Recently, however, a study of chimps in the Tai forest, Cote d'Ivoire, found the answer. Females actively seek out mating partners from adjoining groups. In 13 cases, mother-infant pairs were analyzed for their DNA content and the results compared with DNA profiles obtained for in-group males. The results were startling: In 7 of these 1 3 cases, all the in-group males could be excluded as possible fathers! So, the baby chimps must have been fathered by males outside the mothers' group. Interestingly, all 7 of these females were known to have left their troops during their estrous period, precisely when the infants in question would have been conceived. It is also interesting that such absences are brief-in 4 cases, only one to two days and, moreover, matings with nongroup males must be exceedingly furtive: During 17 years of continuous observations, the sharp-eyed researchers saw nary a one! (Without the DNA analysis, we would have had no way of knowing about this furtive aspect of the love lives of female chimps.)
The researchers suggest that this behavior by females allows them to choose from a wider variety of potential mating partners while still retaining the resources and social support of their in-group males. Another major possibility: They gain toleration of their young when different troops inter act. Males may well say to themselves, in effect: "I remember this female, an old flame from several months ago. So maybe, just maybe, this cute little baby is my kid!"
Paternal perceptions of this sort, whether accurate or not, may turn out to be especially important for the survival of young chimps. An ethically troubling discovery-deriving initially from the pioneering research of primatologist Sarah Hrdy-has been that many animals practice infanticide. In brief, the pattern is as follows: Among polygynous species, when the harem keeping male is eventually deposed, the newly ascendant male not uncommonly embarks on a grisly policy of killing the nursing infants. Although despicable by human moral standards, such behavior makes "good" evolutionary sense, since after their youngsters are eliminated, nursing mothers quickly resume ovulating, whereupon they are likely to mate with the new harem-keeper . . . despite the fact that he murdered their offspring. Insofar as the unfortunate infants were sired by the preceding male, their fate is of no biological concern to the newly ascendant infanticidal male. He is interested in his own progeny, not someone else's.
Female langur monkeys have even evolved an interesting counterstrategy. If a female langur is in the late stages of pregnancy when a male takeover occurs, she may undergo a "pseudo-estrus," developing swollen genitals and a sexual appetite for the new harem-keeper. Then, when her offspring is born, the adult male is more likely to act paternal than infanticidal.
Among many species, including chimpanzees and numerous other primates, the danger of infanticide is not limited to the aftermath of male takeovers. It is ever-present whenever two groups meet. However, as Sarah Hrdy has pointed out, given that even hard-hearted adult males are concerned about their own progeny, it may be that by copulating with more than one male, females introduce a degree of strategic uncertainty (or even erroneous confidence) as to whether a male who has enjoyed a female's sex ual favors may accordingly have fathered her offspring. If so, then EPCs might serve as a kind of infanticide insurance, a means whereby females purchase a degree of immunity for their offspring.
Such a policy is not cheap. If purchased by chimpanzee females, it is at
the cost of substantial risks. For example, females are at risk of predation, especially by leopards, while in transit between groups. Since male chimps are larger than females and physically dominant over them, females can be injured, even killed, when approaching a strange troop. In addition, there is potential risk if home-group members (especially males) discover that a female has been mating outside the social unit, although it isn't clear exactly how they would find out, nor has such a discovery ever been documented.
Bear in mind that in addition to their occasional trysts, female chimps also typically immigrate, as adolescents or occasionally as adults, into a troop different from the one in which they were born. It may be that, at such times, their prominent sexual swellings buy chimpanzee females a crucial degree of social acceptance. (It is an interesting fact that newly immigrant females often keep their sexual swellings for an unusually long time, and when they have recently changed troops, even pregnant females often produce swellings . . . something that does not normally happen when they are less at risk.) It seems likely that estrous swellings are important "safe conduct passes" when an adult female encounters strange males.
—David P. Barash, The Myth of Monogamy, p. 94