Why female moths are big and beautiful.

• Position: Insect Mating

Take a look around in the animal world and you will find that, in most organisms, individuals of one sex are larger than the other of the species. Even though evolutionary biologists long have recognized this discrepancy, called sexual dimorphism, they have struggled for decades to solve a major paradox: How can males and females of one species be of different sizes, given that they share the same genetic blueprints dictating their development and growth? Researchers have discovered that the key to unraveling this mystery lies in the early developmental stages during which the sexes begin to grow apart, and that females can respond to selection on size almost twice as fast as can males.

"In mammals, males usually tend to be larger because there is an advantage in being bigger and stronger when it comes to fighting over who gets the female," explains Craig Stillwell, a postdoctoral fellow at the Center for Insect Science and lead author of the study published in Proceedings of the Royal Society of London Series B.

"In most arthropods, on the other hand, we find the opposite: females are bigger than males. Think of spiders, for example. In some species, the female can be hundreds of times larger than the male. Since there is no difference--at least that we know of--between the male and female genes controlling growth, nobody could figure out why we see what we see in nature: differently sized males and females."

Scientists have known that growth rates do not differ between female and male caterpillars and thus cannot account for the observed size difference. Rather, the sexual dimorphism observed in the adults more likely has to do with differences in the time the two sexes spent as growing larvae.

Stillwell chose to observe the giant...