animals do not have sex for pleasure, or alternatively that humans, pigs, bonobos (and perhaps dolphins
and one or two more species of primates) are the only species that do.
This is sometimes stated as "animals mate only for reproduction". This
view is considered a misconception by some scholars. Jonathan Balcombe
argues that the prevalence of non-reproductive sexual behaviour in
certain species suggests that sexual stimulation is pleasurable. He also
points to the presence of the clitoris in some female mammals, and evidence for female orgasm in primates. On the other hand, it is impossible to know the subjective feelings of animals, and the notion that non-human animals experience emotions similar to humans is a contentious subject.
A 2006 Danish Animal Ethics Council report, which examined current knowledge of animal sexuality in the context of
legal queries concerning sexual acts by humans, has the following
comments, primarily related to domestically common animals:
Even though the evolution-related
purpose of mating can be said to be reproduction, it is not actually the
creating of offspring which originally causes them to mate. It is
probable that they mate because they are motivated for the actual
copulation, and because this is connected with a positive experience. It
is therefore reasonable to assume that there is some form of pleasure
or satisfaction connected with the act. This assumption is confirmed by
the behaviour of males, who in the case of many species are prepared to
work to get access to female animals, especially if the female animal is
in oestrus, and males who for breeding purposes are used to having sperm collected become very eager, when the equipment they associate with the collection is taken out. . . . There is nothing in female mammals' anatomy or physiology that contradicts that stimulation of the sexual organs
and mating is able to be a positive experience. For instance, the
clitoris acts in the same way as with women, and scientific studies have
shown that the success of reproduction is improved by stimulation of
clitoris on (among other species) cows and mares in connection with
insemination, because it improves the transportation of the sperm due to
contractions of the inner genitalia. This probably also applies to
female animals of other animal species, and contractions in the inner
genitals are seen e.g. also during orgasm for women. It is therefore
reasonable to assume that sexual intercourse may be linked with a
positive experience for female animals.
Koinophilia is the love of the "normal" or phenotypically common (from the Greek κοινός, koinós, meaning "usual" or "common"). The term was introduced to scientific literature in 1990, and refers to
the tendency of animals seeking a mate to prefer that mate not to have
any unusual, peculiar or deviant features. Similarly, animals preferentially choose mates with low fluctuating asymmetry. However, animal sexual ornaments can evolve through runaway selection,
which is driven by (usually female) selection for non-standard traits.
Interpretation bias
The field of study of sexuality in non-human species was a long-standing taboo.In the past, researchers sometimes failed to observe, miscategorised or misdescribed sexual behaviour which did not meet their preconceptions—their
bias tended to support what would now be described as conservative
sexual mores. An example of overlooking behaviour relates to
descriptions of giraffe mating:
When nine out of ten pairings occur between males, "[e]very male that sniffed a female was reported as sex, while anal intercourse with orgasm between males was only [categorized as] 'revolving around' dominance, competition or greetings."
In the 21st century, liberal social or sexual views are often
projected upon animal subjects of research. Popular discussions of
bonobos are a frequently cited example. Current research frequently
expresses views such as that of the Natural History Museum at the University of Oslo, which in 2006 held an exhibition on animal sexuality:
Many researchers have described
homosexuality as something altogether different from sex. They must
realise that animals can have sex with who they will, when they will and
without consideration to a researcher's ethical principles.
Other animal activities may be misinterpreted due to the frequency
and context in which animals perform the behaviour. For example,
domestic ruminants display behaviours such as mounting and head-butting.
This often occurs when the animals are establishing dominance
relationships and are not necessarily sexually motivated. Careful
analysis must be made to interpret what animal motivations are being
expressed by those behaviours.
Copulation is the union of the male and female sex organs, the sexual activity specifically organized to transmit male sperm into the body of the female.
Cuckoldry
Small male bluegill sunfishes cuckold large males by adopting sneaker strategies.
Alternative male strategies which allow small males to engage in cuckoldry can develop in species such as fish where spawning is dominated by large and aggressive males. Cuckoldry is a variant of polyandry, and can occur with sneak spawners. A sneak spawner is a male that rushes in to join the spawning rush of a spawning pair. A spawning rush occurs when a fish makes a burst of speed, usually on a near vertical incline, releasing gametes at the apex, followed by a rapid return to the lake or sea floor or fish aggregation. Sneaking males do not take part in courtship. In salmon and trout, for example, jack males
are common. These are small silvery males that migrate upstream along
with the standard, large, hook-nosed males and that spawn by sneaking
into redds to release sperm simultaneously with a mated pair. This behaviour is an evolutionarily stable strategy for reproduction, because it is favoured by natural selection just like the "standard" strategy of large males.
