Back in Hamburg, the scientists tested the preferences of the cave females in darkness by watching them by video camera in infrared light. In some species of fish, the females make obvious signals that they prefer or disdain particular males, but the Atlantic mollies show their preferences by swimming longer near larger males and spending less time around smaller prospects.
Scientists suspect the cave mollies are using a sensory organ known as the lateral line -- a line of pores along the side that, like eardrums, can sense pressure changes -- to find the bigger males. Fish normally use the lateral line to detect objects, and all mollies have such a lateral line.
But scientists found that females from sunny streams were able to use their eyes only to assess which males were bigger and that those females were unable to use their lateral lines to detect the bigger males when given a choice in darkness.
While the cave females have had to evolve a novel way of detecting large mates, they enjoy an advantage in mating with the lights out.
Standing in front of a tank full of mollies from a surface stream population, Plath describes how these zippy males never stop trying to mate with females. Pointing to a large male darting madly after a female that is dashing away, he says, "That's a nip and another nip," referring to the repeated movement of the male's head toward the female in an effort to taste the female's chemical signals.
"There!" he says, pointing out a brief instant when a male swimming alongside a female tried to mate, an action so rapid that it was visible only to well-trained eyes.
"If you don't separate males and females, the females just die," Plath said. "Males constantly harass them. It's copulating, copulating, nothing but copulating and the females can't feed."
But life in caves has calmed the molly males, which do not harass females and are overall a more sluggish bunch than their light-dwelling counterparts. The reason, researchers say, is probably that in the dark, oxygen-poor waters of the cave, simply finding a female is an energy-intensive task, making harassment on top of that too costly.
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