Trade-off. A buff physique means fewer offspring for speckled wood butterflies.

Butterflies' Dilemma: Mate or Disseminate?

Many insects in the Northern Hemisphere have responded to global warming by spreading northwards. The wanderlust comes at a price, though: Some British butterflies that bulk up their flight muscles must settle for smaller broods. This finding could influence how researchers study the effects of climate change.

About 25% of British butterfly species are spreading north as a result of rising temperatures. Studies have shown that crickets and other insects at the edge of a species' range evolve features that help them disperse, such as stronger flight muscles. However, says lead author Clare Hughes, an entomologist at the University of York, U.K., these features may have hidden costs that conservationists should keep in mind. "It's important to know which species can adapt their morphology and behavior" in response to climate change, she says, and collecting data on such phenomena will help set priorities for conserving species.

In butterflies, the thorax is mostly composed of flight muscle while the abdomen is largely dedicated to egg production. Because resources are limited, a bigger thorax should mean a smaller abdomen--and lower fertility, says Hughes. To test the link, Hughes and colleagues netted 71 egg-carrying female speckled wood butterflies (Pararge aegeria) from four woodland sites in fall 2002. Two sites were at the center of isolated, north-spreading populations (one in England, the other in Scotland), and the others were at the northern margins of these populations. The researchers bred the butterflies in the lab and looked at 324 granddaughters to determine their fertility and flight power. Butterflies originating from the northern edge of both ranges had more muscular thoraxes and laid fewer eggs, the researchers report online this week in Biology Letters. Butterflies from York, for instance--which was only colonized by this species about 6 years ago--had 30% heavier thoraxes and laid 26% fewer eggs than those from the population hundreds of kilometers south.

This research "adds to the evidence that studying long-established populations may not be a good predictor" of how a species responds to climate change, comments conservation biologist Chris Thomas of the University of Leeds, U.K. Taking into account those individuals at the leading edge of an expanding range may give a better overall view of how climate is affecting the species.

Related sites
Butterfly Conservation
UK Climate Impacts Programme