Extended female sexuality is where the female of a species
mates
Mates is an English surname, and may refer to:
* Mates (born 1964), British newsreader and journalist
* Michael Mates (born 1934), British politician
* Frederick S. Mates, founded the Mates Investment Fund in 1967 that crashed in the bear market ...
despite being infertile.
In most species, the female only engages in
copulation
Sexual intercourse (or coitus or copulation) is a sexual activity typically involving the insertion and thrusting of the penis into the vagina for sexual pleasure or reproduction.Sexual intercourse most commonly means penile–vaginal penetra ...
when she is
fertile. However, extended sexuality has been documented in
old world primates,
pair bonded birds and some
insects
Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs of j ...
(such as
carrion beetles). Extended sexuality is most prominent in human females who exhibit no change in copulation rate across the
ovarian cycle
The menstrual cycle is a series of natural changes in hormone production and the structures of the uterus and ovaries of the female reproductive system that make pregnancy possible. The ovarian cycle controls the production and release of eggs a ...
.
Although this behaviour incurs costs to females, such as energy and time,
many researchers have proposed reasons for its existence. These hypotheses include the male assistance hypothesis, which proposes that females gain non-genetic benefits (such as food and shelter) in exchange for sexual access.
A sub-hypothesis of this is
Hrdy's, proposing extended female sexuality as an
adaptive
Adaptation, in biology, is the process or trait by which organisms or population better match their environment
Adaptation may also refer to:
Arts
* Adaptation (arts), a transfer of a work of art from one medium to another
** Film adaptation, a ...
process aiming to creating
paternity
Paternity may refer to:
*Father, the male parent of a (human) child
*Paternity (law), fatherhood as a matter of law
* ''Paternity'' (film), a 1981 comedy film starring Burt Reynolds
* "Paternity" (''House''), a 2004 episode of the television seri ...
confusion in males.
Alternative hypotheses, classified as 'male-driven', claim that extended female sexuality occurs due to male adaptations, resulting from an inability to detect fertility status in females or to dampen immune responses against sperm.
Finally, Spuhler's hypothesis suggests that the behaviour may have arisen as an incidental effect of larger adrenal glands in humans.
Occurrence
In non-humans
Although not found in all organisms, researchers have identified sexual intercourse patterns in certain animals that reflect extended female sexuality, such as in some old world primates, birds and insects.
Extensive research has focused on analysing the
musk shrew
The white-toothed shrews or Crocidurinae are one of three subfamilies of the shrew family Soricidae.
The outer layer of these shrews' teeth is white, unlike that of the red-toothed shrews. These species are typically found in Africa and souther ...
's rate of sexual behaviour. The only period that is associated with a drop in female
receptivity
{{Multiple issues, {{refimprove, date=December 2014{{one source, date=December 2014
Receptivity, or receptive agency, is a practical capacity and source of normativity, discussed and developed in various ways by writers such as Ralph Waldo Emers ...
to copulation is during mid to late
pregnancy
Pregnancy is the time during which one or more offspring develops (gestation, gestates) inside a woman, woman's uterus (womb). A multiple birth, multiple pregnancy involves more than one offspring, such as with twins.
Pregnancy usually occur ...
; yet, even at this time, occasional mating is reported. Therefore, researchers have concluded that this animal has similar sexual receptivity across infertile and fertile phases. Within primates, research has consistently found evidence of extended female sexuality in the
rhesus monkey and
chimpanzees. Both of these primates mate at all stages of the ovarian cycle, with only slight increases in sexual receptivity during
fertile stages, and decreases during
menstruation.
In humans
Human females are considered to exhibit the greatest degree of extended female sexuality, with receptivity to sexual intercourse remaining constant across fertile and infertile phases of the reproductive cycle, including during
pregnancy
Pregnancy is the time during which one or more offspring develops (gestation, gestates) inside a woman, woman's uterus (womb). A multiple birth, multiple pregnancy involves more than one offspring, such as with twins.
Pregnancy usually occur ...
,
lactation
Lactation describes the secretion of milk from the mammary glands and the period of time that a mother lactates to feed her young. The process naturally occurs with all sexually mature female mammals, although it may predate mammals. The proces ...
