The Red Queen_ Sex and the Evolution of Human Nature - Matt Ridley [108]
The reason is easy to see. If a female animal mates with several males, then the sperm from each male competes to reach her eggs first; the best way for a male to bias the race in his favour is to produce more sperm and swamp the competition. (There are other ways: some male damsel flies use their penis to scoop out sperm that was there first; male dogs and Australian hopping mice both ‘lock’ their penis into the female after copulation and cannot free it for some time, thus preventing others having a go; male human beings seem to produce large numbers of defective ‘kamikaze’ sperm that form a sort of plug that closes the vaginal door to later entrants.13) As we have seen, chimpanzees live in groups where several males may share a female and therefore there is a premium on the ability to ejaculate often and voluminously, because he who does so has the best chance of being the father. This conjecture holds up across all the monkeys and across all rodents: the more they can be sure of sexual monopoly, like the gorilla can, the smaller their testes; the more they live in multi-male promiscuous groups, the larger their testes.14
It began to look as if Short had stumbled on an anatomical clue to a species’ mating system: big testicles equals polygamous females. Could it be used to predict the mating system of species that had not been studied? For example, very little is known about the societies of dolphins and whales, but a good deal is known of their anatomy, thanks to whaling. They all have enormous testicles, even allowing for their size. The testicles of a right whale weigh more than a ton, and account for two per cent of its body weight. So, given the monkey pattern, it is reasonable to predict that female whales and dolphins are mostly not monogamous, but will mate with several males. As far as is known, this is the case. The mating system of the bottlenose dolphin seems to consist of forcible ‘herding’ of fertile females by shifting coalitions of males and sometimes even the simultaneous impregnation of such a female by two males at the same time – a case of sperm competition more severe than anything in the chimpanzee world.15 Sperm whales, which live in harems like gorillas, have comparatively smaller testicles: one male has a monopoly over his harem and has no sperm-competitors.
Let us now apply this prediction to man. For an ape, man’s testicles are medium-sized – considerably bigger than a gorilla’s. Like those of a chimpanzee, human testicles are housed in a scrotum that hangs outside the body where it keeps the sperm that have already been produced cool and therefore increases their shelf-life, as it were.16 All this would seem to be evidence of sperm competition in man.
But human testicles are not nearly as large as those of chimpanzees, and there is some tentative evidence that they are not, as it were, operating on full power (i.e., that they might once have been bigger in our ancestors): sperm production per gram of tissue is unusually low in man. All in all, it seems fair to conclude that women are not highly promiscuous, which is what we expected to find.17
It is not just monkeys, apes and dolphins that have large testicles when faced with sperm competition. Birds do, too. And it is from birds that the clinching clue comes about the human mating system. Zoologists have long known that most mammals are polygamous and most birds are monogamous. This they put down to the fact that the laying of eggs gives a male bird a much earlier opportunity to help rear his children than a male mammal ever has. A male bird can busy himself with building the nest, with sharing the duties of incubation, with bringing food for the young: the only thing he cannot do is lay the eggs. This opportunity allows junior male birds to offer females a more paternal alternative than merely inseminating them, an offer that will be accepted in species that have