Why do animals fight with members of other species? A nine-year study by UCLA biologists says the reason often has to do with “obtaining priority access to females” in the area. The scientists observed and analyzed the behavior of several species of Hetaerina damselflies, also known as rubyspot damselflies. For the study, published online on March 4, 2015 in Proceedings of the Royal Society B, researchers observed more than 100 damselflies a day in their natural habitat along rivers and streams in Texas, Arizona, and Mexico. Male damselflies always respond aggressively to males of their own species that fly into their territory. Males typically ignore males of another damselfly species when they do not compete for females, but respond aggressively to males of another species that invade their territory and attempt to mate with females. Female damselflies almost always refuse to mate with males of a different species, said Dr. Gregory Grether, a UCLA Professor of Ecology and Evolutionary Biology and senior author of the study. But that doesn’t stop some males from trying, especially in cases where the females of both species have similar coloration. “We were surprised to see how well the degree of reproductive interference — the competition for mates between species — predicts the degree of aggression between species,” said Dr. Jonathan Drury, who was lead author of the study and is now a postdoctoral researcher at the École Normale Supérieure in Paris. Dr. Grether and Dr. Kenichi Okamoto, a postdoctoral scholar at North Carolina State University, developed a mathematical model predicting that as competition for mates increases, male aggression increases, and showing at what point aggression against another species becomes advantageous. Dr. Grether and Dr.
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