Hermaphroditism
Female groupers change their sex to male if no male is available.
Hermaphroditism
occurs when a given individual in a species possesses both male and
female reproductive organs, or can alternate between possessing first
one, and then the other. Hermaphroditism is common in invertebrates but
rare in vertebrates. It can be contrasted with gonochorism,
where each individual in a species is either male or female, and
remains that way throughout their lives. Most fish are gonochorists, but
hermaphroditism is known to occur in 14 families of teleost fishes.
Usually hermaphrodites are sequential, meaning they can switch sex, usually from female to male (protogyny).
This can happen if a dominant male is removed from a group of females.
The largest female in the harem can switch sex over a few days and
replace the dominant male. This is found amongst coral reef fishes such as groupers, parrotfishes and wrasses. As an example, most wrasses are protogynous hermaphrodites within a haremic mating system. It is less common for a male to switch to a female (protandry). A common example of a protandrous species are clownfish—if the larger, dominant female dies, in many cases, the reproductive male gains weight and becomes the female. Hermaphroditism allows for complex mating systems. Wrasses exhibit three different mating systems: polygynous, lek-like, and promiscuous mating systems.
Sexual cannibalism is a behaviour in which a female animal kills and
consumes the male before, during, or after copulation. Sexual
cannibalism confers fitness advantages to both the male and female. Sexual cannibalism is common among insects, arachnids and amphipods. There is also evidence of sexual cannibalism in gastropods and copepods.
During mating, the male Leopard typically immobilizes the female
Sexual coercion behaviour during mating has been documented in a variety of species. In some herbivorous herd species, or species where males and females are very different in size, the male dominates sexually by force and size.
Some species of birds have been observed combining sexual intercourse with violence; these include ducks, and geese. Female white-fronted bee-eaters
are subjected to copulation. When females emerge from their nest
burrows, males sometimes force them to the ground and mate with them.
Such sexual coercion are made preferentially on females who are laying
and who may therefore lay eggs fertilized by the male.
It has been reported that young male elephants in South Africa sexually coerced and killed rhinoceroses. This interpretation of the elephants' behaviour was disputed by one of
the original study's authors, who said there was "nothing sexual about
these attacks".
In elephant seals' sexual intercourse, male elephant seals have
been known to have relations with penguins, attempting to copulate with
them "several times with periods of rest in between". The birds remained
pinned down for the duration. For female elephant seals, physical
injury happens very often. Mating leads to 1 in every 1,000 female
elephant seals getting killed.
Parthenogenesis
Parthenogenesis is a form of asexual reproduction in which growth and development of embryos occur without fertilisation. Technically, parthenogenesis is not a behaviour, however, sexual behaviours may be involved.
Whip-tailed lizard
females have the ability to reproduce through parthenogenesis and as
such males are rare and sexual breeding non-standard. Females engage in
"pseudocopulation" to stimulate ovulation,
with their behaviour following their hormonal cycles; during low levels
of oestrogen, these (female) lizards engage in "masculine" sexual
roles. Those animals with currently high oestrogen levels assume
"feminine" sexual roles. Lizards that perform the courtship ritual have
greater fecundity than those kept in isolation due to an increase in
hormones triggered by the sexual behaviours. So, even though asexual
whiptail lizards populations lack males, sexual stimuli still increase
reproductive success. From an evolutionary
standpoint these females are passing their full genetic code to all of
their offspring rather than the 50% of genes that would be passed in
sexual reproduction.
It is rare to find true parthenogenesis in fishes, where females
produce female offspring with no input from males. All-female species
include the Texas silverside, Menidia clarkhubbsi and a complex of Mexican mollies.
Parthenogenesis has been recorded in 70 vertebrate species including hammerhead sharks, blacktip sharks, amphibians, and lizards.
Unisexuality
Unisexuality occurs when a species is all-male or all-female. Unisexuality occurs in some fish species and can take complex forms. Squalius alburnoides,
a minnow found in several river basins in Portugal and Spain, appears
to be an all-male species. The existence of this species illustrates the
potential complexity of mating systems in fish. The species originated
as a hybrid between two species and is diploid but not hermaphroditic. It can have triploid and tetraploid forms, including all-female forms that reproduce mainly through hybridogenesis.
Interbreeding: Hybrid
offspring can result from the mating of two organisms of distinct but
closely related parent species, although the resulting offspring is not
always fertile. According to Alfred Kinsey, genetic studies on wild animal populations have shown a "large number" of inter-species hybrids.
