From Among Whales, by Roger Payne

From Among Whales, by Roger Payne

Primates are a group of closely related species for which the social systems of many species are well understood and for which the ratio of testes size to body weight is also well known. For example, chimpanzees have a multiple ­male mating system. When she comes into estrus, a female chimpanzee mates with many males. Although the bodies of chimps are smaller than

most human bodies, male chimpanzees have larger testes than we do. 

Goril­las have a single-male mating system. Once he has fought his way to owner­ship of a troupe of females, a silver-back male has sole access to females in his group. Even though they are much larger than chimpanzees, male gorillas have testes so small that during a dissection the testes are difficult to locate.

    When the ratio is plotted of testes weight to body weight for the thirty- three primate species for which both the social systems and these ratios are known, one gets a cloud of points. When a median line is fitted through the cloud, the species above the line all have large testes in relation to their body size, and the species below small testes in relation to their body size.²⁸ 

A fas­cinating fact emerges: the primate species above the line have multiple-male mating systems, while those below it have single-male mating systems. Apparently the mating system determines how large a male’s testes will be relative to his body size. The ratio of testes size to body weight thus provides a valuable clue to the breeding systems of primate species whose mating sys­tems are not known.

    

Potentially, this result has social significance for humans, for we are pri­mates and therefore our testes size should tell us which social system our ancestors favored: polyandry, in which a woman has several mates (the multiple-male mating system); or polygyny, in which a man has several mates (the single-male mating system). It turns out that in humans the ratio of testes size to body weight is neither high nor low. The human ratio lies prac­tically on the line separating polyandrous mating systems from polygynous mating systems. How fascinating: it means that our species never made a clear choice of mating system that would have placed us unambiguously in one of the two systems. Given the shattering effects wrought on our lives throughout human history by this ambiguity, it is interesting that we still seem so undecided.

Among Whales

Payne, Roger(Book - 1995 )