Researchers have shown that when females are more promiscuous, males have to work harder -- at the genetic level, that is. More specifically, they determined that a protein controlling semen viscosity evolves more rapidly in primate species with promiscuous females than in monogamous species. The finding demonstrates that sexual competition among males is evident at the molecular level, as well as at behavioral and physiological levels...Now this is the sort of research I can respect. Of course, it's done by biologists, not psychologists, which may explain why it's good evolutionary research.
Lahn's group studied semenogelin, a major protein in the seminal fluid that controls the viscosity of semen immediately following ejaculation. In some species of primates, it allows semen to remain quite liquid after ejaculation, but in others, semenogelin molecules chemically crosslink with one another, increasing the viscosity of semen. In some extreme cases, semenogelin's effects on viscosity are so strong that the semen becomes a solid plug in the vagina. According to Lahn, such plugs might serve as a sort of molecular "chastity belt" to prevent fertilization by the sperm of subsequent suitors, though they might also prevent semen backflow to increase the likelihood of fertilization.
Lahn and his colleagues compared the SEMG2 gene, which contains the blueprint for semenogelin, from a variety of primates. They began by sequencing the SEMG2 gene in humans, chimpanzees, pygmy chimpanzees, gorillas, orangutans, gibbons, macaques, colobus monkeys, and spider monkeys. These species were chosen because they represent all the major mating systems, including those in which one female copulates with one male in a fertile period (such as gorillas and gibbons); those in which females copulate highly promiscuously (such as chimpanzees and macaques); and those in which mating practices fall somewhere in between (such as orangutans where a female will copulate with the dominant male, but may also copulate with other males opportunistically).
"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. 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.
"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."
Thursday, November 11, 2004
Promiscuous Females = Better Semen
I'm not really a big fan of evolutionary psychology. I don't think it has anything important to say about cognition, and at least up to this point, I've been right. However, there are interesting ways to use evolution to study things like sexual behavior. You have to do good research, though. For instance, this is the sort of research that evolutionary psychologists should be doing, rather than this shit. Here's an excerpt from the article on the study (the good one):