When fruit flies mate, males do not transfer just sperm to females. Instead, male ejaculate fluid contains a cocktail of sperm and sex peptides that have striking effects on female behaviour. For example, sex peptides make females ramp up egg production, while reducing their willingness to mate again. This means that sex peptides help males manipulate females into investing more in their current offspring, even if it carries long-term fitness costs to females. Eleanor Bath and colleagues at the University of Oxford, UK, suspected that male ejaculate might affect another important aspect of female behaviour – aggression.
Females typically fight to protect their offspring and to secure resources to provide for them. This means that fighting should be a flexible trait in females and that females should be most aggressive when they have offspring. Given this, Bath and colleagues predicted that mating might act as a cue to trigger female aggression. To test this idea, the team paired virgin and mated females in a location where food was scarce and recorded how often females butted heads, fenced, shoved or retreated from one another. The result was clear: mated females spent twice as long fighting as virgin flies.
The team then began teasing apart the elements of mating responsible for elevating female aggression. They suspected that the costs of egg laying might drive this elevated aggression, prompting females to fight to secure enough resources to lay eggs. To test this idea, they compared aggression in mated wild-type females and in mutant females that cannot produce any eggs. Surprisingly, levels of aggression were similar in wild-type and mutant females, showing that mating elevates female fighting regardless of whether they produce eggs.
If the costs of egg laying do not trigger elevated aggression, maybe a component of the ejaculate itself is responsible? To find out, the team compared aggression in females mated to wild-type males and male mutants that can transfer seminal fluid proteins but no sperm. Females mated to these spermless males were much less aggressive than control females, showing clearly that sperm stimulates female fighting. To see whether sex peptides also play a role, wild-type females were mated to mutant males that produce sperm but not sex peptides: the females mated to these mutants were more aggressive than virgins, but not as aggressive as females mated to wild-type males. Both sex peptides and sperm are needed to trigger the full increase in female fighting after mating.
These results show that mating triggers fighting in female fruit flies and does so independently of egg laying. However, they can't tell us why male ejaculate triggers fighting in females. Perhaps this is another example of conflict between the sexes, where males try to manipulate female behaviour to elevate their own fitness, even if it carries long-term costs for the females, such as injury? Additionally, the neuronal mechanisms that underpin female responses to seminal peptides are unclear. Understanding the mechanistic basis of how sperm and sex peptides stimulate female aggression and the fitness consequences of female fighting are exciting avenues of future research.