for National Geographic News
Evolutionary biologists have long wondered why females in most species
typically mate with more than one male. An experiment by two researchers
provides the first known evidence that female promiscuity is a hedge
against giving birth to the young of a sibling or close
"Our study suggests that females may benefit from promiscuity because it allows them to reduce the likelihood that their offspring will be inbred," said Tom Tregenza, an evolutionary biologist at the University of Leeds, Great Britain.
The reason males take multiple partners is easy to explain; every mating offers the opportunity for begetting additional offspring.
For females, however, promiscuity looks to be a curious choice. Mating is costly. A female can produce only a limited number of offspring in her lifetime, and bearing and raising young takes a physical toll.
In addition, the female gets plenty of sperm in one matingmore than she can use, in fact. So why seek multiple mates?
Earlier studies of adders, sand lizards, and other insects have shown that females' mating with more than one male is associated with higher egg or offspring viability.
In this study, published in the January 3 issue of the journal Nature, Tregenza and colleague Nina Wedell obtained evidence that by mating with several males, females avoid the low viability of eggs that monogamous females risk if their one mate turns out to be a relative.
Mating with a close relative enhances the possibility of genetic incompatibility, resulting in birth defects, reduced survival rates for offspring, or eggs not hatching at all.
Tregenza and Wedell found that while females might not be able to pick their mates, they were able to choose which sperm to use after mating.
In the study, female field crickets (Gryllus bimaculatus) that mated only with siblings hatched significantly fewer eggs than females who mated with non-relatives, which is what the researchers had expected.
Curiously, the females who mated with a sibling and with a non-relative produced the same number of viable eggs as a female who mated with two non-relatives. The order in which she mated with the sibling and non-sibling had no impact on egg viability.
The fact that hatching success was not affected when a cricket mated with both a close relative and a non-relative led the researchers to conclude that a female could choose which sperm to use to fertilize her eggs after mating. The exact process by which she does this is still unknown.
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