Multiple partners are good for sperm (Image: Dietmar Nill/Foto Natura/Minden/FLPA)

After only 12 generations of practising polygamy male mice are much more fertile than their monogamous brethren.

That is the conclusion of the first study to provide concrete evidence for the evolutionary theory which proposes that competition for mates will increase male fertility, says Leigh Simmons at the University of Western Australia, Australia.

House mice (Mus domesticus) can swap between being polygamous and monogamous. To see if this had an impact on their sperm quality, Simmons and his colleague Renee Firman created polygamous and monogamous breeds and compared their fertility.


For the “monogamous” mice, they paired 18 males with 18 female mice, then took two offspring from each monogamous couple – one male and one female – and bred them with the offspring of another monogamous couple. The experiment was repeated for 12 generations.

For “polygamous” mice, each female was sequentially mated with three males, and this, too, was repeated for 12 generations. At the end of breeding, all mice were the same size and weight.

Sperm success

Next came the ultimate test: would sperm from polygamous males be more successful at producing offspring than sperm from monogamous mice?

To find out, 16 females in heat were each mated with a different monogamous male, quickly followed by a polygamous male. The experiment was reversed in another group of 16 females: polygamous males were mated first. Embryos from the 32 females were then genetically tested to determine paternity.

The team found that polygamous males produced more offspring in both experiments: they fathered 76 per cent of the offspring when they mated first and 58 per cent when they mated last.

Simmons and Firman think they know why the polygamous males are more fertile. In previous experiments, they had analysed the sperm quality of the mice after only eight generations of mating. Males from the polygamous lines produced more sperm with better motility than monogamous males.

The sperm might also be getting bigger. Using previous literature Maximilian Tourmente of the National Museum of Natural Sciences in Madrid, Spain, compared the testicle mass of 226 species with their sperm length. In general, animals with larger testes had longer sperm.

Big testes, big sperm

Testes are known to be larger in polygamous animals, so Tourmente reasoned polygamous mating selects for longer sperm.

“As sperm become longer, sperm swimming velocity increases,” says Tourmente. Since sperm speed is a major factor in successful fertilisation, “this explains why it is targeted by sexual selection so efficiently”, he says.

While Simmons did not see larger sperm in his polygamous mice after eight generations, he says it could be “the next step” in their evolution. “These mice only had a few generations to adapt, perhaps in the following generations we will begin to see larger sperm,” he says.

Russell Bonduriansky, an evolutionary biologist at the University of New South Wales, Australia, says, these studies provide “strong evidence” that the appearance of sperm “evolves quite rapidly” in response to intense sperm competition.

Journal Reference: Simmons: BMC Evolutionary Biology (in press), Tourmente: BMC Evolutionary Biology, DOI: 10.1186/1471-2148-11-12