Imagine a naked man standing next to a chimpanzee, a gorilla and an orangutan. Which one has the biggest testes? Most people would say the gorilla, and most people would be wrong. The largest ape, at 350 - 450 pounds, has testicles that weigh, together, only a bit more than an ounce. Humans and orangutans, although lighter than gorillas, are slightly better endowed, with 1.5 ounces. Surprisingly, the smallest male in our line-up, the 100 - pound chimpanzee, has over 4 ounces worth of testicles.
For decades, scientists puzzled over these differences in the apes' sexual anatomy. But the answers eluded them because there was so little information on the natural behavior of the animals.
By the 1970's, however, field biologists observing wild primates were revealing the secrets of their subjects' sexual lives. Professor Alexander Harcourt, now in the Dept. of Anthropology at UCDavis, was one of those biologists, studying wild gorillas with the late Dian Fossey. Since then, Harcourt has collaborated with other researchers to understand how an animal's sexual anatomy relates to its sexual behavior and overall mating system - equivalent to humans' marriage system. The result of these studies is now known as sperm competition theory.
In a nutshell, it goes like this: males of a species will have large testicles when they need to make lots of sperm in order to fertilize females. And they will need to make lots of sperm when other males are mating at the same time with the same females. Think of fertilization (and hence, siring offspring) as a lottery competition. Large testicles allow a male to enter more tickets (sperm cells) into the lottery and therefore give him a better crack at the prize. But not all mating systems are lottery competitions.
Take, for example, gorillas and chimpanzees. Gorillas live in cohesive groups usually consisting of one fully adult male, two to three adult females and their offspring. When a gorilla female is ready to mate, normally only one adult male is there as a partner. In groups with more than one, the alpha male can often monopolize females. The 400-pound male gorilla can therefore afford to have relatively tiny testes (relative to his body size) because the only sperm racing for the female's ovum will be his own.
Compare this with the chimpanzee that lives in loosely structured multi-male, multi-female communities. When a female chimpanzee comes into estrus, she is often attended by a host of males, many or most of whom will mate with her on the same day. That means lots of sperm from different individuals all competing to be the first one to reach the egg. The male with the biggest testicles will produce the most sperm. He has 'bought the most lottery tickets'. And so he is most likely to fertilize the female.
To expand the comparison beyond the apes, Harcourt and his colleagues examined the data on testicle size and mating system for a wide variety of primates, from lemurs to baboons to humans. The results are dramatic. In most species classified as multi-male, that is, where a female has more than one partner during any one given estrous period (equivalent to ovulation), males' testicles are larger than you would predict from their body size. In contrast, single-male systems, that is, monogamous primates, or those where just one male defends a harem of females, such as the gorilla, have testicles that are either smaller than you would predict from body size, or right on the line. I know what you're wondering, and the answer is: humans fall right on the line, suggesting that sperm competition has played a minor role in the evolution of our sexual anatomy and behavior.
The relation is so clear that if you knew the typical body size of a species, along with the size of its testicles, you could make a pretty good guess at its mating system, even if you had no data from the wild. Furthermore, researchers stimulated by the primate work have found the relation in other groups of animals including birds and butterflies.
But there are many questions about the evolution of sexual equipment
and behavior that remain unanswered. We are a long way from understanding
the penis, for example, and the ways in which its size and shape
relate to mating systems. It is likely that females hold many
of the answers, says Professor Harcourt. The influence of females'
behavior and sexual anatomy on those of males is a fertile field
for study of sperm competition and its role in evolution.
Dept. of Anthropology, University of California, Davis, CA 95616.
e-mail: kjstewart@ucdavis.edu