Overview of related topics
Figure 1. Fundamentals in the evolution of primates
Each one of us is result of a complex synergy among these and other variables. Our differences result from differences in our genetic information, personal experiences, and importantly for these factors -- small differences in hormones and other biochemical factors.
Primate mating systems
Number of males mating per female cycle
Type of sexual relationship
Long term exclusive
Long term but other females mate with resident male
Two or more
Short term, non-exclusive, gregarious
Two or more
(from Dixon, 1998, p.27)
one sex has multiple partners. Two forms are possible.
Polygyny - one male with several females -- is common in primates. Polygynous species tend to have greater sexual dimorphism (M>>F) and maybe lesser MPI (below).
Polyandry - one female with several males -- is very rare in primates.
parental investment (_PI)
estimates of costs -- usually very metaphorical -- of parenting
Always very high since mammal mothers supply food and care
Minimal in many species, limited to costs of a few million sperm cells. In gibbons and humans, male investment is much higher.
differences in male and female bodies due to pre and postnatal hormones. May arise from diverse causes, e.g. male competition, advantages to larger females in some species.
One's fitness is enhanced if close relatives sharing your genes also passes those genes into the next generation. So it may pay to help them out!
when females mate with several males, reproductive success hinges on quantity as well as quality of the males' sperm. To the extent more sperm increases odds of fertilization, the competitors will tend to have larger testes -- as evidenced in chimpanzees.
inter-male competition for mating opportunities
Males may compete for control of females directly (e.g. gorillas and orangs, maybe humans). Or they compete for dominance over each other within the group (e.g. chimps) for preferential mating, and/or at the level of sperm when the female mates with several males over a short time span.
Darwin (1871) noted that females can play a fundamental role in the evolution of species simply by choosing which males to mate with. Primate females actively solicit preferred males, and not necessarily the highest ranking ones. See Small (1993) who suggests novelty is a virtue for many primate females in choosing mates.
Females' sexual behavior in some primate species is highly controlled by hormones on their ovarian cycle. Yet in many other species, especially apes and humans, this is much less so.
Concealed or advertised ovulation?
Concealed ovulation refers to the lack of any obvious signs of oestrus in some species. Usually in regard to humans, it is suggested that concealed ovulation offers females a means of maintaining male interest over a longer time frame. However most primates do not obviously advertise ovulation and humans are no exception. Perhaps the question is why have chimps evolved those unmistakeable swellings?
Females may solicit sex with various activities -- posturing, touching genitals, etc.
Females may indicate willingness to copulate by lack of resistance in addition to any proceptive behaviors. In some species --notably common chimps --males harass females apparently with the result of making the females more receptive.
Sexual attractiveness (erotic imagery, pubertal eroticism)
Female attractiveness may be indicated in terms of behaviors, appearance -- notably estrogen dependent sexual skin swellings in chimpanzees - - and possible olfactory cues.
Male attractiveness involves appearance, rank, behaviors, and possible olfactory cues. Of course each species has its own "sexual or erotic imagery" characterizing desirable mates. There may be common themes indicating fertility and health including body symmetry and behaviors indicative of health.
Male chimps try to isolate fertile females, leading them away from the group for several hours to several weeks. Females must cooperate, e.g. not responding to other males vocalizations. Obviously this reduces "sperm competition" at a critical time.
Killing a nursing infant returns female to fertility, offering another male a reproductive opportunity. We see this as a dramatic beastly behavior but Dixon (1998) notes "Infantcide is neither widespread nor of general importance in the evolution of primate social or sexual behaviour. P.70"
This infantcide "strategy" works because nursing reduces fertility. For maximum effect nursing has to be continuous, hence it is not a reliable means of contraception for humans who typically do not have infants clinging to them 24 hours a day.
See Wrangham and Peterson's chapter "relationship violence" in Demonic Males. It appears more common in orangutans, where sub-dominant males still have physical strength advantages over females, and much less so in bonobos where females have close bonds among themselves and are rarely alone.
Harrassment and interruption of copulation
Dominant males may try to prevent lessers from mating. (Males may harass females with the goal of intimidating them and gaining mating advantages.)
Food for sex?
"Some chimpanzees press the attack, others focus the quarry's retreat, others wait in ambush, and soon the hunter apes have killed four monkeys.
A big male sits with meat in hand. Three other chimpanzees cluster around him, reaching eagerly with outstretched hands, watching anxiously for any sign of favor. One of the supplicants is his ally, his friend in the community's ongoing male status contests. So the meat owner tears a piece off his prize and drops it into his friend's hand. Encouraged by these signs of generosity, a female supplicant turns and invites the meat owner to mate. He does so, at the same time holding his valued property high to prevent a greedy hand from taking any. Then, after settling back, he rewards the willing female with a chew."
(from ch.1 Wrangham and Peterson (1997) Demonic Males.)
Also see the Stanford (TO) reading.
(See Goodall video notes on this topic. A recent finding based on DNA evidence is that a certain percentage of chimp fathers are not from the female's group. Also see Pusey in TO) The main idea though is that even in groups with dominant males, females may choose lesser males to mate with but only out of "sight" of that dominant male. (See "Family of chimps" video for examples.)
dispersal from natal (birth) groups
Depending on species, males or females leave their birth group at adolescence. Typically primate males leave with the important exceptions of gorilla and chimps, as well as red colobus monkeys and baboons. Gorillas of both sexes may leave their natal group.