Prostitution: There are reports that animals occasionally engage in prostitution.
A small number of pair-bonded females within a group of penguins took
nesting material (stones) after copulating with a non-partner male. The
researcher stated "I was watching opportunistically, so I can't give an
exact figure of how common it really is." It has been reported that "bartering of meat for sex ... forms part of
the social fabric of a troop of wild chimps living in the Tai National
Park in the Côte d'Ivoire."
Pavlovian conditioning: The sexualisation of objects or locations is recognised in the animal breeding world. For example, male animals may become sexually aroused
upon visiting a location where they have been allowed to have sex
before, or upon seeing a stimulus previously associated with sexual
activity such as an artificial vagina. Sexual preferences for certain cues can be artificially induced in rats
by pairing scents or objects with their early sexual experiences. The primary motivation of this behaviour is Pavlovian conditioning, and the association is due to a conditioned response (or association) formed with a distinctive "reward".
Viewing images: A study using four adult male rhesus macaques (Macaca mulatta) showed that male rhesus macaques will give up a highly valued item, juice, to see images of the faces or perineum of high-status females. Encouraging captive pandas to mate is problematic. Showing young male pandas "panda pornography"
is credited with a recent population boom among pandas in captivity in
China. One researcher attributed the success to the sounds on the
recordings.
Copulatory wounding and traumatic insemination:
Injury to a partner's genital tract during mating occurs in at least 40
taxa, ranging from fruit flies to humans. However, it often goes
unnoticed due to its cryptic nature and because of internal wounds not
visible outside.
There is a range of behaviours that animals perform that appear to be
sexually motivated but which can not result in reproduction. These
include:
Masturbation: Some species, both male and female, masturbate, both when partners are available and otherwise.
Oral sex: Several species engage in both autofellatio and oral sex. This has been documented in brown bears, Tibetan macaques, wolves, goats, primates, bats, cape ground squirrels and sheep. In the greater short-nosed fruit bat,
copulation by males is dorsoventral and the females lick the shaft or
the base of the male's penis, but not the glans which has already
penetrated the vagina. While the females do this, the penis is not
withdrawn and research has shown a positive relationship between length
of the time that the penis is licked and the duration of copulation.
Post copulation genital grooming has also been observed.
Homosexuality:
Same-sex sexual behaviour occurs in a range of species, especially in
social species, particularly in marine birds and mammals, monkeys, and
the great apes. As of 1999, the scientific literature contained reports of homosexual behaviour in at least 471 wild species. Organisers of the Against Nature? exhibit stated that "homosexuality has been observed among 1,500 species, and that in 500 of those it is well documented." Besides humans, sheep are the only known mammal to exhibit exclusive homosexual behavior.
A male black and white tegu mounts a female that has been dead for two days and attempts to mate.
Genital-genital rubbing:
This is sexual activity in which one animal rubs his or her genitals
against the genitals of another animal. This is stated to be the
"bonobo's most typical sexual pattern, undocumented in any other
primate".
Inter-species mating: Some animals opportunistically mate with individuals of another species.
Sex involving juveniles: Male stoats (Mustela erminea) will sometimes mate with infant females of their species. This is a natural part of their reproductive biology—they have a delayed gestation period, so these females give birth the following year when they are fully grown. Juvenile male common chimpanzees have been recorded mounting and copulating with immature chimps. Infants in bonobo societies are often involved in sexual behaviour.
Necrophilia:
This describes when an animal engages in a sexual act with a dead
animal. It has been observed in mammals, birds, reptiles and frogs.
Bisexuality: This describes when an animal shows sexual behaviour towards both males and females.
Seahorses,
once considered to be monogamous species with pairs mating for life,
were described in a 2007 study as "promiscuous, flighty, and more than a
little bit gay". Scientists at 15 aquaria
studied 90 seahorses of three species. Of 3,168 sexual encounters, 37%
were same-sex acts. Flirting was common (up to 25 potential partners a
day of both sexes); only one species (the British spiny seahorse)
included faithful representatives, and for these 5 of 17 were faithful,
12 were not. Bisexual behaviour was widespread and considered "both a
great surprise and a shock", with big-bellied seahorses of both sexes
not showing partner preference. 1,986 contacts were male-female, 836
were female-female and 346 were male-male.
Bonobo
Among bonobos, males and females engage in sexual behaviour
with the same and the opposite sex, with females being particularly
noted for engaging in sexual behaviour with each other and at up to 75%
of sexual activity being non-reproductive, as being sexually active does
not necessarily correlate with their ovulation cycles. Sexual activity occurs between almost all ages and sexes of bonobo societies. Primatologist Frans de Waal believes that bonobos use sexual activity to resolve conflict between individuals. Immature bonobos, contrariwise, perform genital contact when relaxed.
