April 24, 2015
Biologists have long wondered why some animals ignore other species and others aggressively combat interlopers from another species.
A new study from UCLA researchers has revealed that rubyspot damselfly males fight off males from other damselfly species because of competition for mating females – even though female rubyspot damselflies are not interested in inter-species mating.
“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 study author Jonathan Drury, currently a postdoctoral researcher at the École Normale Supérieure in Paris.
Damsels in dsitress
In the study, researchers daily tracked more than 100 damselflies along rivers and streams in Texas, Arizona and Mexico. They found a substantial link between inter-species violence and similar coloration patterns. The biologists recorded some instances where aggression between species disappeared due to substantial divergence in wing coloration. However, in most of the pairs of species they studied, there is very little alteration in color, and males are as hostile to males of a different 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,” said study author Gregory Grether, a UCLA professor of ecology. “I think that’s the root cause of the persistence of male territorial aggression.”
After their observations, the team designed and confirmed a mathematical simulation projecting that as competition for mates rises, male aggression rises, and displaying at what point hostility against a different species becomes beneficial.
“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.
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