Variety really is the spice of life, and not just for people. Two new studies, one appearing in Nature and the other in Animal Behavior, show that both male and female guppies like to mate with new partners if they can--and have ways of recognizing, and ignoring, potential mates they have encountered before.
Most biologists assume that for many male animals, it pays to be promiscuous: Because they have a bountiful supply of sperm, every new sex partner increases their chances of having offspring. To test whether male guppies actively try to increase their number of partners, behavioral ecologist Jennifer Kelley of the University of St. Andrews in the United Kingdom and her colleagues collected schools of guppies from three rivers in Trinidad, one of which had such low flow that the fish had become trapped in large pools; they also used fish that had lived in aquariums for 6 weeks. The researchers put males from each group into a tank with females from their own environment or with new acquaintances. In each experiment, they monitored courtship behavior for 10 minutes.
Males that had gotten to know their female companions in the lab aquaria or in natural pools showed a significant preference for fresh mates, Kelley's team reports in this week's Nature. But males from free-flowing rivers courted new females and the ones that had been collected from the same river almost as enthusiastically. The experiment implies that male guppies not only like variety, but also can tell females apart, even though they appear to look all the same, says Kelley. Perhaps male guppies notice subtle differences, she says, or recognize females from chemical cues.
"It's an elegant experiment," says Kimberly Hughes, a population geneticist at Arizona State University West in Phoenix. "There hasn't been much work at all showing that males might prefer novel females." In this month's Animal Behavior, Hughes reports that the opposite is also true: Female guppies prefer mating with novel males as well. That result is harder to explain, she notes, because females have only so many eggs and need only one male to fertilize them. Perhaps the female's desire for novelty "is a mechanism for maintaining a lot of genetic variation," says Hughes, which can be quite useful over the long run for a species, even if it doesn't do the female herself much good.