Male guppies are beautiful, but what female could be satisfied with just one?

Pisces Swingers Bear Better Babies

A wandering eye appears to reward female guppies with more talented offspring, according to a new study. Having multiple sexual partners is worth all the trouble--although it's still not clear how the fish manage to improve their reproductive success.

Males have an obvious motive for mating with many females: They spread their genes far and wide by fathering as many young as possible. Although females of many species mate with multiple males, the benefits aren't as obvious: Researchers generally assumed that one male should provide enough sperm to fertilize plenty of eggs. And promiscuity has its drawbacks--all that time spent mating could be better spent feeding or avoiding predators, it seems. To investigate why female guppies still entertain multiple suitors despite the dangers, Jonathan Evans and Anne Magurran of the University of St. Andrews in Scotland played matchmaker with 76 females.

Promiscuity paid off big time. On average, females mated to four different males gave birth to 73% more young and had 20% shorter gestation times than females who mated with only a single male repeatedly, the researchers report in the 22 August issue of the Proceedings of the National Academy of Sciences. In addition, the young from females who had multiple partners were more adept at schooling (swimming in tandem with another fish) and escaping from trouble, critical survival skills for a young fish.

The authors speculate that females who mate with many males have a better shot at finding ones with larger sperm counts--perhaps one male doesn't always provide enough sperm to fertilize all the available eggs after all. They also suggest that the female may control her gestation period; after mating with her first beau, she may delay fertilization in the hope that something better comes along.

The differences in offspring from singly and multiply mated females are "astonishing," says evolutionary and behavioral biologist Anne Houde of Lake Forest College in Illinois. But she cautions that it remains to be seen whether multiple mating has the same benefits in the wild, where other mating behaviors come into play.

Related sites
Fish Behaviour and Biodiversity Group, University of St. Andrews