Seminal Discovery: Promiscuous females speed sperm evolution

New evidence suggests that Mother Nature has her own means of handling cheating wives. Researchers have found that a gene responsible for semen viscosity has evolved more rapidly in primate species with promiscuous females than in monogamous species. The changes may affect which male’s sperm impregnates a female with multiple partners.

Previous studies had shown that when a species has a high rate of polyandry—the mating of one female with more than one male—a variety of adaptations have evolved that increase the odds that an individual male’s sperm meets up with an egg. For example, compared with males in monogamous species, those from many polyandrous species have larger testes capable of producing and storing more semen, and their sperm contain more mitochondria, the energy-producing organelles that power sperm’s motility.

Furthermore, the semen of many polyandrous species becomes more viscous after ejaculation than that of other primates, forming a solid plug in extreme cases. Although the plug increases the chance of fertilization by preventing semen from leaking out of the vagina, it can also act as a biological chastity belt, thwarting a rival male’s sperm from getting through.

To see whether sexual competition shows up at the molecular level, Bruce Lahn of the University of Chicago and his colleagues sequenced SEMG2, a gene that contributes to semen viscosity. The researchers chose nine primate species that represent the major mating systems. Among gorillas, for example, one female copulates with one male during a given fertile period. In chimpanzees and macaques, females have many sex partners at any time. Some other species, including humans, fall somewhere in between.

Using statistical methods, Lahn’s team tracked how fast SEMG2 had changed in each species. When the researchers compared these results with rates of female promiscuity, they found a strong correlation: The more freewheeling a species’ females, the faster SEMG2 evolved. The team published its results in the December Nature Genetics.

A Harvard University team reported a similar difference between a promiscuous and a monogamous species in another primate semen-viscosity gene in the August 2003 Journal of Molecular Evolution. According to team member Sarah Kingan, such adaptations are “fascinating, not least because of the inherent human interest in ourselves and our closest relatives.”

In the new report, the rate of human SEMG2 evolution was roughly in the middle of the rates measured for the other primates studied. Lahn notes that these results confirm the prevailing view that people aren’t as promiscuous as some other primates, nor are they strictly monogamous. Five to 10 percent of paternity tests indicate that the woman’s apparent partner isn’t the father, he says.