MONKEY SEX: PAYING FOR IT, MASTURBATION, HOMOSEXUALITY

PRIMATE SEX ORGANS

blue testicles of the male vervet monkey

Male primates have a low-hanging penis and testes that descend into a scrotum. According to Encyclopedia Britannica: The functions of the individual organs of reproductive systems are fairly uniform throughout the primates, but, in spite of this physiological homology, there is a remarkable degree of variation in minor detail of organs between groups—particularly in the external genitalia. [Source: Encyclopedia Britannica]

The 19th-century zoologist George Mivart described primate penis as “pendulous” and the testes as “scrotal.” In contrast to most other mammals the primate penis is not attached to the abdominal wall but hangs free. The testes, with a few exceptions among the lemurs, in which they are withdrawn seasonally, lie permanently in the scrotal sac, to which they migrate from their intra-abdominal position some time before birth (in humans) or after birth (in nonhuman primates). In all primates except modern humans, tarsiers, and some South American monkeys, the penis contains a small bone called the baculum, a typically mammalian character. The uterus of female primates shows all grades of transition between the two-horned (bicornuate) uterus, typical of most mammals, to the single-chambered (simplex) uterus of the higher primates and humans.

Variations between primate taxa are demonstrated most strikingly by the glans penis, scrotum, and perineum of the male and by the clitoris and labial folds of the female vulva. In the clitoris, there is in most primates a small bone, the baubellum, homologous with the baculum of the penis. The length and form of the clitoris, which when elongated mimics the penis (as in spider monkeys, for instance), are a potent source of confusion in determining the sex of certain New World primates. The coloration of the male scrotum in forest-living primates, particularly of the guenon (genus Cercopithecus) and in drills and mandrills (genus Mandrillus), shows an infinite range of variations and provides a species-recognition signal of considerable effectiveness.

The external appearance of the genitalia undergoes seasonal variation in a number of primates. In the male, swellings of the testes and colour changes of the scrotum occur, and, in the female, swelling and coloration of the vulva and perineal region herald ovulation, sometimes most obtrusively. Turgidity and excessive vascularity of the tissues of the perineum are probably characteristic of all mammals, but there are certain primate species in which this engorgement reaches monstrous proportions, notably baboons, mangabeys, some macaques, and chimpanzees.

Regions other than the primary sex organs may also be affected by hormones circulating at certain periods of the reproductive cycle. For instance, in the gelada (Theropithecus), the skin on the front of the female chest, which normally bears a string of caruncles resembling the beads of a necklace, becomes engorged and brightly coloured. A German zoologist, Wolfgang Wickler, has suggested that this is a form of sexual mimicry, the chest mimicking the perineal region. The observation that geladas spend many hours a day feeding in a sitting posture provides a feasible, Darwinian explanation of this curious physiological adaptation.

Monkeys Get Their Rock Off with Rocks

rainbow rump of a "fatted" dominant make mandrill

In a study published online in August 2022 in the journal Ethology, scientists how monkeys in Indonesia engage in "self-directed tool-assisted masturbation" — getting their rocks off so to speak — by using actual rocks, supporting what's known as the sex toy hypothesis. Patrick Pester wrote in Live Science: Researchers studying long-tailed macaques (Macaca fascicularis) found that the monkeys repetitively tapped and rubbed their genitals with stones to pleasure themselves.[Source: Patrick Pester, Live Science, August 19, 2022]

The team found that males and females of different age groups all used stones to play with themselves, but there was some variation among the groups: Female monkeys were pickier about the stones they used, while young males engaged in the activity the most. Researchers watching the monkeys usually didn't have to wait long to observe the behavior. "You do see this genital stone tapping and rubbing quite regularly," lead author Camilla Cenni, a doctoral candidate at the University of Lethbridge in Alberta, Canada, told Live Science. "They are not, of course, constantly doing it, but if you stop and see them and they start playing with stones, they are likely going to do it."

Some macaque populations regularly manipulate stones as part of their behavioral repertoire, seemingly as a form of play. They carry stones around, rub them on surfaces and bash them together. This stone manipulation is likely cultural, because its only seen in certain populations, Cenni said. The "self-directed tool-assisted masturbation" described in the new study likely stems from this wider stone use. However, it's been documented in only one population of macaques in Bali, Indonesia. "When we talk about tool use in animals, we normally think about survival-dependent instances," Cenni said. For example, chimpanzees (Pan troglodytes) use stones to crack nuts so they can eat them. "There is an increasing number of studies that are suggesting that using objects as tools doesn't have to be a matter of survival. This is clearly an example."