, and in adolescence. In a study of 20,000 women from 13 countries, frequency of copulation was reportedly the same across all stages of the ovarian cycle. The only notable drop in sexual behaviour occurred during
menstruation. Therefore, women largely showed the same level of sexual behaviour in the non-fertile phases of their ovarian cycles as in the fertile phases.
Researchers have investigated the effect of
hormonal contraceptive use in women on the frequency of sexual intercourse. Many of these contraceptives mimic a pregnancy state in females by altering hormone levels. Therefore, women who use these contraceptives do not experience the fertile phases of their cycles. In a systematic review, it appeared that the frequency of sexual intercourse was unaffected by contraceptive use in the majority of women. Although artificially created, this adds to the literature documenting the existence of copulation in humans during non-fertile periods.
Impact of concealed ovulation
In order to encourage
mating
In biology, mating is the pairing of either opposite- sex or hermaphroditic organisms for the purposes of sexual reproduction. ''Fertilization'' is the fusion of two gametes. ''Copulation'' is the union of the sex organs of two sexually reprod ...
in non-conceptive periods and to encourage males to maintain their proximity, males must possess imperfect knowledge of the female's
fertility
Fertility is the capability to produce offspring through reproduction following the onset of sexual maturity. The fertility rate is the average number of children born by a female during her lifetime and is quantified demographically. Fertili ...
status. This is achieved through
concealed ovulation in most animals that exhibit extended female sexuality.
A review of studies revealed that, in humans, females only exhibit subtle changes during
estrus
The estrous cycle (, originally ) is the set of recurring physiological changes that are induced by reproductive hormones in most mammalian therian females. Estrous cycles start after sexual maturity in females and are interrupted by anestrous ...
, making it difficult for males to assess fertility with precision.
In the
!Kung tribe, for instance, females lack any overt signals of fertility and are continuously
receptive to sexual intercourse, encouraging males to remain and provide resources.
Explanations
Male assistance hypothesis
Mating outside the fertile window of their
ovarian cycle
The menstrual cycle is a series of natural changes in hormone production and the structures of the uterus and ovaries of the female reproductive system that make pregnancy possible. The ovarian cycle controls the production and release of eggs a ...
may incur considerable costs for females, such as in time and energy usage. To counteract these costs, the male assistance hypothesis argues that females exhibit extended sexuality to obtain resources from males. These resources vary between species, but can include food, social alliance, and protection of the female and her offspring.
For example, in the
Trobriand
The Trobriand Islands are a archipelago of coral atolls off the east coast of New Guinea. They are part of the nation of Papua New Guinea and are in Milne Bay Province. Most of the population of 12,000 indigenous inhabitants live on the main isla ...
tribe, men give women gifts in exchange for sexual access. From this hypothesis, three predictions can be made.
Male provision of non-genetic resources
Firstly, in species that demonstrate extended female sexuality, there should be evidence that the males provide non-genetic resources to females. This prediction is supported in a variety of animals, with reviews revealing that male assistance (such as food or protection), is provided to certain
mammals and communally breeding birds in exchange for mating outside the conceptive period. For example, female
blackbirds that solicit mating outside the fertile period have increased
mate guarding
In biology, mating is the pairing of either opposite- sex or hermaphroditic organisms for the purposes of sexual reproduction. ''Fertilization'' is the fusion of two gametes. ''Copulation'' is the union of the sex organs of two sexually reprodu ...
from their pair-bonded partner. This increases protection against other
sexually coercive males and ensures the provision of other material benefits.
In a review investigating primates that exhibit extended female sexuality, it was noted that females engaging in extended sexuality benefited from increased offspring protection and paternal care from males.
Enhanced reproductive success
Secondly, in order to outweigh the aforementioned costs,
mating
In biology, mating is the pairing of either opposite- sex or hermaphroditic organisms for the purposes of sexual reproduction. ''Fertilization'' is the fusion of two gametes. ''Copulation'' is the union of the sex organs of two sexually reprod ...
during infertile phases should increase females'
reproductive success
Reproductive success is an individual's production of offspring per breeding event or lifetime. This is not limited by the number of offspring produced by one individual, but also the reproductive success of these offspring themselves.
Reproduct ...
by increasing the number of offspring produced. Current research has only investigated this factor indirectly, and it has predominantly been investigated in insects. For example, when male insects deliver material benefits in exchange for sexual access, the reproductive success of the females increases with the number of matings.
