Male Barbary Macaques Like Their Lovers Loud

Part of the experience of living in an apartment involves occasionally being subjected to the sounds of members of our own species mating. While the torrid love affairs of our neighbors might keep us up at night, though, there's a good reason why they do it (just as there's a good reason why there's a whole business based upon the proclivity of some men to drop loads of cash to listen to a woman pretend to have orgasms over the phone), at least if we're anything like Barbary Macaques (Macaca sylvanus). In a new study published in the Proceedings of the Royal Society B, Pfefferle et al. explain how what once was thought to be a signal of fertility turns out to be a useful behavior in encouraging male ejaculation.

Barbary macaques are promiscuous primates, and like many other primates (including our own species), they can get pretty loud during mating. The problem, however, has been determining just what the mating calls mean. In order to try and figure this out, the team of researchers collected fecal samples for hormone analysis and recorded vocalizations of a group of free-ranging, habituated barbary macaques at the Upper Rock Nature Reserve, Gibraltar. While much of the debate over the purpose of the mating calls has been based around the mating behavior, knowing when the females were most fertile would be of primary importance in testing the "fertility signaling" hypothesis, and male rank would have to be taken into account via behavioral observation as well. What the researchers found, however, contradicted the idea that the female macaques were advertising fertility.

While the hormone testing would have to wait until after the behavioral studies were made, the researchers were able to record the female vocalizations and observe the behavioral of males during mating, and a mating that resulted in ejaculation being signaled by the presence of an "ejaculatory pause" or the presence of fresh sperm (and usually both were seen). When the data from these behaviors were analyzed (and it could be ruled out that female fertility cycles and male rank influenced the calls), it became apparent that when females called during mating (86% of all copulations) the males were observed to have ejaculated 59% of the time. During the 14% of matings in which the females were relatively silent, less than 2% of those matings resulted in male ejaculation. The connection seems clear; the males are much more likely to ejaculate if they're with a vocal female than not. Could it be that the females were just calling more because of more intense copulations that were more likely to result in ejaculations anyway? The researchers kept track of male thrust rate before the onset of the female vocalizations and compared it with call intervals and found that thrust rate was influenced by call intervals and not vice versa, again showing that the females are helping stimulate the males.

Of further interest is that the female macaques don't get choosy when they're close to ovulation; they keep up the promiscuity and actually distribute their matings a little more evenly throughout the male group. This would suggest, then, that the females are confusing paternity by encouraging a larger number of males to have successful matings with them but why females would do this is still an open question. If male macaques were infanticidal the advantage of confusing paternity would be fairly straightforward, but male barbary macaques have been known to contribute to child care, even of infants that are not their own. Could the females be confusing paternity to help ensure this contribution to child care? More study is required before that can be determined, and hopefully similar studies will be carried out in other groups of primates to help determine how female mating vocalizations and consequent confusion of paternity (if the females are promiscuous during the time when they're most fertile) can change group dynamics. In baboons, for instance, the mating system is different and female vocalizations appear to lead to increased mate guarding, so female mating vocalizations are more like a social tool with various uses depending on context rather than a behavior with the same consequence every time.

Indeed, female vocalizations during mating can have different advantages or consequences in different social settings. In some cases it might allow a female to lessen the chances that an undesirable male will fertilize her (in the case of macaques over 98% of "silent" copulations did not result in male ejaculation) or females could confuse paternity by encouraging as many males as possible to ejaculate. Still, what always is niggling at my brain during such discussions is a question that we might never get a good answer to; what are these primates thinking about their own behaviors? We might be able to study what they do and identify certain consequences of certain behaviors, but do the female barbary macaques, for instance, know that they're confusing paternity (and perhaps providing a benefit of increased paternal care to their offspring)? Such questions could easily keep a naturalist up at night, but for now I will let sleeping dogs lie and say that this is certainly an interesting study that also provides a model for similar studies in other primate groups.

References;

Pfefferle, D.;Brauch, K.; Heistermann, M.; Hodges, J.K.; Fischer, J. (2007) "Female Barbary macaque (Macaca sylvanus) copulation calls do not reveal the fertile phase but influence mating outcome." Proceedings of the Royal Society B Online Tuesday, Dec. 18, 2007.