The calls that female chimpanzees make during mating may help to protect their offspring.

By NICHOLAS WADE

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 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 Zuberbuhler, 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 in the scientific journal PLoS One.
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 higherranking 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.
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.
Female chimpanzees have sexual swellings during their receptive period. They remain visible for several days, but chimps ovulate on 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. Zuberbuhler said. “But she doesn’t. She conceals the time of ovulation by calling throughout her cycle.