My Fishy Valentine

For many species, the perfect mate is a symmetrical mate

• Hannah Schardt
• Feb 01, 2006

IT SEEMS that some fish females agree with what many human females have known for years: The qualities they find appealing in mates while young do not necessarily remain so during middle age--and that includes looks. A new study by Ohio University researchers found that female swordtail fish begin to favor males with asymmetrical markings as they grow older. Most studies of mate preferences among all species--including humans--have found that females tend to prefer mates with symmetrical features. But when biologist Molly Morris noticed that the swordtails she was studying showed, on average, no preference for either symmetrical or asymmetrical markings, she decided to take a closer look.

She found that, in fact, female swordtails show a strong early preference for symmetry, then switch mid-life to prefer imperfect mates. Morris says that once she thought about it, preference for asymmetry was not entirely surprising. "We think of females as preferring symmetry so that they can get better mates," she says. "But it's not just good genes that matter. Maybe symmetrical males are easier to see and so they attract more predators. Or maybe symmetrical males are more popular so they don't have as much sperm left."

Or maybe asymmetrical males are just better conversationalists. Whatever the reason, Morris says that the phenomenon of changing mate preferences mid-life is unlikely to be limited to swordtails. "This idea that females switch preferences is starting to be found in all types of organisms," she says.