It is important to note that the
mating
In biology, mating is the pairing of either opposite- sex or hermaphroditic organisms for the purposes of sexual reproduction. ''Fertilization'' is the fusion of two gametes. ''Copulation'' is the union of the sex organs of two sexually reprod ...
behaviour assessed was not limited to extended female sexuality. Hence, it only provides indirect support for the second prediction.
Shifting mate preferences and behaviour across the ovarian cycle
The final prediction of the male-assistance hypothesis has been extensively investigated. It predicts that females will exhibit differing
mate preferences during fertile and non-fertile periods. Specifically, when fertile, the females will be sensitive to indicators of high genetic quality to increase the genetic quality of her
offspring
In biology, offspring are the young creation of living organisms, produced either by a single organism or, in the case of sexual reproduction, two organisms. Collective offspring may be known as a brood or progeny in a more general way. This ca ...
.
Conversely, outside of the fertile period, females will show a preference for males who can provide resources for her and her offspring. In most species, males of higher genetic quality offer fewer non-genetic resources (such as shelter and food) than those of lower quality, so females are likely to choose different males at each stage.
Evidence for this prediction has been found in many different species. In
hens, those near peak fertility show a preference for socially dominant roosters with large
combs. Larger combs have been reliably associated with health and
offspring
In biology, offspring are the young creation of living organisms, produced either by a single organism or, in the case of sexual reproduction, two organisms. Collective offspring may be known as a brood or progeny in a more general way. This ca ...
survival so reliably indicate the genetic quality of the
rooster
The chicken (''Gallus gallus domesticus'') is a domesticated junglefowl species, with attributes of wild species such as the grey and the Ceylon junglefowl that are originally from Southeastern Asia. Rooster or cock is a term for an adult m ...
. In the non-fertile phases, hens mate more indiscriminately with less regard for the larger comb size. A similar pattern emerges in humans. A review of relevant studies on female preferences across the
ovarian cycle
The menstrual cycle is a series of natural changes in hormone production and the structures of the uterus and ovaries of the female reproductive system that make pregnancy possible. The ovarian cycle controls the production and release of eggs a ...
reveals that women show a greater preference for
masculine traits in fertile phases, especially for
short term mating intention. This finding occurs across a variety of traits, such as masculine
voices, body and facial features,
scent
An odor (American English) or odour (Commonwealth English; see spelling differences) is caused by one or more volatilized chemical compounds that are generally found in low concentrations that humans and animals can perceive via their sense ...
, and
behavioural displays.
The level of masculinity acts as an indicator of genetic quality through its association with high
testosterone
Testosterone is the primary sex hormone and anabolic steroid in males. In humans, testosterone plays a key role in the development of male reproductive tissues such as testes and prostate, as well as promoting secondary sexual characteristi ...
. Similarly, women show a preference for higher levels of
symmetry, which is thought to indicate underlying developmental stability. However, during infertile phases, women prefer males with lower masculinity and symmetry, as they tend to be more willing to offer material benefits.
![Big red rooster](https://upload.wikimedia.org/wikipedia/commons/f/f1/Big_red_rooster.jpg)
In addition to impacting
mating preferences, females have been found to exhibit differing mating behaviour at different cycle stages. An analysis of 121 studies with female birds showed that most mate outside their
pair bond
In biology, a pair bond is the strong affinity that develops in some species between a mating pair, often leading to the production and rearing of offspring and potentially a lifelong bond. Pair-bonding is a term coined in the 1940s that is freque ...
at a higher rate when fertile, especially when the primary partner possesses indicators of low quality
gene
In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
s. During infertile phases, birds showed reductions in this behaviour, suggesting that the function of extended female sexuality is not to increase the genetic quality of offspring. In humans, females show increased motivation for mating with other males at
mid-cycle without an accompanying increase in
copulation
Sexual intercourse (or coitus or copulation) is a sexual activity typically involving the insertion and thrusting of the penis into the vagina for sexual pleasure or reproduction.Sexual intercourse most commonly means penile–vaginal penetra ...
with their long-term partner, especially if the partner was less physically attractive.
