On average, men are bigger, stronger and more aggressive than women. The behavioral and physiological differences are the result of sexual competition: Males tend to fight among themselves for females, and so tend to be bigger and stronger. The pattern holds across most species.
Here’s another tendency: If a female is promiscuous, a male is going to have to work harder if he wants to monopolize her than if she is monogamous.
If, say, a man has any intention of starting a family with a promiscuous woman, he will want to monopolize her, since — in a Darwinian sense at least — he won’t want to waste his resources raising someone else’s children.
This is Darwin’s process of sexual selection. It gives us an explanation for the profound differences in behavior and physiology between males and females.
However, a seminal discovery reported by scientists in the United States this week shows that when females are promiscuous, males have to work harder at a genetic level, too.
And this really is a seminal piece of work: The scientists determined that a protein controlling semen viscosity evolves more rapidly in primate species with promiscuous females than in monogamous species. In other words, the more free love in a primate species, the thicker the semen used to consummate that love.
In the case of some promiscuous primates such as chimps, semen can even become so viscous that it acts as a mating plug, as it does with some insects. The plug may be a kind of proteinaceous chastity belt, preventing the sperm of rival males gaining access to the eggs. Or it might simply be a plug preventing leakage.
The U.S. research was led by geneticist Bruce Lahn, of the Howard Hughes Medical Institute at the University of Chicago, who explained: “The idea is that in species with promiscuous females, there’s more selective pressure for the male to make his semen more competitive.
“It’s similar to the pressures of a competitive marketplace. In such a marketplace, competitors have to constantly change their products to make them better, to give them an edge over their rivals — whereas, in a monopoly, there’s no incentive to change.”
Lahn found evidence for this competitive change by looking at a gene called SEMG2, which contains the instructions for making semenogelin, a protein in the seminal fluid that controls the viscosity of semen immediately following ejaculation. Lahn and his team sequenced SEMG2 from humans, chimpanzees, bonobos (formerly known as pygmy chimpanzees), gorillas, orangutans, gibbons, macaques, colobus monkeys and spider monkeys. This range of species includes all the major primate mating systems.
At the monogamous end of the scale, there are gorillas and gibbons. In these species one female copulates with one male during one cyclical fertile period. Moving along the scale, we find orangutans, whose females copulate with the dominant male, but who will squeeze in a quickie with other males if they get the chance. At the promiscuous end of the scale we find chimpanzees and macaques, whose females mate with many males each breeding season. Humans fall in the middle of the scale.
It’s hard to place humans more precisely, though, because cultural practices have such a strong influence on our mating system: In some cultures, women may be effectively part of a harem, with access only to one man; in other cultures, it’s possible for women to behave as promiscuously as chimps.
The researchers measured protein-evolution rates by counting the number of amino-acid changes in the protein, then scaling it to the amount of evolutionary time taken to make those changes.
“When we plotted data on the evolution rate of the semenogelin protein against the level of female promiscuity, we saw a clear correlation whereby species with more promiscuous females showed much higher rates of protein evolution than species with more monogamous females,” said Lahn.
It is well known that chimps have bigger testes relative to their body size than humans, who themselves have relatively bigger testes than gorillas. Humans are halfway between promiscuous chimps (who will copulate frequently, going for pretty much anything with a pulse) and gorillas (who, being physically able to defend their females from other males, copulate far less frequently). But this is the first time that sexual competition has been shown to produce different genetic effects.
Lahn likened the high speed of genetic change to an “arms race” between males: “The genes have to adapt faster for any given male to gain an edge over his competitors,” he said.
Work such as this is to be welcomed. Not only do we now know that the rate of genetic change correlates with the intensity of sexual selection, but we are forcibly reminded of how we evolved. We are closely related to chimps and gorillas, and the size of male humans’ testes and the rate of evolutionary change of their semen proteins is governed by the same principles that govern theirs.
It’s easy enough to think that humans are exempt from the rest of nature; this mentality is deeply embedded in our culture — in Western culture, at least. Anything that snaps us out of this view is a good thing, even something as frankly weird and apparently obscure as the thickness of primate semen.