Polygamy produces more virile mice: study

Having multiple sexual partners leads mice to produce more fertile offspring, a new study has found.

The research, published in BioMed Central's open access journal BMC Evolutionary Biology, shows that sperm from polygamous mice are better competitors in the race for the egg.

Renee Firman and co-author Leigh Simmons at the Centre for Evolutionary Biology at the University of Western Australia used the naturally polygamous house mice to show that sperm from rival males compete to fertilize females.

Firman said she is assessing the potential benefits that a female gains from mating with more than one male in a single reproductive cycle (polyandry). Polyandrous behaviour creates competition between sperm of rival males.

The results indicate that over several generations, polygamy gains an advantage over monogamy by selecting for mice that produce more sperm, and sperm that are better swimmers.

"Sperm competition has also been shown to influence the evolution of testes size and efficiency," the researchers wrote.

Firman and Simmons said this is the first time anyone has demonstrated that polygamy selects for superior sperm competitiveness in a vertebrate.

12 generations

In the study, they paired 18 males and 18 females in monogamous relationships. They then took two offspring — one male and one female — from each union and bred them with offspring from another monogamous pair. The pattern was repeated for 12 generations.

A second group of polygamous females were each mated with three males. This was also repeated for 12 generations.

The researchers then competed 12th generation males against each other, to mate with a female from each group. Each female mated successively with one monogamous and one polygamous male.

They found 53 per cent of the litters were of mixed paternity, but of the remainder there was "a significant paternity bias towards males from the polygamous selection lines."

"Males from the polygamous lines gained exclusive paternity of 33 per cent of the litters, while males from the monogamous lines gained exclusive paternity of just 14 per cent of the litters," Firman and Simmons wrote.

They said their study is evidence that the expression of genes for sperm competitiveness can respond to selection, and that improved sperm quality translates to greater competitive fertilization success in mammals.

"Males from the polygamous lines may have evolved an intrinsic trait that improved zygote quality and ensured higher rates of implantation," they wrote. A zygote is a fertilized egg, and it must implant in the uterus for a normal pregnancy to take place.

Firman and Simmons said the next step will be to further assess the paternity success of the polygamous males. They will also be looking at ways to separate sperm competitiveness from other factors.