Context is everything, it seems, even when the subject is the mating habits of the small fish known as the Atlantic molly.

As described in Current Biology, male mollies choose one female if they are alone, but alter their choice to a less-preferable female if another male is around. Because males are known to copy the mate choices of other males, the behavior suggests that the fish try to deceive rivals by steering them away from their preferred mates.

Martin Plath, of the University of Potsdam in Germany and the University of Oklahoma, and colleagues uncovered the misleading behavior as part of experiments to see whether a fish’s social environment affects mating preferences, which are an important element of sexual selection.

“We weren’t preoccupied with the idea that we’d find deception,” Dr. Plath said. “But we found that the mere presence of a rival has a very strong effect.”

Credit Chris Gash

In the experiments, males initially chose larger females over smaller ones, and females of the same species over those of a related asexual species, the Amazon molly. But when a second male was put into the tank (inside a clear cylinder to avoid contact), the first male switched mates.

The effect was so strong — almost all the males switched, and interacted sexually with the alternate females — that it seemed unlikely that the preferred mates had suddenly become less attractive, the researchers said.

Such misleading behavior can offer advantages by reducing sperm competition with other males. But in this case the deception may also play a role in maintaining a balance between Atlantic and Amazon mollies.

Although Amazon mollies are asexual, they rely on insemination by Atlantic mollies to trigger embryogenesis, which results in female-only offspring.

Because no Atlantic molly genes are passed on, there would seem to be no advantage to an Atlantic molly to mate with the other species. But without such mating, the asexual fish would eventually die out. So by deceiving rivals, the male Atlantic mollies may ensure that some mating with Amazon mollies occurs.

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