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Cricket Study Offers Clues to Female Promiscuity in Some Species

Hillary Mayell
for National Geographic News
January 14, 2002
 
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
relative.

"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 mating—more 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.

Avoiding Incest

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.

In most insects, sperm is passed to a female in a special packet, not simply as a fluid, explained Wedell, who is also an evolutionary biologist and co-author of the study. In crickets, the male attaches this packet to the female's genitalia. After mating, the sperm packet remains attached to the female for half an hour or so, gradually pumping sperm into her.

Whether a female simply doesn't store the sperm packets of siblings, or whether some chemical process occurs in her reproductive tract that causes the sperm to be rejected or rendered useless, remains a mystery.

"Possibly females can only tell for sure during mating that they have picked a sibling as a mate and so only then can they choose not to store sperm," said Tregenza. "Alternatively, perhaps only the intimate association between the females reproductive tract and the sperm themselves allows females to tell related from unrelated males."

How often sibling mating actually occurs in the wild is also unknown. In the laboratory, the female crickets could distinguish closely related males by smell, so the researchers aren't sure why the females don't simply avoid mating with relatives.

The sheer number of eggs laid by each female may make sibling mating in the wild an inevitable hazard. "Females typically lay around 100 eggs in three days, but they can go on for much longer," said Tregenza.

"Also," he added, "it could be that although females can avoid matings [mating requires the female to crawl onto the male's back], males continue to harass them by singing and chasing them around. So it could actually be less costly for females to give up and mate with a male than continue to resist his advances."

Implications for Other Species

Multiple mating by females occurs in nearly every species. The role of female choice in mating behavior has broad implications for understanding how species evolve, competition between males, mate choice, and why males typically produce so many millions of tiny sperm.

Tregenza and Wedell believe that whatever drives female crickets to mate with lots of different males is likely to be important in other species as well.

Because incest is a risk in many species, it is likely that crickets are not alone in their ability to use promiscuity to their advantage, said Tregenza.

"If the ability of female field crickets to avoid using sperm from related males is shared by other species, this form of genetic incompatibility avoidance may be an important factor promoting female promiscuity across taxa," the researchers concluded.
 

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