This avoids incest. Goodall (1986) reports copulation between siblings is rare. Also see Pusey in TO.
"When it comes to having sex, a female chimpanzee isn't normally very picky. She finds most males attractive, or at least tolerable. One kind of relationship, however, stops her in her tracks. She doesn't like to mate her maternal brothers. Even when those males court elaborately, with shaking branches and rude stares and proud postures, female chimpanzees refuse their brothers.
Normally, the female's reluctance to mate with her brother marks the end of it. But occasionally a brother can't stand being denied. She resists and avoids him. He becomes enraged. He chases and, using his greater size and strength, beats her. She screams and then rushes away and hides. He finds her and attacks again. He pounds and hits and holds her down, and there's nothing she can do. Out in the woods, there's a rape."
Wrangham and Peterson (1997)
This is reflected in the success of one's genes -- the more that make it into the next generation, the more successful one is. See "kin selection" for a similar idea.
characteristics of large primates
See overhead from EHE, p.154 "Human mating patterns." This shows how unique each species is from the other. Also de Waal above/below.
social-sexual arrangements vary by species
(also see overhead from deWaal (1995) Sci. Am.)
The apes certainly illustrate this; each species has a different arrangement. Even among a given species there may be variation. For example gorillas are described as having a harem arrangement, yet in 40% another younger male--often the brother or son of the silverback-- may mate with the 6 to 8 females of the "harem."
Mating behavior itself is extremely variable; gorillas copulate infrequently with a copulation lasting 80 seconds while female bonobos mate almost continuously and engage in lots of sex activity, including homosexual, as a means of gaining food and forming alliances. DeWaal (199x) reports that male bonobos at the San Diego zoo have penile erections about 5% of the time except at feeding time when the percentage increases to over 50%. Sex is a fundamental part of that species food sharing and tension reduction behavior.
Orangs present another system altogether (though it may be seen as a wide-ranging harem in which the females are not in contact except on rare occasions when food (e.g. figs) is sufficient for a number of females and her recent offspring.)
Female chimpanzees come into estrus once a month. They have a 35-day menstrual cycle and can breed at anytime of the year. The gestation period is more than seven months long, and a single offspring is produced.
function of orgasm
Small (1993) argues "orgasm for human females is the necessary feature of our reproductive biology which evolved to take the place of estrus...anticipated pleasure for BOTH sexes is most likely an initiator to begin mating behavior. p.148"
Other proposals include the idea that orgasm facilitates conception, sucking the sperm into the egg vortex; another suggests female orgasms are epiphenomenal -- just a counterpart of the male orgasm resulting from erection and ejactulation of sperm.
on what basis do female NHPs "choose" mates?
While size, dominance, fathering skills may play a role in a given species, the evidence is sparse that this really matters in general. Small (1993) summarizes research on rhesus monkeys--the best studied primate--and reports the females prefer a variety of mates, especially outsiders, new to their group. This of course is a natural protection against excessive inbreeding. "Their interest in novelty, therefore, might be a primitive urge to find mates lurking at the periphery of their group with genes different, but not completely different from their own. p.171" Thus choice on this basis increases the genetic diversity of the species, hence its overall fitness. (Of course this may not always be a virtue if successful adaptations are LOST; cf role of birdsong dialects)
A variety of mates with a promiscuous female also ensures that enough sperm are available at the moment of ovulation, since repeated matings with the same male will not provide the same number of sperm each mating.
Small says, while these are speculative explanations, they "pull these females out of the negative light of promiscuous sex-crazed harlots into the arena of strategizing creatures trying to improve their individual reproductive success. p.182"
Small suggests the wild orangs support her idea that in general female nonhuman primates show a preference for fully mature, sexually able males by resisting efforts by younger males to mate with them. p.155
"Status, hierarchies, and rank are certainly part of our primate heritage, but unlike ourselves, nonhuman female primates seem to focus less on status then we do. A fancy car won't do a male ape much good.....p.160"
sex in our closest relatives--chimps & bonobos
Male chimps are usually unkind to females and Goodall suggests females prefer the "nicer" males.
"Bonobos have sex more often and in more combinations than most people of any culture, and most of the time bonobo sex has nothing to do with reproduction. p.175"
de Waal relates many instances of tension reduction apparently the primary function of a sexual encounter. Generally these involve genital stimulation but apparently no orgasm or ejaculation.
Attraction in humans
This has been widely studied for both sexes; there are few surprises though it is unclear as to the origin of the "erotic image" of a desirable mate. Early experience probably plays a critical role as well as some instinctual factors preferring symmetrical faces, and shapes indicating health and fertility in females and strength and fertility in males. See Pusey on female breast functions.
Much research has examined shape, size, smell and other predictors of attraction in humans. A recent study on predicting attractiveness of silouette shapes suggest the following formula predicts best:
take her volume in cubic metres and divide it by the square of her height. The researchers call the figure her volume-height index, or VHI.
Darwin discussed this issue; he thought cultural factors played a much greater role in reproduction than mere physical attraction. This may be true in some cultures with extensive arranged marriages -- but maybe not!
Buss's "evolutionary psychology" of mating
David Buss has summarized cross-cultural evidence on human mate choice and preferences. Some (not surprising) findings are:
woman value resources in mates
infidelity in women major cause of divorce
selectivity increases for both in offspring context
males value younger attractive women (maybe unlike chimp males!)