Intricate as the mating dance may be among people, for other primates like chimpanzees and baboons it is even more complicated. This is evident from the work of researchers who report that the distinctive calls made by female chimpanzees during sex are part of a sophisticated social calculation.
Biologists have long been puzzled by these copulation calls, which can betray the caller’s whereabouts to predators. To compensate for this hazard, the calls must confer a significant evolutionary advantage, but what?
The leading explanation involves the way female primates protect their offspring. Male chimps and baboons are prone to kill any infant they believe could not be theirs, so females try to blur paternity by mating with as many individuals as possible before each conception. A side benefit is that by arranging to have sperm from many potential fathers compete for her egg, the female creates conditions for the healthiest male to father her child.
The calls that female chimps make during sex seemed to be just part of this strategy. By advertising a liaison in progress, biologists assumed, females stood to recruit many more partners.
But the study, by Simon Townsend, Tobias Deschner and Klaus Zuberbühler, shows that in making calls or not, the females take the social situation into account.
The researchers monitored the lively love lives of seven female chimps in the Budongo Forest of Uganda, making audio recordings of nearly 300 copulations. In two-thirds of these encounters, they found, the female made no sound at all. This finding undermines the thesis that the principal purpose of copulation calls is to instigate rivalry among males, the researchers reported online Tuesday in the scientific journal PLoS One.
Unlike female baboons, who give a staccato whoop at each copulation, the chimps seem much more aware of the social context. Chimps are particularly likely to be silent and conceal their liaisons when higher-ranking females are nearby. They were most acoustically exuberant when cavorting with a high-ranking male.
The reason may be that other higher-ranking males are likely to be around, too, and by advertising her availability to them a female chimp may gain many influential protectors for her future infant.
The calculus changes when higher-ranking females are around because they are likely to attack the caller and break up the fun. To avoid incest, young females leave their home group and try to integrate with neighbors by offering themselves to socially important males. But the resident females tend to be obstructive, perhaps because they see them as competitors for male protectors and desirable feeding areas.
A similar use of copulation calls could once have existed in the human lineage but if so, it may have lost its evolutionary advantages when human societies developed their distinctive system of pair bonding and made intercourse a largely private activity.
Dorothy Cheney, an animal behavior expert at the University of Pennsylvania, said that copulation calls usually occurred in primate species where the females have visible sexual swellings during their receptive period. Because swellings do not occur in humans, it is hard to speculate about the relevance of chimp sexual calls to human behavior, Dr. Cheney said.
Though human vocalizations during intercourse have not been much studied, they do have “a quite elaborate acoustical structure, which suggests some kind of communicative function,” said Dr. Townsend, who is at the University of St. Andrews in Scotland. Copulation calls are not a feature of public life in Western societies, but the situation could be different in hunter-gatherer groups, which enjoy little privacy.
“I can imagine that these sort of signals may still be very much perceived by other group members and give a female a high degree of control over her willingness to copulate or let others know her sexual state,” said Dr. Zuberbühler, also of the University of St. Andrews.
The female primate’s strategy of blurring paternity could be useful in human societies, too, especially when the rate of illegitimacy is high. “Whether or not this happens in humans I don’t know,” he said. “I wouldn’t be surprised if something similar went on.”
Female chimpanzees have sexual swellings that remain visible for several days, but they ovulate on just one day. A female gives her copulation calls throughout the period, concealing her most fertile time from the males.
“If she was truly interested in meeting with the best males, she should do all her calling during that narrow window when it matters,” Dr. Zuberbühler said. “But she doesn’t. She conceals the time of ovulation by calling throughout her cycle.”