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Sexual orientations |
Non-human animals |
Animal sexual behaviour takes many different forms, including within the samespecies. Commonmating orreproductively motivated systems includemonogamy,polygyny,polyandry,polygamy andpromiscuity. Other sexual behaviour may be reproductively motivated (e.g.sex apparently due to duress or coercion andsituational sexual behaviour) ornon-reproductively motivated (e.g.homosexual sexual behaviour,bisexual sexual behaviour,cross-species sex,sexual arousal from objects or places,sex with dead animals, etc.).
When animal sexual behaviour is reproductively motivated, it is often termedmating orcopulation; for most non-humanmammals, mating and copulation occur atoestrus (the most fertile period in the mammalian female's reproductive cycle), which increases the chances ofsuccessful impregnation.[1][2] Some animal sexual behaviour involvescompetition, 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.[3]
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 speciesmasturbate and mayuse 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).
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Insociobiology andbehavioural 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:
| The four basic mating systems[4]: 160–161 [5] | ||
|---|---|---|
| Single female | Multiple females | |
| Single male | Monogamy | Polygyny |
| Multiple males | Polyandry | Polygynandry |
Monogamy occurs when one male and one female mate exclusively with each other. A monogamous mating system is one in which individuals formlong-lasting pairs and cooperate in raising offspring. These pairs may last for a lifetime, such as inpigeons,[6] or it may occasionally change from one mating season to another, such as inemperor penguins.[7] In contrast withtournament species, these pair-bonding species have lower levels of male aggression, competition and littlesexual dimorphism.Zoologists andbiologists 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 withextra-pair partners.[8][9][10][11] This includes previous examples, such asswans. Sometimes, theseextra-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.[9][12][13][14] 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.[15]
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.[10][16] This stands in contrast to mammals. Only 3% of mammalian species are socially monogamous, although up to 15% of primate species are.[10][16] Social monogamy has also been observed inreptiles, fish, andinsects.
Sexual monogamy is also rare among animals. Many socially monogamous species engage inextra-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."[17]Patricia Adair Gowaty has estimated that, out of 180 different species of socially monogamous songbirds, only 10% are sexually monogamous.[18]
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 thefairy-wrens, lovely tropical creatures technically known asMalurus splendens andMalurus cyaneus. More than 65% of all fairy-wren chicks are fathered by males outside the supposed breeding group.[16]p. 12
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 termpolygamy 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 waspApoica flavissima.[citation needed] In some species, polygyny and polyandry is displayed by both sexes in the population. Polygamy in both sexes has been observed inred flour beetle (Tribolium castaneum). Polygamy is also seen in manyLepidoptera species includingMythimna unipuncta (true armyworm moth).[19]
Atournament species is one in which "mating tends to be highly polygamous and involves high levels of male-male aggression and competition."[20] Tournament behaviour often correlates with high levels ofsexual dimorphism, examples of species includingchimpanzees andbaboons. Most polygamous species present high levels of tournament behaviour, with a notable exception beingbonobos.[citation needed]
Polygyny occurs when one male gets exclusive mating rights with multiple females. In some species, notably those withharem-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.[21] The new male may achieve this in many different ways, including:
Von Haartman specifically described the mating behaviour of theEuropean pied flycatcher as successive polygyny.[23] 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.[24]
Polygynous mating structures are estimated to occur in up to 90% of mammal species.[25] As polygyny is the most common form of polygamy among vertebrates (including humans), it has been studied far more extensively than polyandry or polygynandry.
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Polyandry occurs when one female gets exclusive mating rights with multiple males. In some species, such asredlip blennies, bothpolygyny andpolyandry are observed.[26]
The males in some deep seaanglerfishes are much smaller than the females. When they find a female they bite into her skin, releasing anenzyme that digests the skin of their mouths and her body and fusing the pair down to the blood-vessel level. The male then slowlyatrophies, losing first his digestive organs, then his brain, heart, and eyes, ending as nothing more than a pair ofgonads, which releasesperm in response tohormones in the female's bloodstream indicatingegg release. This extremesexual dimorphism ensures that, when the female is ready to spawn, she has a mate immediately available.[27] 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 matingchimpanzees andbonobos. 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.[28] This mutual promiscuity is the approach most commonly used by spawning animals, and is perhaps the "original fish mating system."[4]: 161 Common examples areforage fish, such asherrings, which form huge matingshoals in shallow water. The water becomes milky with sperm and the bottom is draped with millions of fertilised eggs.[4]: 161
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 ofanisogamy, wheresperm are smaller and much less costly (energetically) to produce thaneggs. 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 asBateman's principle.[29] Many females also have extra reproductive burdens in thatparental care often falls mainly, or exclusively, on them. Thus, females are more limited in their potentialreproductive success.[30] In species where males take on more of the reproductive costs, such assea horses andjacanas, 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 ofplanarians, sexual behaviour takes the form ofpenis 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.[31] Post mating,banana slugs will some times gnaw off their partners penis as an act ofsperm competition called apophallation.[32] 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.[33] In thegrey 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.[34]
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 andexternal fertilisation likecorals,sea urchins andclams, the timing of the common spawning is the only externally visible form of sexual behaviour. In areas with continuously highprimary 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.
