"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," said Stillwell.
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 (Manduca sexta), 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."
Stillwell and Davidowitz discovered that female caterpillars initiate this fundamental change a bit later than the males. By the time the female caterpill
|Contact: Daniel Stolte|
University of Arizona