Hrdy's hypothesis
![Baby ginger monkey](https://upload.wikimedia.org/wikipedia/commons/2/27/Baby_ginger_monkey.jpg)
Hrdy's hypothesis is an extension of the male assistance hypothesis, in that both hypotheses argue that women have evolved this adaptation to gain some tangible benefit from males. According to Hrdy's hypothesis,
extended female sexuality is an
adaptive
Adaptation, in biology, is the process or trait by which organisms or population better match their environment
Adaptation may also refer to:
Arts
* Adaptation (arts), a transfer of a work of art from one medium to another
** Film adaptation, a ...
process with an aim of creating paternity confusion in their male counterparts.
Paternity confusion refers to the male being unsure as to whether
offspring
In biology, offspring are the young creation of living organisms, produced either by a single organism or, in the case of sexual reproduction, two organisms. Collective offspring may be known as a brood or progeny in a more general way. This ca ...
are genetically his own. If the female mates with different males (at all points of her
ovarian cycle
The menstrual cycle is a series of natural changes in hormone production and the structures of the uterus and ovaries of the female reproductive system that make pregnancy possible. The ovarian cycle controls the production and release of eggs a ...
) whilst concealing
fertility
Fertility is the capability to produce offspring through reproduction following the onset of sexual maturity. The fertility rate is the average number of children born by a female during her lifetime and is quantified demographically. Fertili ...
, then the males will inevitably have paternity confusion.
Paternity confusion is proposed to be an adaptive function for preventing
infanticide. Thus, if the female can successfully create paternity confusion, males will be less likely to kill her offspring, as the lack of paternity certainty means that they run the risk of killing their own genetic
offspring
In biology, offspring are the young creation of living organisms, produced either by a single organism or, in the case of sexual reproduction, two organisms. Collective offspring may be known as a brood or progeny in a more general way. This ca ...
. Additionally, the males, in turn, are likely to protect the same female's offspring from infanticide that may be committed by other adults within the species. Once again, this is because they are uncertain about paternity, and aim to protect infants that are genetically their own.
Paternity confusion in primates
Researchers have analysed the behaviour of
chimpanzees, with particular reference to
copulation calling. Copulation calling is a type of vocalisation used to attract mates. The calls are vocalised either before, during, or after sexual intercourse. Copulation calling, in line with Hrdy's hypothesis, may then be one way to ensure that the female can mate with as many different males as possible, causing paternity confusion. Indeed, the authors found that the probability of copulatory calling in female chimpanzees was not modulated by the ovarian phase of the female caller, thereby aiding paternity confusion. This paternity confusion ultimately ensures that the woman has access to the resources of a number of different males. These resources can be utilised by her, as well as by her offspring.
Criticism of Hrdy's hypothesis emerges from evidence which suggests that male primates can discriminate between their own offspring and the offspring of others. In one study, researchers analysed the
DNA of 75 juvenile
baboon
Baboons are primates comprising the genus ''Papio'', one of the 23 genera of Old World monkeys. There are six species of baboon: the hamadryas baboon, the Guinea baboon, the olive baboon, the yellow baboon, the Kinda baboon and the chacma ...
s to conclude who fathered them. They found that males selectively cared for their own offspring, particularly when their offspring became involved in aggressive confrontations which posed the possibility of injuries or a threat to their
social standing
Social stratification refers to a society's categorization of its people into groups based on socioeconomic factors like wealth, income, race, education, ethnicity, gender, occupation, social status, or derived power (social and political). As ...
. Evidently, if males can discriminate between their own offspring and the offspring of others, then there is no purpose in the female attempting to create paternity confusion during the pregnancy stage. This would be counter-intuitive, as, once the offspring is born, the males will know whether the offspring is, or is not, their own. Future research will need to be conducted in this vein to investigate whether males in other species show the ability to discriminate between their own offspring and the offspring of others before coming to any decisive conclusions.
Concealed estrus as a function of paternity confusion in primates
Research is fairly consistent in the finding that species with concealed estrus mate at all stages of their ovarian cycle. For instance, mating activity in
assamese macaques (''Macaca assamensis'') has been investigated. The researchers analysed the levels of
progesterone in the monkeys, in order to establish the ovarian stage of the female, as progesterone peaks following the fertile window. They found that the females concealed estrus and were
sexually receptive
Animal sexual behaviour takes many different forms, including within the same species. Common mating or reproductively motivated systems include monogamy, polygyny, polyandry, polygamy and promiscuity. Other sexual behaviour may be reprodu ...
during the entire mating season. Concealed estrus and sexual receptivity (at all times of the ovarian cycle) aids paternity confusion. This is because the males are unsure of who mated with the female during her fertile period, and so do not know the identity of the father.
