Why female moths are bigger than the males

Washington, March 12 (ANI): Researchers from the University of Arizona (UA) have discovered why female moths are bigger and more beautiful than the male species.

The researchers 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.

"The question we asked was, 'how do females and males come to be different in size?'" said Craig Stillwell, lead author of the study and a UA Center for Insect Science postdoctoral fellow in the lab of Goggy Davidowitz, an assistant professor of entomology at the UA.

Many biologists have tried to solve this puzzle over time, but when Stillwell and Davidowitz looked at the literature, they realized something was missing in the picture.

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 adult animals more likely has to do with differences in the time the two sexes spent as growing larvae.

Even in light of that, nearly all research has focused on the adult animals.

"We are the first ones to look at the larvae with this question in mind," Stillwell said.

Stillwell and Davidowitz chose the giant hawk moth, a species native to Arizona, as a model organism, mostly because this insect species is well-studied, easily bred in the lab and large enough to allow for ease of handling and measuring.

The researchers followed more than 1,200 caterpillars from the time they hatched, all the way through four molts and until they pupated.

They weighed and measured the animals at different times during development and fed the data into a complex statistical model they developed.

For most of their lives as caterpillars, females and males do not appear much different.

"The final larval stage is when it all happens," Stillwell said.

"There is a point in the caterpillar's life when an inner clock and environmental cues tell the animal it's time to become an adult. Hormonal changes make them stop feeding and wander around looking for a place to pupate. Within a few hours they develop into a pupa, from which the adult moth will emerge a few weeks later," he added.

Stillwell and Davidowitz discovered that female caterpillars initiate this fundamental change a bit later than the males.

By the time the female caterpillars pupate, they are larger, making for larger moths when they emerge. (ANI)