by SARAH FECHT
IMAGE/IFlickr/grandslam05
A 17-year-long study upends the most common evolutionary explanation of female infidelity
Infidelity is easy to explain in males. By sleeping around, a guy can potentially impregnate more females and sire more offspring than if he just had one mate. But females cheat, too, even though a woman will only be able to have roughly one baby per year no matter how many male sex partners she has had.
One leading evolutionary hypothesis suggests that a female who mates with multiple males ensures the genetic diversity and quality of her offspring; having higher-quality offspring could theoretically give her more grandchildren later. A 17-year study, published in the June issue of The American Naturalist, challenges this hypothesis.
“This is one of the most careful and most robust studies to explore whether polyandry is adaptive in females,” says Tommaso Pizzari, a University of Oxford biologist who was not involved in the research. “The answer is: not really.”
Previous studies tested the “quality” hypothesis indirectly. In socially monogamous species, researchers would compare the legitimate and illegitimate offspring of cheating females by asking: Which offspring were larger? Which lived longer? But a better way to understand why female promiscuity evolved, says Jane Reid, a biologist at the University of Aberdeen in Scotland and author on the new study, is to determine whether those illegitimate offspring actually have more babies.
Reid and her team studied the isolated population of wild song sparrows on Mandarte Island in Canada. Like their mainland counterparts, these birds are socially monogamous. Males and females pair up for an entire breeding season or for several breeding seasons, and work together to feed the hatchlings and defend the nest. But they’re not always faithful; blood tests showed that in this particular population, 28 percent of hatchlings were fathered by other males.
Scientific American for more