Why do animals fight members of other species?

Why do animals fight members of other species?
Mating damselflies. Researchers found that male damselflies often have difficulty distinguishing between females of their own species and another species when making quick decisions about whether to pursue a mate. Credit: Mark Bjorklund

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 , also known as rubyspot damselflies. For the study, published this month in the print edition of 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.

Male damselflies always respond aggressively to 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 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 Jonathan Drury, who was lead author of the study and is now a postdoctoral researcher at the École Normale Supérieure in Paris.

Grether and 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. Grether and Drury tested and confirmed their model with help from Christopher Anderson, an assistant professor of biological sciences at Dominican University. (Drury and Anderson were all previously doctoral students in Grether's laboratory.)

It's common to find two species of damselflies in one location. The biologists documented some cases where aggression between species has essentially 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, and males are 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," Grether said. "I think that's the root cause of the persistence of male territorial aggression."

The researchers sectioned off a part of the river, marked the damselflies for identification, and observed and analyzed rates of fighting within and between species. Territorial battles between two males can last a few hours, the biologists found.

Damselflies typically live only a couple of weeks, and have few mating opportunities.

"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.

The researchers also conducted experiments in which they captured damselflies and flew them, tethered with a transparent thread, into the nearby territories of other damselflies in order to measure the responses.

A male damselfly often rammed into a tethered male intruder (see 1st video) of the same species more than 100 times in two minutes, they found, while blithely ignoring (see 2nd video) a tethered male of a species that differed substantially in wing coloration.

A male rubyspot damselfly (Hetaerina occisa) perching on his territory and then fighting with a tethered male intruder of the same species.
A male rubyspot damselfly (Hetaerina occisa) perching on his territory and ignoring a tethered male intruder of a species (Hetaerina titia) that differs in wing coloration.
Grether believes the findings about territorial aggression are likely to hold true with other species that have mating territories, including reptiles, amphibians, insects and some species of birds. He wants to extend the research to species that are in competition for resources besides mates, such as birds, which compete for food and nesting sites.

Implications for humans

As for humans, Grether thinks reproductive interference and aggression between species may well have played an important role in our evolutionary past. Modern humans have existed for at least 200,000 years, he noted, and Neanderthals did not disappear until approximately 40,000 years ago.

"There is genetic evidence of interbreeding between the two ," 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 and its evolutionary impact are understudied subjects, Grether said.


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Apr 23, 2015
B.S
Females do fight too!

Apr 23, 2015
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


I would like to see an example of this type of warfare in traditional human groups as I have never heard of what is being referred to here. Of course humans fight wars, but to say that humans do it due to inter-mating.. when has that ever been the case in recent history?

Apr 24, 2015
I agree with the articles sentiment exactly. Traditional human groups are always starting wars over pressure for mates as well as pressure for resources. I cannot understand Tandgent2 above not noticing this.


Apr 24, 2015
In this world all living organisms have a definite function and vital niche, and live in harmony and cooperation, providing bio balance in the natural environment of man. In fact there is no competition. Darwinism is unnatural flawed ideology that have no manifestation in nature, that is the foundation of modern civilization. It causes humanity to encounter so unsolvable problems and to exist risk of self destruction of humanity. The desision is in the love and cooperation on free will basis. Not in competition.

Apr 24, 2015
Traditional human groups are always starting wars over pressure for mates


This is exactly what I am not understanding.. show me an example from history where this was the case? Of course human groups start wars over resources, but pressure for mates? I have never heard of any human group starting a war over mating pressures and hence I question this 'sentiment'.

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