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Correlates of Exaggerated Sexual Swellings
Several characteristics have been identified as being correlates of sexual swellings in primates. Exaggerated sexual swellings are commonly found in primates which live in large social groups with multiple males. It’s noted that those species with such swellings have a higher average of males per group than those without; over twice as many, and of the ‘Old World’ primate species which have multi-male social systems 71% show exaggerated swellings. Additionally, primates with sexual swellings have been shown to demonstrate non-seasonal breeding patterns, longer mating periods and longer ovulation cycles. Of the 23 species which are both non-seasonal breeders, and live in multi-male societies, 91% have sexual swellings.

Exaggerated sexual swellings also increase gradually in size throughout the female’s cycle beginning after menstruation, for example research on baboons showed that after 14 days of gradual increase swellings peaked for 2 days before reducing. It is thought that these swellings are controlled hormonally, and it has been shown in ovariectomized chimpanzees, swelling can be induced by estrogen and inhibited by progesterone. Peak swelling is likely to coincide with the highest potential of ovulation but this is not a perfect association. For example, research on West African Chimpanzees showed that the higher probabilities of ovulation tended to be within 7 to 9 days of the onset of maximum swelling of sexual skin. A study into wild white-handed gibbons showed that maximum swelling size and ovulation overlapped closely in 80% of menstrual cycles

Male Response to Sexual Swellings
Male primates as highly attracted to females when their sexual swellings are at their most swollen, and demonstrate preferential mating during periods of maximal swelling. Males tend to compete more for females whose swellings are at their maximum point. Male-male competition peaks and males attempting to mate with the females with the largest swellings receive increased levels of aggression from other males as a result. Observations of chimpanzees have revealed that the presence of at least one female who was maximally swollen prompted higher levels of aggression between males in a group, as well as increased levels of sexual behaviour.

In general, females' sexual swellings are responded to by males in the group as though they provide indications of female fertility and males therefore use these swellings in order to decide their level of investment and effort in courting females. In male baboons, mating effort is determined by the size of the females' swellings, and this in turn impacts levels of male-male aggression and competition and fighting behaviours, as well as how much time is invested in grooming and courting the female. Peak swelling also correlates with higher levels of mate guarding behaviour with males preferring to guard those females whose swellings are close to or at maximal swelling, as well as more perineal inspections

The greater access to the most swollen females is usually granted to and won by the most dominant males in the group. Those further down the hierarchy tend to only be able to gain access and mate with females outside of these periods of peak swelling, when the competition for them is reduced and the attention of more dominant males has shifted to the more swollen females in the group. In baboons, the more mature and dominant males chase away younger males from the most receptive females at peak swelling, but young males do get access to mate with females outside of these peak swelling points (e.g. early in the estrous cycle).

Best-Male Hypothesis
The Best-Male Hypothesis for sexual swellings, as an explanation for the function of sexual swellings in primates, is one of the longest standing. The hypothesis proposes that sexual swellings incite competition between males for access to a female by indicating her fertility and receptivity so that the female is able to identify the eventual winner as the 'best male' with higher fitness and the best genes to pass on to her offspring. This hypothesis treats sexual swellings as signals by which females, through advertising that they are receptive to insemination by males, aim to increase their chances of high quality offspring by inciting competition between males in a group. Under the best-male hypothesis, the male with whom the female eventually copulates is the result of indirect mate choice as the female does not have to directly assess the fitness of each potential partner, rather her sexual swelling attracts competing males and results in the benefit of increased viability of her offspring.

