Sperm is cheap and eggs are precious, the usual story goes. That's why, in the animal world, males tend to be promiscuous and females more discerning. As a consequence, males have evolved bright colors, ornate horns, absurd tails and other ornaments to strut their stuff. But a paper in the 16 October early edition of the Proceedings of the National Academy of Sciences shows that, under certain circumstances, females can be just as eager to impress.
Evolutionary biologist Trond Amundsen of the Norwegian University of Science and Technology in Trondheim and his Swedish colleague Elisabet Forsgren of Göteborg University studied the two-spotted goby (Gobiusculus flavescens), a small fish in which the females sport a bright orange-red belly during the mating season. Males establish nests to attract females and later protect the eggs, so fatherhood is not a hit-and-run affair. Moreover, toward the end of the breeding season, males mysteriously die off, leaving the remaining males in the enviable position of choosing from many available females.
This female ornamentation led Amundsen and Forsgren to test if male gobies are indeed doing the selecting toward the end of the mating season. They put males in tanks where each could see (but not touch) two females: one with a bright red belly, the other less bright. The males directed 80% of their displays toward the red-bellied females, all but ignoring the drabber ones--the first ever proof that females evolve traits to attract males, Amundsen says. To double-check, the researchers beautified the bellies of understated females with a red felt-tip marker. When given the choice between these and untreated drab females, the males once again courted the red ones. "We were astonished that the results matched so well," says Amundsen, who hopes that their study will open a new "female beauty" chapter in sexual selection research.
The experiments are "elegant," says an impressed Iain Barber, who investigates mate choice in fish at the University of Wales in Aberystwyth. He adds that the logical next step would be to find out if redness correlates with some other female quality, like the size or number of eggs.