The culmination of a nine-year study by biologists finds that an animal of one species often battles with one from another species to...get an edge with the local females.
Scientists from the University of California, Los Angeles (UCLA) eyeballed the activities of more than 100 damselflies that daily flitted about their natural habitat beside streams and rivers in Arizona, Texas, and Mexico. They comprised members of several species of Hetaerina damselflies, commonly known as rubyspot damselflies.
Males would consistently become aggressive toward males of their same species that encroached into their territory. Communicating with males of another species is a different story. While they generally ignored the other males when no competition for females was at stake, they became aggressive to males of another species that encroached and made a beeline for the females.
Despite the fact that female damselflies shut down mating advances from males of a different species, the males persisted in their efforts, according to Gregory Grether, a UCLA professor of ecology and evolutionary biology and senior author of the study.
Sometimes the females' wings from the two species were colored alike.
"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," says Jonathan Drury, lead author of the study and now a postdoctoral researcher at the École Normale Supérieure, Paris.
Grether and Kenichi Okamoto, postdoctoral scholar, North Carolina State University, created a mathematical model predicting that as the contest for mates heats up, male aggression increases, and they showed at what point aggression against another species works to one's advantage.
Two species of damselflies often live in the same area. When the color of their wings deviates, the aggressive behavior sometimes fades away, the biologists found.
"Male damselflies often have difficulty distinguishing between females of their own species and another species when making split-second decisions about whether to pursue a female," Grether says. "I think that's the root cause of the persistence of male territorial aggression."
Grether says he believes the discoveries about territorial aggression probably endure with other species that have mating territories, such as reptiles, amphibians, insects, and some species of birds. He would like to broaden the research to species that compete for resources in addition to mates.
Do these findings translate to the evolution of humans?
Grether says he believes reproductive interference and aggression between species may have been in the mix in our evolutionary past. Modern humans have been around for a minimum of 200,000 years, he notes, and Neanderthals did not vanish until about 40,000 years ago.
"There is genetic evidence of interbreeding between the two species," Grether says. "Interbreeding and warfare with modern humans are usually viewed as completely different explanations for the demise of the Neanderthals, but they might not be different explanations after all. Fighting between Homo sapiens and Homo neanderthalensis groups might well have been motivated in part by intermating, just as it is in some cases of warfare between traditional human groups."
The article, "Reproductive interference explains persistence of aggression between species," is published in the print edition of the journal Proceedings of the Royal Society B.