Animal sexual behaviour

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 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): 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. 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: 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. Polygamy 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 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. • harassment to miscarriage: amongst wild horses and baboons, the male will continually attack pregnant females until they miscarry. • Pheromone-based spontaneous abortion • 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 '' 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 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. ==Parental investment and reproductive success==

, suspended from a slime thread 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. ==Seasonality==

s typically spawning in connection with the full moon every August 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. 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, 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: ==Koinophilia==

Koinophilia is the love of the "normal" or phenotypically common (from the Greek , , 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. 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: 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: 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. ==Types of sexual behaviour==

Reproductive sexual behaviour Copulation 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 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 s 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. It is less common for a male to switch to a female (protandry). Hermaphroditism allows for complex mating systems. Wrasses exhibit three different mating systems: polygynous, lek-like, and promiscuous mating systems. Sexual cannibalism 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. Sexual coercion 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.