Voyeurs put male fish off their ideal mate

Male molly fish Poecilia mexicana normally prefer to mate with large females who produce more eggs. In mate choice experiments, a male will spend 80% of its time near large females and only 20% near smaller ones.But when Martin Plath at the
University of Potsdam in Germany and colleagues stuck a glass container holding another male into the tank to let him watch the show, the first molly changed his mind. Under the gaze of the intruder, he began to pay equal attention to both large and small females. Being watched by a green swordtail (Xiphophorus helleri ), on the other hand, did not faze the mollies at all – they only slightly reduced their preference for large females."We think that the molly does this to avoid sperm competition," Plath told New Scientist. "It's likely that the other male will share the preference for large females, so it makes sense for the molly to not invest all his sperm into one female.

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