miércoles, 18 de junio de 2008

Chimp’s Sex Calls May Reflect Calculation

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.”

Study: Chimps calm each other with hugs, kisses

WASHINGTON (AP) — For most folks, a nice hug and some sympathy can help a bit after we get pushed around. Turns out, chimpanzees use hugs and kisses the same way. And it works. Researchers studying people's closest genetic relatives found that stress was reduced in chimps that were victims of aggression if a third chimp stepped in to offer consolation.

"Consolation usually took the form of a kiss or embrace," said Dr. Orlaith N. Fraser of the Research Center in Evolutionary Anthropology and Paleoecology at Liverpool John Moores University in England.

"This is particularly interesting," she said, because this behavior is rarely seen other than after a conflict.

"If a kiss was used, the consoler would press his or her open mouth against the recipient's body, usually on the top of the head or their back. An embrace consisted of the consoler wrapping one or both arms around the recipient."

The result was a reduction of stress behavior such as scratching or self-grooming by the victim of aggression, Fraser and colleagues report in Tuesday's edition of Proceedings of the National Academy of Sciences.

Dr. Frans de Waal of the Yerkes Primate Center at Emory University in Atlanta said the study is important because it shows the relationship between consolation and stress reduction. Previous researchers have claimed that consolation had no effect on stress, said de Waal, who was not part of Fraser's research team.

"This study removes doubt that consolation really does what the term suggests: provide relief to distressed parties after conflict. The evidence is compelling and makes it likely that consolation behavior is an expression of empathy," de Waal said.

De Waal suggested that this evidence of empathy in apes is "perhaps equivalent to what in human children is called 'sympathetic concern.'"

That behavior in children includes touching and hugging of distressed family members and "is in fact identical to that of apes, and so the comparison is not far-fetched," he said.

While chimps show this empathy, monkeys do not, he added.

There is also suggestive evidence of such behavior in large-brained birds and dogs, said Fraser, but it has not yet been shown that it reduces stress levels in those animals.

Previous research on conflict among chimps concentrated on cases where there is reconciliation between victim and aggressor, with little attention to intervention by a third party.

Fraser and colleagues studied a group of chimps at the Chester Zoo in England from January 2005 to September 2006, recording instances of aggression such as a bite, hit, rush, trample, chase or threat.

The results show that "chimpanzees calm distressed recipients of aggression by consoling them with a friendly gesture," Fraser said.

Consolation was most likely to occur between chimpanzees who already had valuable relationships, she added.

The research was supported by the Leakey Trust.