The new research builds on a study led by Cenni and published in the journal Physiology & Behavior in 2020. The study first proposed the sex toy hypothesis in male macaques, while the new research looked at males and females and their potential motivation. Young males spent significantly more time engaging in the activity than mature males did. Mature males, in fact, were the least partial to stone masturbation, possibly because they had access to mature females. However, there was a lot of variation among individual macaques of both sexes. "Within those groups, you have monkeys that do it way more than others," Cenni said.

The monkeys were urban-dwelling macaques living in and around the Sacred Monkey Forest Sanctuary in the town of Ubud. They are free-roaming and fed by people. The researchers suggested that the feeding might relax pressures on the monkeys to forage, leading them to engage more in the stone behaviors. In other words, they have more time on their hands than other monkeys, and they choose to spend it with the stones.

Male Monkeys Pay for Sex

According to the paper, "Payment for Sex in a Macaque Mating Market," published in the December 2008 issue of Animal Behavior male crab-eating macaques in Indonesia "paid" for sexual access to females — and that the going rate for such access dwindled as the number of available females went up. [Source: Krista Mahr, Time, January 7, 2008]

AFP reported: “Selling sex is said to be humankind's oldest profession but it may have deep evolutionary roots, according to a study into our primate cousins which found that male macaques pay for intercourse by using grooming as a currency. Michael Gumert of Nanyang Technological University in Singapore made the discovery in a 20-month investigation into 50 long-tailed macaques in Kalimantan Tengah, Indonesia, New Scientist reported.

On average, females had sex 1.5 times per hour. But this rate jumped to 3.5 times per hour immediately after the female had been groomed by a male -- and her partner of choice was likely to be the hunky monkey that did the grooming. Market forces also acted on the value of the transaction. If there were several females in the area, the cost of buying sex would drop dramatically -- a male could "buy" a female for just eight minutes of nit-picking. But if there were no females around, he would have to groom for up to 16 minutes before sex was offered.

Gumert told the New York Times that about 89 percent of the male-grooming-female episodes observed “were directed at towards sexually active females. When males groomed these females it appears they expected sex or at least genital inspection. By contrast when mothers groomed their young it appeared they did so to clean and sooth them.

Gumert told the New York Times, “What led me to think of grooming as a form of payment was seeing how it changed across different market conditions. When there were fewer females around, the male would groom longer, and hwn there were lots of females, the grooming times whet down. Males also grooomed females of high-rank considerably longer than ones of lower rank.

The work supports the theory that biological market forces can explain social behaviour, the British weekly says. "There is a very well-known mix of economic and mating markets in the human species itself," said Ronald Noe of France's University of Strasbourg. "There are many examples of rich old men getting young attractive ladies."

Homosexuality Among Monkeys

Paul Vasey, a researcher at the University of Lethbridge in Alberta, Canada, has spent many years studying homosexuality among Japanese macaques. The Economist reported: “Female macaques often form homosexual consortships. These are temporary but exclusive relationships that involve frequent sexual activity. Females in a consortship will mount each other tens or hundreds of times. In one group that Dr Vasey observed, females mounted each other as often as once every two minutes. Yet his observations suggest these consortships serve no adaptive function. He has spent many years testing hypotheses that might explain the behaviour, such as alliance forming, the relief of social tension and the communication of dominance. There is, he says, not a shred of evidence for any of them. Female mounting behaviour may have evolutionary roots, but he reckons the reason for it now is sexual gratification. That gratification is involved is known because when a female mounts another female she thrusts her pelvis against the mountee and masturbates her clitoris using her tail. [Source: The Economist , February 22, 2003]

“This activity, of course, excludes males. In one study, Dr Vasey found that when male monkeys courted a female involved in a homosexual consortship, 95 percent of such females rebuffed him and chose to remain with their girlfriend. This suggests, he says, that it is not simply males who are competing for sexual partners, as Darwin's theory predicts, but both males and females. And homosexual behaviour is documented in at least 15 other species, including Canada geese, gorillas, chimpanzees and humans.

”Such examples may not be enough to topple sexual selection, and it is likely that this part of Darwin's theory does indeed hold good for many species. But as Dr Roughgarden warns supporters of that theory, although any one of these problems with it might be overlooked, the “sheer number of difficulties is hard to deny. If these are not enough to falsify sexual-selection theory, then what would be?” Sex, it seems, has come a long way since Darwin.”

Image Sources: Wikimedia Commons

Text Sources: Animal Diversity Web animaldiversity.org ; National Geographic, Live Science, Natural History magazine, David Attenborough books, New York Times, Washington Post, Los Angeles Times, Smithsonian magazine, Discover magazine, The New Yorker, Time, Reuters, Associated Press, AFP, Lonely Planet Guides, Wikipedia, The Guardian, Top Secret Animal Attack Files website and various books and other publications.

Last updated December 2024