Fish lie

Males don’t show their true preferences in front of rivals

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Science now reveals a fish dating scene worse than junior high school.

That fish over there chasing the small female — he probably doesn’t really like her. He’s just acting that way because another guy’s watching.

This scenario plays out among the small silvery-gray fish called Atlantic mollies (Poecilia mexicana), says Martin Plath of the University of Potsdam in Germany and the University of Oklahoma in Norman. A male molly tends to switch his mating preferences to the opposite of his usual ones when a rival male molly shows up, Plath and his colleagues report in the Aug. 5 Current Biology.

When a male molly pursues a female, other males tend to chase her, too, Plath says. Thus he and his colleagues propose that switching preferences in front of a rival could deflect the competition’s interest to a less desirable female.

The finding adds to a growing body of evidence that social environment affects mating preferences, Plath says, adding another layer of complexity to the study of sexual choices and evolution.

Or it could be that mollies offer a rare example of documented deception among fish, Plath says. “You expect it among ravens,” he says, since the cognitively advanced birds trick observers while hiding food. “The funny thing here is that it’s a small gray fish.”

The fake mate preferences sound plausible, says theoretical ecologist Stephen Ellner of CornellUniversity, who has written about the potential for lying among animals. “Why not try to deceive a possible competitor?”

To test male molly preference behavior, researchers first put a male in a tank with two females. A male molly doesn’t do anything Plath would call courtship. He just swims up to a female, nips at her rear and then swims alongside her while inserting an organ that transfers sperm in barely a second.

When researchers presented males with females of two sizes, 90 percent of the time the males chose the larger female for the first sexual gesture. This preference made sense to researchers: Molly females give live birth about once a month and larger females have more babies than smaller ones.

The researchers repeated the experiment, this time adding a second male, caged in a Plexiglas cylinder, to half the tanks. In tanks with two males, the free-swimming one directed his attention first to the smaller female 90 percent of the time. In tanks without the observer, the solitary male displayed his original preference.

Likewise, the same patterns emerged when researchers repeated the experiments, only this time letting a male choose between a female of his own or another species. The other species, an Amazon molly, is an all-female species that needs the sperm of other species’ males only to stimulate embryo formation — a bad deal for the male Atlantic mollies. Without a rival observing, the males chased their own species first. But when the rival appeared, most often the male mollies chased after the Amazons first.

“It would have made this paper stronger to verify that the blatantly ‘wrong’ choices that the focal male was making had an impact on subsequent behavior by the male who was watching,” Ellner says.

Another question Plath, himself, brings up. “There is ample evidence animals lie,” he says. What’s harder to explain is why they don’t.

Susan Milius

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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