Fish in the dark still size up mates.

Female fish living in a cave still prefer a mate with a nice, big body, even though it's too dark to see him.

In plenty of species, females choose large males, so that preference in the Atlantic mollie (Poecilia mexicana) comes as no surprise. What interested Martin Plath of Hamburg University in Germany and his colleagues was what happens when muffles adapt to life in a pitch-black cave.

The researchers collected Atlantic mollies from water in a cave, at the cave entrance, and in a portion of the river outside of the cave. Researchers raised offspring of these fish for lab tests. Females then got a chance to evaluate two males, one larger than the other.

When there was plenty of light for the fish to see one another, females from all three locations tended to hang around more with the larger male. However, when the researchers repeated the tests in darkness, only the cave dwellers still showed the size preference. In adapting to cave life, the fish evidently had come to rely on some nonvisual sensory system--probably their lateral lines, which detect slight water displacement--to make their choices, say Plath and his coworkers. Their results appear in an upcoming issue of Behavioral Ecology and Sociobiology.

--S. M.