Support for this hypothesis argues that it accounts for some of the correlates and characteristics of of sexual swellings, such as the proximity of peak swelling to ovulation and increased male-male competition over females at peak swelling. Criticism of the best-male hypothesis, however, states that it cannot account for the exaggerated nature of these swellings, as Pagel (1994) argued smaller swellings would be sufficient to incite competition between males since this behaviour was calculated to be an evolutionarily stable strategy. Others have criticised the hypothesis' assumption that the male which is successful in competition for swollen females (i.e. the most dominant) would also be the females own choice of partner, citing observations that some females choose subordinate males with whom to mate

Obvious-Ovulation Hypothesis
The obvious-ovulation (or paternity confidence) hypothesis of sexual swellings was first proposed by Hamilton in 1984. The hypothesis proposes that exaggerated swellings indicate the timing of ovulation and as a result increase paternal certainty, allowing males to assess if they have been successful in siring that female's offspring. This has benefits for the female and her offspring as paternal certainty has been frequently associated with the level of paternal care and investment. The obvious-ovulation hypothesis therefore proposes the function of sexual swellings as being a means by which ovulation can be indicated to males who can then be assured of paternity of offspring, in order to encourage the male to invest preferentially in that female's offspring. This is similar to the paternal care hypothesis which proposes that sexual swellings enable a male to determine the likelihood of having sired a particular female's offspring by signalling her ovulation status, allowing them to subsequently allocate investment accordingly based on their assessment of whether they have achieved paternity.

In support of this hypothesis, in his 1999 review, Nunn argued that the obvious-ovulation explanation of sexual swelling is consistent with the high likelihood of ovulation to coincide with maximum swelling. Further support that males used swellings in the way set out by this hypothesis to assess the timing of ovulation, comes from observations that in wild longtailed macaques (Macaca fascicularis), males are more aroused and find females more attractive at peak swelling. Others have been critical. According to Stallman and Froelich's assessment, the hypothesis predicts monandry (i.e. that females will have only one mating partner) which runs counter to observations of species such as Barbary macaques (Macaca sylvanus) which have exaggerated sexual swellings, yet have been seen to be promiscuous and polyandrous (i.e. mating with multiple males) in their mating behaviours.

Function of sexual swellings (brief intro draft)
Though much is understood about the physical characteristics of sexual swellings, their exact functional significance remains controversial. The role of sexual selection in the evolution of these swellings features in many hypotheses and has, since Darwin, been assumed to play a significant role. Hypothesised functions of sexual swellings often focus on swellings in terms of female mating strategies. These range from advertising fertility and quality (e.g. Reliable Indicator), maximising potential mates to confuse paternity of offspring (e.g. Many-Males), to aiding in a female's assessment of the best possible mating partners (e.g. Best-Male) and even assuring paternity certainty (e.g. Obvious-Ovulation) and all aim to account for aspects of exaggerated sexual swellings. This section covers the range of proposed functions for these swellings.

Male Services Hypothesis
The Male Services Hypothesis proposes that sexual swellings lead to direct benefits for the female by encouraging dominant males to engage in consortship behaviours (i.e. forming a partnership). Swellings illicit mate guarding behaviours from males who want to increase their chances of siring the swollen female's offspring, resulting in dominant males acting like bodyguards, reducing and preventing harassment from other males in the social group. Females may also benefit in that dominant males may later protect the resulting offspring, reducing the threat of infanticide from other males. It has been observed that male primates will attempt to monopolise, or gain exclusive sexual access to, a female early on in the duration of sexual swellings to ensure he has sexual access at maximum swelling when she is most likely to be ovulating. Though the males in these situations are guarding the females for their own gains (i.e. to ensure they will be able to sire the female's offspring) it has been observed in rhesus macaque (Macaca mulatta) that females also benefit from these consortships and are harassed less by subordinate males when they are with a dominant one.

A review by Nunn supported this hypothesis, citing its ability to account for the tendency of swellings to coincide with ovulation. Nunn also highlighted that females benefit from the lack of precision with which swellings signal ovulation as this is likely to extend the duration of mate guarding and consortship behaviours outlined by the hypothesis. Stallman and Froelich's review outlines an issue with this hypothesis in that, despite a reduction in harassment from subordinate males when with dominant males, females do not always choose to mate with these stronger and more dominant mates.