Many animals behave aggressively towards one another. This is usually when they are fighting for something like territory, mates or food. However, an animal’s decision to become aggressive isn’t a simple on-off switch and many factors feed into how aggressive an animal is. For example, many animals become less aggressive after losing a fight against another individual and are therefore more likely to avoid or lose fights in the future (the ‘loser effect’). On the other hand, winning a fight can have the opposite effect: the individual can become more aggressive, self-assured and more likely to win fights in the future (the ‘winner effect’).

For fruit flies, both males and females behave aggressively towards one another. The males fight each other for territories used to attract females, and the females fight each other for good spots to lay eggs on. But do female and male flies respond to winning and losing fights in the same way?

To look into this, Benelli and colleagues carried out an experiment where they paired a male or female fly with a much weaker or stronger opponent to fight against. They then looked to see if, after winning or losing once or twice in a row, whether those individuals would be more likely to win or lose in a fight against a new individual.

They found that both males and females that won one or two fights in a row displayed the ‘winner effect’: they were very aggressive (‘hyperaggressive’) and were much more likely to then win their next fight.

Similarly, males and females that lost a fight showed the ‘loser effect’, becoming less aggressive and being more likely to lose their next fight. Surprisingly, flies that lost two fights didn’t become even bigger losers, instead they became hyperagressive.

Male and females were actually pretty similar in how they responded to winning and losing fights. The main difference between the sexes was that females tended to fight for much longer than did males.

The authors suggest a rather intriguing application for their findings in pest management. One effective way to control fruit flies is to introduce sterile males. These males then mate with females who do not reproduce successfully. However, one limitation to this technique is that sterile males are often less good at competing with regular males, due to the stress they go through during sterilization, shipping and release. If these flies were to be kept in higher densities in their cages, this would increase the number of competitions each male had. Whether he kept winning or kept losing would presumably create hyperagressive behaviour, and therefore potentially make him more likely to win a fight against a non-sterile male out in the real world.

Photo credits

Aggressive cat: Steve Hardy

Fruitfly Ceratitis capitata on hand & Fruitfly Ceratitis capitata alone: Eran Finkle

Two males of Ceratitis capitata fighting for an apricot leaf: Giovanni Benelli


Benelli et al. (2015) Sex differences in fighting-induced hyperaggression in a fly. Animal Behaviour, 104: 165-174.