Battling butterflies: Motivation determines the winner

"Normally when two animals fight or size each other up, the larger of the two is the one who wins. But with butterflied neither size, age, nor energy reserves seem to have any impact on who will win the battle. Instead it's a matter of motivation. The notion that individual motivation can have such an influence is an entirely new and exciting finding," says zoologist Martin Bergman, who presents the study on territorial fighting among butterflies in his dissertation "The evolution of territoriality in butterflies."

If you walk about outdoors during the summer you can see that certain butterflies, such as brimstones and orange tips, are in constant movement, flying over huge areas, whereas others, such as small tortoise shells and peacocks, are closely bound to a site, spending most of their time looking out from a particular vantage point. These are two strategies for a male butterfly to find a mate, either to fly around and search or to sit still and watch. In species that use the sitting strategy, the males are often highly territorial and chase away other males from the site.

"We have studied the behavior of the speckled wood butterfly, where the males establish their territory in large sunny spots in forests. The males sit on the ground in the sunny spots and check out the females that fly by. These sunny spots are defended against intruders via extended flying duels. The flight contests, where the males circle around each other, can last up to 90 minutes. The winner gains access to the sunny spot as a reward, while the loser has to go and look for another suitable sunny spot," says Martin Bergman.

A male who monitors a good sunny spot has a greater chance of discovering females flying by and thereby a better chance of mating. It has long been believed that sunny spots serve as a meeting place for males and females, but it has not been proven until now, largely because many female butterflies mate only once in their lifetime and most often nearly immediately after they hatch.

"To study the territorial battles of butterflies, we have reared caterpillars in captivity and then released the adult butterflies in large cages at the Stockholm University research station at Tovetorp. There we have been able to observe the butterflies' behavior under largely natural conditions," says Martin Bergman.