How do we know this? Unlike physical artifacts or skeletal remains, human social behavior like mating does not leave fossil records. So how do we know that women have not been sexually exclusive to their husbands throughout human evolutionary history? It turns out that clear evidence of women’s promiscuity has been left on the man’s body, his genitals, to be precise.

The first piece of evidence is the relative size of the testes. Across species, the more promiscuous the females are, the larger the size of the testes relative to the male’s body weight. This is because when a female copulates with multiple males within a short period of time -- in other words, when she is promiscuous -- sperm from different males must compete with each other to reach the egg to inseminate it. This process is known as “sperm .” One good way to outcompete others is to outnumber them. Male gorillas, whose females live in a harem tightly controlled by one silverback male and therefore do not have many opportunities for extra-pair copulations (“affairs”), have relatively small testes (0.02% of body weight) and produce a very small number of sperm per ejaculate (50 million). They don’t have to produce a lot of sperm to impregnate their females because their sperm are not likely competing with anyone else’s.

On the other extreme, male chimpanzees, whose females are highly promiscuous and do not attach themselves to any single male (in other words, they don’t have “ ”) have relatively large testes (0.3% of body weight) and produce a very large number of sperm per ejaculate (600 million). So, compared to gorillas, chimpanzees have testes 15 times as large, and produce 12 times as many sperm per ejaculate. They have to, if they have any hope of outnumbering sperm from other males and inseminating the egg before them.

On this scale, humans lie somewhere between the gorilla and the chimpanzee, but closer to the former than the latter. Men’s testes are about 0.04% to 0.08% of their body weight, and the approximate number of sperm per ejaculate is 250 million. So women have been more promiscuous than gorilla females in their evolutionary history, but not nearly as promiscuous as chimpanzee females. The evidence of women’s promiscuity throughout evolutionary history is in the relative size of men’s testicles. Men would not have such large testicles and produce so many sperm per ejaculate had women not been so promiscuous. But then, their testicles would have been much larger and they would have produced even more sperm per ejaculate had women been more promiscuous.

But enough about the testicles. In the next post, I will talk about the penis, and what men do with it.