The behavior, which offers some of the first evidence that selecting a desirable partner is energetically costly for females, is surprising because it runs against conventional scientific wisdom.

Researchers often assume that choosiness demands comparatively little exertion from females. While males put a great deal of effort into courtship -- in the iguanas' case, they sidle up to the females, hunch their backs and walk sideways while rocking back and forth -- all the females have to do is amble into the males' territory, watch this flamboyant behavior and choose the male they want, while simply avoiding other suitors.

However, avoiding energetic but unwanted males is itself exhausting, said lead researcher Maren Vitousek, and it often results in smaller eggs whose young might be less likely to survive. The team found that over a month-long mating period, particularly choosy females lost as much as 25 percent of their body weight -- far more than the 14 percent lost by the average female. The longer they spent in the company of the attractive males, the more weight they lost. (Males lost 17 percent of their weight on average during the same period.)

Vitousek said that choosiness may also decrease a female's chances for survival. During the dry weather of El Niño years, marine iguanas have a hard time finding food, and those that start the season at a low body weight are less likely to live through it.

Why all this effort on the part of the females is worthwhile is unclear, because male iguanas do not seem to offer females anything other than fertilization for their eggs.

"It's possible that attractive males provide females with better genes, or perhaps other benefits that we have not yet identified," said Vitousek, a graduate student in Princeton's Department of Ecology and Evolutionary Biology. "We're certain they're not getting food, protection or better sunning spots from the males. Understanding the costs of being choosy should help to illuminate the process of sexual selection, which is one of the primary forces driving evolution."-Princeton University