Hrdy's hypothesis has been criticised, however, on the basis that some female
primate
Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians ( monkeys and apes, the latter including ...
s show both extended female sexuality and
sexual swellings. In terms of Hrdy's hypothesis, these two concepts are incompatible. Sexual swellings appear only during the most fertile phase of the female's ovarian cycle, with the purpose of advertising fertility. In sharp contrast, according to Hrdy, extended sexuality is adapted to
conceal fertility and ensure mating across all stages of the ovarian cycle, to aid paternity confusion.
Male-driven hypotheses
One of the alternative explanations is that extended female sexuality is 'male-driven'. This hypothesis is theoretically based on male uncertainty regarding the fertility status of females.
Although some
physiological changes occur during the fertile period that may act as reliable indicators (e.g. the concentration of
oestrogen
Estrogen or oestrogen is a category of sex hormone responsible for the development and regulation of the female reproductive system and secondary sex characteristics. There are three major endogenous estrogens that have estrogenic hormonal acti ...
can change female
scent
An odor (American English) or odour (Commonwealth English; see spelling differences) is caused by one or more volatilized chemical compounds that are generally found in low concentrations that humans and animals can perceive via their sense ...
), most species have not evolved signals advertising fertility (e.g.
sexual swellings). Therefore, males will be unable to detect fertility with any precision. As a result, extended female sexuality is proposed as a male
sexually selected trait. Males will pursue sexual access throughout the entirety of the
ovarian cycle
The menstrual cycle is a series of natural changes in hormone production and the structures of the uterus and ovaries of the female reproductive system that make pregnancy possible. The ovarian cycle controls the production and release of eggs a ...
to increase their chances of
impregnating the female. According to this hypothesis, females lack any benefit from this activity due to their inability to conceive, yet will be
coerced by males to engage in
sexual intercourse.
The training hypothesis
Another hypothesis claiming that extended female sexuality has evolved to benefit males' interests is the 'training hypothesis'. It has been shown that women's
immune system
The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splint ...
s attack foreign
antigen
In immunology, an antigen (Ag) is a molecule or molecular structure or any foreign particulate matter or a pollen grain that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune respons ...
s found in
sperm.
This may reduce the likelihood of conception and so reduce the
reproductive success
Reproductive success is an individual's production of offspring per breeding event or lifetime. This is not limited by the number of offspring produced by one individual, but also the reproductive success of these offspring themselves.
Reproduct ...
for the males. However, prior exposure to sperm antigens can dampen the immune response to increase the chances of successful conception and
implantation. Therefore, males may have evolved to train and 'condition' women's immune systems by copulating with females during infertile periods, in order to reduce the likelihood of her immune system reacting against the sperm's antigens from lack of exposure. Indeed, research has identified findings consistent with this hypothesis.
Couples who had used condoms before trying to conceive were more likely to suffer complications, such as
pre-eclampsia, during pregnancy as a result of the woman's immune system having no prior exposure to the antigens in the sperm. In non-humans, an increased frequency of copulation in crickets has been shown to reduce the female's immune response to sperm.
This hypothesis has received much criticism . For example, as all mammals experience the same immune system responses to sperm antigens, this hypothesis predicts that all mammals should exhibit extended female sexuality. However, only few species of mammals exhibit sexual behaviour outside the conceptive period. On the other hand, eclampsia is virtually unique to humans and thus the Training Hypothesis may only be relevant to extended female sexuality in humans.
Spuhler's hypothesis
Spuhler's hypothesis is a stand-alone hypothesis of extended female sexuality. Spuhler suggests that extended female sexuality has evolved as a by-product of an adaptation in females that increases the levels of
adrenal
The adrenal glands (also known as suprarenal glands) are endocrine glands that produce a variety of hormones including adrenaline and the steroids aldosterone and cortisol. They are found above the kidneys. Each gland has an outer cortex whic ...
hormones. He proposed that secretion of higher levels of adrenal hormones were initially selected in women in order to increase endurance for walking or running. This hypothesis suggests that the larger
adrenal gland
The adrenal glands (also known as suprarenal glands) are endocrine glands that produce a variety of hormones including adrenaline and the steroids aldosterone and cortisol. They are found above the kidneys. Each gland has an outer cortex whic ...
s may have contributed to the development of extended female sexuality as they are also the main source of '
libido
Libido (; colloquial: sex drive) is a person's overall sexual drive or desire for sexual activity. Libido is influenced by biological, psychological, and social factors. Biologically, the sex hormones and associated neurotransmitters that act u ...
hormones', which increase the female's sexual drive.
