To everything there is a season. Even the sexual proclivities of butterflies.
Yale researchers have found that male butterflies do not always take the lead in courting females at mating times. In some instances it’s the females that open the negotiations, and, curiously, the deciding factor seems to be the conditions in which they grew up as larvae—whether it was the dry, cool season, or the wet and warm season.
When certain caterpillars are raised in warm, moist conditions they grow into what some would consider traditional roles — males pursuing demure females. But new research has found that when they are raised in dry, cool conditions, it’s the ladies that become aggressive adults, actively courting the guys. [AP]
Among squinting bush brown butterflies, the species tracked in this study, both male and female sport what look like eye spots on their wings. The white spots in the center (the pupil of the eye) reflect light in the ultraviolet range, which appears to be the key to the butterflies‘ mating behavior.
“Cool temperatures increase the UV reflectance of female sexual ornaments, warmer temperatures increase the UV reflectance of male sexual ornaments. These changes are not visible to humans because we do not see UV,” explains postdoctoral fellow Dr Kathleen Prudic. However butterflies can see UV, so by developing more attention-grabbing eye spots, females born in the dry season are able to attract males. [BBC News]
When the team studied butterflies reared in the larval stage at about 80 degrees Fahrenheit versus those subjected to temperatures 20 degrees cooler, they indeed found this effect.
Why, though, should something so simple as the season of birth affect who chases whom in the butterfly mating game? Reverse-engineering an explanation for evolved traits is a tricky business, but Prudic’s team has a working hypothesis that food is the driving force.
In addition to delivering sperm during the mating process, male butterflies also deliver nutrients. In less than optimal times for reproduction (the dry, cool season), these male offerings appear to lead to increased female longevity. [LiveScience]
Thus, males—as the bringers of sustenance—get to be more choosy in the cool season, while the females are put in the position of madly batting those wing spots and trying to catch some attention.
Prudic’s study appears in this week’s edition of Science.
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Image: Science / AAAS