Females That Change Color Don't Always Have Sex on Their Minds

Getting duller.
In these three species, males (second column) retained the ancestral wing patterns (first column), whereas females (third column) now mimic toxic butterflies (fourth column).

There's no confusing male and female mallard ducks. The male's emerald head stands in stark contrast to the female's dull brown coloring. Males evolved their sheen in order to attract the opposite sex. At least that's what Biology 101 tells us. But a new study suggests that, in some species, once-colorful females were the ones to change, sometimes dulling down as a protective measure.

Charles Darwin was the first to propose sexual selection, the idea that evolution could drive males and females to look and act very differently to attract the opposite sex. But his contemporary rival, Alfred Russel Wallace, took a different tack: He argued that such differences could arise because females face different risks than males, being burdened with motherhood. He noticed that in certain swallowtail butterflies, females, but not males, had changed their coloration to mimic a toxic species, presumably because females carrying eggs were less able to make quick escapes and needed a different way to avoid being eaten. For the most part, the idea was ignored.