Macaque
Similar same-sex sexual behaviours occur in both male and female macaques. It is thought to be done for pleasure as an erect male mounts and thrusts upon or into another male. Sexual receptivity can also be indicated by red faces and shrieking. Mutual ejaculation after a combination of anal intercourse and masturbation has also been witnessed, although it may be rare. In comparison to socio-sexual behaviours such as dominance displays,
homosexual mounts last longer, happen in series, and usually involve
pelvic thrusting.
Females are also thought to participate for pleasure as vulvar,
perineal, and anal stimulation is part of these interactions. The
stimulation can come from their own tails, mounting their partner,
thrusting or a combination of these.
Dolphin
Male bottlenose dolphins
have been observed working in pairs to follow or restrict the movement
of a female for weeks at a time, waiting for her to become sexually
receptive. The same pairs have also been observed engaging in intense sexual play with each other. Janet Mann, a professor of biology and psychology at Georgetown University, argues that the common same-sex behaviour among male dolphin
calves is about bond formation and benefits the species evolutionarily.
Studies have shown the dolphins later in life are bisexual and the male
bonds forged from homosexuality work for protection as well as locating
females with which to reproduce.
In 1991, an English man was prosecuted for allegedly having sexual contact with a dolphin. The man was found not guilty after it was revealed at trial that the
dolphin was known to tow bathers through the water by hooking his penis
around them.
Hyena
The female spotted hyena has a unique urinary-genital system, closely resembling the penis of the male, called a pseudo-penis.
Dominance relationships with strong sexual elements are routinely
observed between related females. They are notable for using visible
sexual arousal as a sign of submission but not dominance in males as
well as females (females have a sizeable erectile clitoris). It is speculated that to facilitate this, their sympathetic and parasympathetic nervous systems may be partially reversed in respect to their reproductive organs.
Mammals mate by vaginal copulation. To achieve this, the male usually mounts the female from behind. The female may exhibit lordosis in which she arches her back ventrally to facilitate entry of the penis, which is particularly present in elephants, felids, and rodents. Amongst the land mammals, other than humans, only bonobos mate in a face-to-face position, as the females' anatomy seems to reflect, although ventro-ventral copulation has also been observed in Rhabdomys. Some sea mammals copulate in a belly-to-belly position. Some camelids mate in a lying-down position. In most mammals ejaculation occurs after multiple intromissions, but in most primates, copulation consists of one brief intromission. In most ruminant species, a single pelvic thrust occurs during copulation. In most deer species, a copulatory jump also occurs.
The copulatory behaviour of many mammalian species is affected by sperm competition.
Some females have concealed fertility, making it difficult for
males to evaluate if a female is fertile - humans are amongst these
species. This is costly as ejaculation expends much energy.
Courting garden snails. The one on the left has fired a love dart into the one on the right.A male star coral releases sperm into the water.
Invertebrates are often hermaphrodites. Some hermaphroditic land snails
begin mating with an elaborate tactile courting ritual. The two snails
circle around each other for up to six hours, touching with their
tentacles, and biting lips and the area of the genital pore, which shows
some preliminary signs of the eversion of the penis. As the snails
approach mating, hydraulic pressure builds up in the blood sinus
surrounding an organ housing a sharpened dart. The dart is made of calcium carbonate or chitin, and is called a love dart.
Each snail manoeuvres to get its genital pore in the best position,
close to the other snail's body. Then, when the body of one snail
touches the other snail's genital pore, it triggers the firing of the
love dart. After the snails have fired their darts, they copulate and exchange
sperm as a separate part of the mating progression. The love darts are
covered with a mucus that contains a hormone-like substance that facilitates the survival of the sperm.
Penis fencing is a mating behaviour engaged in by certain species of flatworm, such as Pseudobiceros bedfordi. Species which engage in the practice are hermaphroditic, possessing both eggs and sperm-producing testes. The species "fence" using two-headed dagger-like penises which are
pointed, and white in colour. One organism inseminates the other. The
sperm is absorbed through pores in the skin, causing fertilisation.