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 asoestrus but commonly described as being "in season" or "in heat". Sexual behaviour may occur outside oestrus,[35] and such acts as do occur are not necessarily harmful.[36]
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 havepenile spines. Upon withdrawal of acat's penis, the spines rake the walls of the female'svagina, which may cause ovulation.[37][38]
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 theRana clamitans (green frog), spend from June to August defending their territory. In order to protect these territories, they use five vocalizations.[39]
Like manycoral reef dwellers, theclownfish 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.
Variousneurohormones stimulate sexual wanting in animals. In general, studies have suggested thatdopamine is involved in sexual incentive motivation,oxytocin andmelanocortins in sexual attraction, andnoradrenaline in sexual arousal.[40]Vasopressin is also involved in the sexual behaviour of some animals.[41]
The mating system ofprairie voles ismonogamous; after mating, they form a lifelong bond. In contrast,montane voles have apolygamous mating system. When montane voles mate, they form no strong attachments, and separate after copulation. Studies[42] 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.[citation needed]
Mother rats experience apostpartum 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 oxytocinreceptor antagonist, they no longer experience these maternal motivations.[43]
Prolactin influences social bonding in rats.[43]
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.[citation needed][44]
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.[citation needed]
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.[45][46][47][48][49][50][51][52][53][54]
It is often assumed that animals do not have sex for pleasure, or alternatively thathumans,pigs,bonobos (and perhapsdolphins 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.[55][56]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 theclitoris in some female mammals, and evidence for female orgasm in primates.[57] On the other hand, it is impossible to know the subjective feelings of animals,[40] and the notion thatnon-human animals experience emotions similar to humans is a contentious subject.[58][59][60][61]
A 2006Danish Animal Ethics Council report,[62] 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 tohaving sperm collected become very eager, when the equipment they associate with the collection is taken out. . . . There is nothing infemale mammals' anatomy or physiology that contradicts thatstimulation 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 Greekκοινός,koinós, meaning "usual" or "common").[63] 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.[63] Similarly, animals preferentially choose mates with lowfluctuating asymmetry.[64] However, animal sexual ornaments can evolve through runaway selection, which is driven by (usually female) selection for non-standard traits.[65]
The field of study of sexuality in non-human species was a long-standingtaboo.[66][unreliable source?] In the past, researchers sometimes failed to observe, miscategorised or misdescribed sexual behaviour whichdid 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 ofgiraffe mating:
When nine out of ten pairings occur between males, "[e]very male that sniffed a female was reported as sex, whileanal intercourse withorgasm between males was only [categorized as] 'revolving around'dominance, competition or greetings."[66]
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 theNatural 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.[66]
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.[67]
Copulation is the union of the male and femalesex organs, the sexual activity specifically organized to transmit male sperm into the body of the female.[68]
Alternative male strategies which allow small males to engage incuckoldry can develop in species such as fish wherespawning is dominated by large and aggressive males. Cuckoldry is a variant ofpolyandry, and can occur withsneak spawners. A sneak spawner is a male that rushes in to join the spawning rush of a spawning pair.[69] A spawning rush occurs when a fish makes a burst of speed, usually on a near vertical incline, releasinggametes at the apex, followed by a rapid return to the lake or sea floor or fish aggregation.[70] 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 intoredds to release sperm simultaneously with a mated pair. This behaviour is anevolutionarily stable strategy for reproduction, because it is favoured by natural selection just like the "standard" strategy of large males.[71]
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 withgonochorism, 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 ofteleost fishes.[72]
Usually hermaphrodites aresequential, meaning they canswitch 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.[72] This is found amongstcoral reef fishes such asgroupers,parrotfishes andwrasses. As an example, mostwrasses areprotogynous hermaphrodites within a haremic mating system.[73][74] It is less common for a male to switch to a female (protandry).[4]: 162 A common example of a protandrous species areclownfish—if the larger, dominant female dies, in many cases, the reproductive male gains weight and becomes the female.[75][76] Hermaphroditism allows for complex mating systems. Wrasses exhibit three different mating systems: polygynous,lek-like, and promiscuous mating systems.[77]
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.[78] Sexual cannibalism is common among insects, arachnids[79] andamphipods.[79] There is also evidence of sexual cannibalism ingastropods andcopepods.[80]
Sexual coercion behaviour during mating has been documented in a variety of species. In someherbivorous herd species, or species where males and females are very different in size, the maledominates sexually by force and size.[citation needed]
