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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