Thus, extended sexuality is little to do with sexual behavior, or evolutionary advantage, but rather it is just a by-product of hormones.
[Williams, G. C. (1966). Adaptation and Natural SelectionPrinceton University Press. ''Princeton, NJ''.] However, this hypothesis cannot account for extended female sexuality in invertebrates, which lack adrenal systems.
Additionally Spuhler highlighted the existence of larger
thyroid
The thyroid, or thyroid gland, is an endocrine gland in vertebrates. In humans it is in the neck and consists of two connected lobes. The lower two thirds of the lobes are connected by a thin band of tissue called the thyroid isthmus. The thy ...
and
adrenal gland
The adrenal glands (also known as suprarenal glands) are endocrine glands that produce a variety of hormones including adrenaline and the steroids aldosterone and cortisol. They are found above the kidneys. Each gland has an outer cortex whic ...
s in humans compared to other primates. However, no empirical evidence has established the link between hormones and endurance walking as an adaptation and extended female sexuality as a natural incidental effect. As there are costs to extended sexuality, it can be argued that selection would serve the dissociation of extended sexuality from the mechanism that affects endurance walking.
See also
*
Alternative mating strategy An alternative mating strategy is a strategy used by male or female animals, often with distinct phenotypes, that differs from the prevailing mating strategy of their sex. Such strategies are diverse and variable both across and within species. Anim ...
*
Animal sexual behaviour
Animal sexual behaviour takes many different forms, including within the same species. Common mating or reproductively motivated systems include monogamy, polygyny, polyandry, polygamy and promiscuity. Other sexual behaviour may be reprodu ...
*
Human female sexuality
Human female sexuality encompasses a broad range of behaviors and processes, including female sexual identity and Human sexual activity, sexual behavior, the physiological, psychological, social, cultural, political, and spiritual or religious ...
*
Human mating strategies
*
Strategic pluralism
*
Human sexual activity
Human sexual activity, human sexual practice or human sexual behaviour is the manner in which humans experience and express their sexuality. People engage in a variety of sexual acts, ranging from activities done alone (e.g., masturbation) t ...
*
Human sexuality
Human sexuality is the way people experience and express themselves sexually. This involves biological, psychological, physical, erotic, emotional, social, or spiritual feelings and behaviors. Because it is a broad term, which has varied ...
*
Major histocompatibility complex and sexual selection
The major histocompatibility complex in sexual selection concerns how major histocompatibility complex (MHC) molecules allow for immune system surveillance of the population of protein molecules in a host's cells. In 1976, Yamazaki et al. demonstr ...
*
Mating system
A mating system is a way in which a group is structured in relation to sexual behaviour. The precise meaning depends upon the context. With respect to animals, the term describes which males and females mate under which circumstances. Recognised ...
*
Ovulatory shift hypothesis
The ovulatory shift hypothesis holds that women experience evolutionarily Adaptation, adaptive changes in subconscious thoughts and behaviors related to mating during different parts of the Menstrual cycle, ovulatory cycle. It suggests that what ...
*
Reproduction
References
{{Reflist, 30em
Further reading
* Buss, D. M. (2015). ''The Handbook of Evolutionary Psychology, Foundation'' (Chapter 13). New York: John Wiley & Sons.
* Campbell, L. (2009, June 17)
Comparison of the sexuality of humans, common chimpanzees and bonobos
* Shackelford, T. K., & Hansen, R. D. (2015).
The Evolution of Sexuality' (Chapter 8). New York: Springer.
* Simpson, J. A., & Campbell, L. (2013).
The Oxford Handbook of Close Relationships' (Chapter 17). Oxford: University Press.
* Thornhill, R., & Gangestad, S. W. (2008). ''The Evolutionary Biology of Human Female Sexuality'' (Chapter 3). Oxford: University Press.
Animal sexuality
Evolutionary biology
Human sexuality