Some species of birds have been observed combining sexual intercourse with violence; these includeducks,[81][82] andgeese.[83] Femalewhite-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.[84]
It has been reported that young male elephants in South Africa sexually coerced and killed rhinoceroses.[85] 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".[86]
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.[87]
Parthenogenesis is a form of asexual reproduction in which growth and development of embryos occur without fertilisation.[88] 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"[89] to stimulateovulation, 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 anevolutionary 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.[citation needed]
It is rare to find true parthenogenesis in fishes, where females produce female offspring with no input from males. All-female species include the Texassilverside,Menidia clarkhubbsi[90] and a complex of Mexicanmollies.[4]: 162
Parthenogenesis has been recorded in 70 vertebrate species[91] includinghammerhead sharks,[92]blacktip sharks,[93] amphibians[94]and lizards.[95]
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 isdiploid but not hermaphroditic. It can havetriploid andtetraploid forms, including all-female forms that reproduce mainly throughhybridogenesis.[96]
_Coyote_dog_mating.jpg)
There is a range of behaviours that animals perform that appear to be sexually motivated but which can not result in reproduction. These include:
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".[126] Scientists at 15aquaria 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.[126]
Amongbonobos, males and femalesengage 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.[119] Sexual activity occurs between almost all ages and sexes of bonobo societies.[127][128] Primatologist Frans de Waal believes that bonobos use sexual activity to resolve conflict between individuals.[28][129] Immature bonobos, contrariwise, perform genital contact when relaxed.[128]
Macaque
Similar same-sex sexual behaviours occur in both male and female macaques.[130] It is thought to be done for pleasure as an erect male mounts and thrusts upon or into another male.[130][131] Sexual receptivity can also be indicated by red faces and shrieking.[130] Mutual ejaculation after a combination of anal intercourse and masturbation has also been witnessed, although it may be rare.[131] In comparison to socio-sexual behaviours such as dominance displays, homosexual mounts last longer, happen in series, and usually involve pelvic thrusting.[130]
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.[132]
Malebottlenose 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 inintense sexual play with each other. Janet Mann, a professor of biology and psychology at Georgetown University, argues[133] that the common same-sex behaviour among maledolphin 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.[133]
In 1991, an English man was prosecuted for allegedly having sexual contact with a dolphin.[134] 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.[134]
The femalespotted hyena hasa unique urinary-genital system, closely resembling the penis of the male, called apseudo-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 erectileclitoris).[135] It is speculated that to facilitate this, theirsympathetic andparasympathetic nervous systems may be partially reversed in respect to their reproductive organs.[136]
Mammals mate by vaginal copulation. To achieve this, the male usually mounts the female from behind.[137] The female may exhibitlordosis 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,[138][better source needed] as the females' anatomy seems to reflect,[119] although ventro-ventral copulation has also been observed inRhabdomys.[139] Some sea mammals copulate in a belly-to-belly position.[140][141] Somecamelids mate in a lying-down position.[142] In most mammals ejaculation occurs after multiple intromissions,[143] but in most primates, copulation consists of one brief intromission.[144] In most ruminant species, a singlepelvic thrust occurs during copulation.[145][146] In most deer species, a copulatory jump also occurs.[147][148]
During mating, a "copulatory tie" occurs in mammals such asfossas,[149]canids with the exception ofAfrican wild dogs,[150] andJapanese martens.[151] A "copulatory lock" also occurs in some primate species, such asGalago senegalensis.[152]
The copulatory behaviour of many mammalian species is affected bysperm competition.[153]
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.[22]
Invertebrates are oftenhermaphrodites. Some hermaphroditicland 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 ofcalcium carbonate orchitin, and is called alove 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.[154] 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 ahormone-like substance that facilitates the survival of the sperm.[155][156]
Penis fencing is a mating behaviour engaged in by certain species offlatworm, such asPseudobiceros bedfordi. Species which engage in the practice are hermaphroditic, possessing both eggs and sperm-producing testes.[157] 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 bothgonochoristic (unisexual) andhermaphroditic, each of which can reproduce sexually and asexually. Reproduction also allows corals to settle new areas. Corals predominantly reproducesexually. 25% ofhermatypic corals (stony corals) form single sex (gonochoristic) colonies, while the rest arehermaphroditic.[158] About 75% of all hermatypic corals "broadcast spawn" by releasinggametes –eggs andsperm – into the water to spread offspring. The gametes fuse during fertilisation to form a microscopiclarva called aplanula, typically pink and elliptical in shape.[159]Synchronous spawning is very typical on the coral reef and often, even when multiplespecies 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.[158] Synchronous spawning may form hybrids and is perhaps involved in coralspeciation.[160]