Corals can be both gonochoristic (unisexual) and hermaphroditic,
each of which can reproduce sexually and asexually. Reproduction also
allows corals to settle new areas. Corals predominantly reproduce sexually. 25% of hermatypic corals (stony corals) form single sex (gonochoristic) colonies, while the rest are hermaphroditic. About 75% of all hermatypic corals "broadcast spawn" by releasing gametes – eggs and sperm – into the water to spread offspring. The gametes fuse during fertilisation to form a microscopic larva called a planula, typically pink and elliptical in shape. Synchronous spawning is very typical on the coral reef and often, even when multiple species
are present, all corals spawn on the same night. This synchrony is
essential so that male and female gametes can meet. Corals must rely on
environmental cues, varying from species to species, to determine the
proper time to release gametes into the water. The cues involve lunar
changes, sunset time, and possibly chemical signalling. Synchronous spawning may form hybrids and is perhaps involved in coral speciation.
Butterflies spend much time searching for mates. When the male spots a mate, he will fly closer and release pheromones. He then performs a special courtship dance
to attract the female. If the female appreciates the dancing she may
join him. Then they join their bodies together end to end at their abdomens. Here, the male passes the sperm to the female's egg-laying tube, which will soon be fertilised by the sperm.
Many animals make plugs of mucus to seal the female's orifice
after mating. Normally such plugs are secreted by the male, to block
subsequent partners. In spiders the female can assist the process. Spider sex is unusual in that males transfer their sperm to the female
on small limbs called pedipalps. They use these to pick their sperm up
from their genitals and insert it into the female's sexual orifice,
rather than copulating directly. On the 14 occasions a sexual plug was made, the female produced it
without assistance from the male. On ten of these occasions the male's
pedipalps then seemed to get stuck while he was transferring the sperm
(which is rarely the case in other species of spider), and he had great
difficulty freeing himself. In two of those ten instances, he was eaten
as a result.
In the orb-weaving spider species Zygiella x-notata,
individuals engage in a variety of sexual behaviors including male
choosiness, mate guarding, and vibrational signaling in courtship.
Genetic evidence of interspecies sexual activity in humans
Research into human evolution confirms that, in some cases, interspecies sexual activity may have been responsible for the evolution of new species (speciation). Analysis of animal genes found evidence that, after humans had diverged from other apes, interspecies mating nonetheless occurred regularly enough to change certain genes in the new gene pool. Researchers found that the X chromosomes of humans and chimps may have diverged
around 1.2 million years after the other chromosomes. One possible
explanation is that modern humans emerged from a hybrid of human and
chimp populations. A 2012 study questioned this explanation, concluding that "there is no
strong reason to involve complicated factors in explaining the autosomal
data".
When close relatives mate, progeny may exhibit the detrimental effects of inbreeding depression. Inbreeding depression is predominantly caused by the homozygous expression of recessive deleterious alleles. Over time, inbreeding depression may lead to the evolution of inbreeding avoidance
behaviour. Several examples of animal behaviour that reduce mating of
close relatives and inbreeding depression are described next.
Reproductively active female naked mole-rats tend to associate with unfamiliar males (usually non-kin), whereas reproductively inactive females do not discriminate. The preference of reproductively active females for unfamiliar males is interpreted as an adaptation for avoiding inbreeding.
When mice inbreed with close relatives in their natural habitat, there is a significant detrimental effect on progeny survival. In the house mouse, the major urinary protein (MUP) gene cluster provides a highly polymorphic scent signal of genetic identity that appears to underlie kin recognition and inbreeding avoidance. Thus there are fewer matings between mice sharing MUP haplotypes than would be expected if there were random mating.
Meerkat females appear to be able to discriminate the odour of their kin from the odour of their non-kin. Kin recognition is a useful ability that facilitates both cooperation
among relatives and the avoidance of inbreeding. When mating does occur
between meerkat relatives, it often results in inbreeding depression.
Inbreeding depression was evident for a variety of traits: pup mass at
emergence from the natal burrow, hind-foot length, growth until
independence and juvenile survival.
The grey-sided vole (Myodes rufocanus) exhibits male-biased dispersal as a means of avoiding incestuous matings. Among those matings that do involve inbreeding the number of weaned
juveniles in litters is significantly smaller than that from non-inbred
litters indicating inbreeding depression.
In natural populations of the bird Parus major
(great tit), inbreeding is likely avoided by dispersal of individuals
from their birthplace, which reduces the chance of mating with a close
relative. Dispersing to avoid inbreeding is a common behavior amongst animals, such as felids and canids, although inbreeding can still occur, albeit rarely.
Toads display breeding site fidelity, as do many amphibians. Individuals that return to natal ponds to breed will likely encounter siblings as potential mates. Although incest is possible, Bufo americanus
siblings rarely mate. These toads likely recognise and actively avoid
close kins as mates. Advertisement vocalisations by males appear to
serve as cues by which females recognise their kin.
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