Species vs Species: Males Fight for Female Attention

Scientists have long been curious as to why it's common for animals to fight with members of other species, and now a new study has found the answer, blaming it on females, of course.

According to a team of UCLA biologists, males of different species often fight for female attention to gain "priority access" to them for mating purposes. At least, that's what they observed when studying the behavior of several species of Hetaerina damselflies, also known as rubyspot damselflies.

During the study, published in the journal 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.

While it's typical for male damselflies to be aggressive toward males of their own species that encroach on their territory, the researchers were surprised to find that they also act antagonistic toward males of other damselfly species, who attempt to mate with females.

Female damselflies almost always refuse to mate with males of a different species, but like any persistent male, that doesn't stop them 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," Jonathan Drury, the study's lead author, said in a statement.

In fact, the researchers came up with a model that predicts as competition for mates increases, male aggression increases. It also shows at what point aggression against another species becomes advantageous.

What they found during their study was that species versus species competition virtually disappeared because of substantial divergence in wing coloration. However, in most of the pairs of species they studied, there is very little difference in color. In these cases, males are just as aggressive to males of another species as to males of their own species.

"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," explained Gregory Grether, a UCLA professor of ecology and evolutionary biology and senior author of the study. "I think that's the root cause of the persistence of male territorial aggression."

And these acts of aggression can last even up to a few hours, biologists found.

These territorial battles between two males are most likely so violent because damselflies typically live only a couple of weeks, and have few mating opportunities. They cannot afford to let someone else swoop in and steal their women right from under them.

"Low levels of reproductive interference are associated with low levels of aggression, and high levels of reproductive interference are associated with high levels of aggression," Grether said.

While these findings are indeed enlightening into the world of species versus species battles, they also may shed light on human evolution. That is, Neanderthals may have mated with modern humans about 50,000 years ago, so reproductive interference and aggression between species may well have played an important role in our evolutionary past.

"There is genetic evidence of interbreeding between the two species," Grether said. "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 inter-mating, just as it is in some cases of warfare between traditional human groups."

Interspecies aggression and its evolutionary impact are still little understood, and researchers hope to provide more insight into the subject in future studies.

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