Butterflies spend much time searching for mates. When the male spots a mate, he will fly closer and releasepheromones. He then performs a specialcourtship 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 theirabdomens. Here, the male passes the sperm to the female's egg-laying tube, which will soon be fertilised by the sperm.[161]
Many animals makeplugs of mucus to seal the female'sorifice after mating. Normally such plugs are secreted by the male, to block subsequent partners. In spiders the female can assist the process.[162] 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.[162] 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.[162]
In the orb-weaving spider speciesZygiella x-notata, individuals engage in a variety of sexual behaviors including male choosiness, mate guarding, and vibrational signaling in courtship.[163][164]
Research intohuman evolution confirms that, in some cases, interspecies sexual activity may have been responsible for the evolution of new species (speciation). Analysis of animalgenes found evidence that, afterhumans had diverged from otherapes, interspecies mating nonetheless occurred regularly enough to change certain genes in the newgene pool.[165] Researchers found that theX chromosomes of humans and chimps may havediverged 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.[166] A 2012 study questioned this explanation, concluding that "there is no strong reason to involve complicated factors in explaining the autosomal data".[167][dubious –discuss]
When close relatives mate, progeny may exhibit the detrimental effects ofinbreeding depression. Inbreeding depression is predominantly caused by thehomozygous expression of recessive deleterious alleles.[168] Over time, inbreeding depression may lead to the evolution ofinbreeding avoidance behaviour. Several examples of animal behaviour that reduce mating of close relatives and inbreeding depression are described next.
Reproductively active femalenaked mole-rats tend to associate with unfamiliar males (usually non-kin), whereas reproductively inactive females do not discriminate.[169] 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.[170] In the house mouse, themajor urinary protein (MUP) gene cluster provides a highly polymorphic scent signal of genetic identity that appears to underliekin recognition andinbreeding avoidance. Thus there are fewer matings between mice sharing MUPhaplotypes than would be expected if there were random mating.[171]
Meerkat females appear to be able to discriminate the odour of their kin from the odour of their non-kin.[172] 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 ininbreeding 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.[173]
Thegrey-sided vole (Myodes rufocanus) exhibits male-biased dispersal as a means of avoiding incestuous matings.[174] 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 birdParus major (great tit), inbreeding is likely avoided by dispersal of individuals from their birthplace, which reduces the chance of mating with a close relative.[175] Dispersing to avoid inbreeding is a common behavior amongst animals, such asfelids andcanids, although inbreeding can still occur, albeit rarely.
Toads displaybreeding site fidelity, as do manyamphibians. Individuals that return to natal ponds to breed will likely encountersiblings as potential mates. Althoughincest 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.[176]
{{cite journal}}:Cite journal requires|journal= (help)nineteen was invoked but never defined (see thehelp page).'Selv om det evolutionsmæssige formål med at parre sig kan siges at være reproduktion, er det ikke selve det, at dyrene får afkom, der i første omgang får dem til at parre sig. Det er til gengæld sandsynligt, at de parrer sig, fordi de er motiverede for selve parringsakten, og at denne er forbundet med en positiv oplevelse. Det er derfor rimeligt at antage, at der er en eller anden form for behag eller tilfredsstillelse forbundet med akten. Denne antagelse bekræftes af adfærden hos handyr, der for mange arters vedkommende er parate til at arbejde for at få adgang til hundyr, især hvis hundyret er i brunst, og handyr der i avlsøjemed er vant til at få tappet sæd – de viser stor ivrighed, når det udstyr, de forbinder med sædopsamlingen, tages frem.' [Translation to English] 'Although the evolutionary purpose of mating can be said to be reproduction, it is not the very fact that animals have offspring that causes them to mate in the first place. Rather, it is likely that they mate because they are motivated by the act of mating itself and that this is associated with a positive experience. It is therefore reasonable to assume that there is some form of pleasure or satisfaction associated with the act. This assumption is confirmed by the behaviour of male animals, which for many species are prepared to work to gain access to females, especially if the female is in heat, and male animals which for breeding purposes are used to having their semen collected - they show great eagerness when the equipment they associate with semen collection is taken out.'
{{cite journal}}:Cite journal requires|journal= (help)This makes O. aries (ram) only the second mammal known, apart from humans, capable of displaying exclusive homosexuality.
Perhaps the bonobo's most typical sexual pattern, undocumented in any other primate, is genito-genital rubbing (or GG rubbing) between adult females. One female facing another clings with arms and legs to a partner that, standing on both hands and feet, lifts her off the ground
{{cite book}}:|journal= ignored (help)
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