For females (Animals), sex can be traumatic. In some insects, insemination involves wounding females and infecting them with dangerous microbes. In many other species, mating reduces females' lifespans.
Given that in most species, a single mating is generally enough to fertilize all a female's eggs, she has little incentive to mate again. And yet in many species, females mate multiple times with different males.
Researchers at the University of East Anglia found that in inbreeding populations the female becomes more promiscuous to screen out sperm from genetically incompatible males.
In a study of flour beetles, the team found the breeding success of females in regular populations was identical, whether mating with one male or five.
However, when they conducted the same tests with an inbred population, females mating with just one male showed a 50% reduction in the number of surviving offspring produced, the Daily Mail reported. "Polyandry" - where a female's eggs are fertilized by multiple fathers - is the norm in most species, from chimpanzees to chickens, the scientists said.
The results, they said, showed that females possess mechanisms that allow them to filter in the genetically most compatible sperm to produce more viable offspring. The team then deliberately created "genetic bottlenecks" of inbred beetles and found that after 15 generations the females began to mate more frequently and with more partners. Lead researcher prof Matthew Gage said: "By generating inbred populations, we were able to create real risks of high genetic incompatibility between reproducing males and females, and expose the mechanisms that females possess to promote fertilisation by the most compatible males and their sperm.
"These exciting results show how this common but paradoxical mating pattern can evolve if females use it to avoid reproducing with genetically incompatible males."
According to Gage, how females filter the most compatible sperm is still a mystery.