Fruit Flies (Yes, Fruit Flies!) Reveal New Insights into Animal Aggression

By Alana Wilcox

3 July 2019

Julia Kilgour, a PhD candidate in Integrative Biology, studies the evolution of aggressive behaviour (photo courtesy of J. Kilgour)

Julia Kilgour, a PhD candidate in Integrative Biology, studies the evolution of aggressive behaviour (photo courtesy of J. Kilgour

A small leap and then he charges. A tussle ensues as the winged combatant pummels his competitor, ultimately emerging victorious. It’s an epic battle on a tiny scale involving a common kitchen pest and popular lab animal: the fruit fly.    

“Fruit flies will actively – and aggressively – defend their food patches in the laboratory,” says Julia Kilgour, a PhD candidate with Profs. Ryan Norris and Andrew McAdam in the Department of Integrative Biology. These bouts of aggression mimic behaviour observed in the wild and are helping researchers better understand how any one fly can have a competitive advantage over the rest.

Kilgour has long had an interest in the role of aggression in the animal kingdom. She recently carried out a study to determine if aggressive behaviour in fruit flies provides a competitive advantage and improves their chance of survival.

“Fruit flies are ideal for understanding how different group sizes and compositions may favour aggressive behaviour. It’s nearly impossible to go out into the field and manipulate groups of wild animals. But with fruit flies, we can order strains off the Internet and experimentally create whatever group combinations we want,” explains Kilgour.

The aim was to test two leading theories of why aggressive and non-aggressive behaviours are both maintained in populations. The first has to do with population density, the other has to do with what types of fruit flies are in the group. 

When individuals of a group are spread out, there is little need to compete for food, but cluster these same individuals together and it is likely that competition will increase. In these situations, it’s predicted that the most aggressive individuals will “win” the competitions and gain access to the food. Of course, these individuals risk injury if there are too many competitions, and this trade-off could be one reason why both aggressive and non-aggressive behaviours persist.

But density isn’t the only factor that could influence whether individuals will be aggressive or shy away from a fight. In some cases, the relative abundance, or frequency, of a particular behaviour in a population determines whether this behaviour will be successful. Occasionally, behaviours that don’t often occur can still provide an important advantage for survival.

A fruit fly feeding on a banana (photo by Sanjay Acharya, CC BY-SA 4.0)

A fruit fly feeding on a banana
(photo by Sanjay Acharya, CC BY-SA 4.0)

“We generally expect the most aggressive individuals to be the winners, in the same that way bullies are able to get whatever they want. But then everyone would be aggressive, and that isn’t the case,” Kilgour notes. “So in this experiment, we wanted to take a step back and see how both group size/density and group composition influenced the success, or survival, of both aggressive and non-aggressive types.”

Kilgour examined two types of fruit flies, one with a high and one with a low aggression level. Inside large test tubes she mixed individuals of each type at different frequencies. Some tubes contained an equal numbers of the two types of flies, while others had a 3:1 or 1:3 ratio of aggressive and non-aggressive individuals. She then varied the total density of flies from 30 to 300 individuals and observed how the population changed – in other words, which type survived – over the four-day experiment.

So, at what point is it beneficial to be aggressive?

Kilgour found that as the density of aggressive flies increased, survival declined, which was expected because the food supply was limited. But she also found that when the aggressive individuals were rare in the population, they had a greater chance of survival.

Even more surprisingly, the same pattern was found in the non-aggressive flies! They showed greater survival when they were rare, suggesting that there is an advantage to being non-aggressive. These results show that both the density and frequency of individuals in a population are important for maintaining behavioural variation.

“This was really exciting!” Kilgour exclaims, noting that it is the first study to show both aggressive and non-aggressive behaviours can be maintained in a population. “Our results show the importance of the social group in influencing an individual’s survival.”


Dr. Gustavo Betini, University of Guelph, also contributed to the study.


Read the full study in the journal Journal of Animal Ecology.

Read about other CBS Research Highlights.