The cost of aggression in an animal without weapons

Abstract To understand the prevalence and conditional use of aggression among animals, one has to know its costs and benefits. The obvious cost of aggression in animals that possess teeth, claws or other specialized weaponry is injury. Many species, however, do not have such body parts and thus cannot readily injure others. The cost of aggression in these animals is not well studied. We tested whether aggression has a fitness cost in fruit flies, which can serve as a model species for animals without weapons that engage in aggression. In three experiments employing distinct protocols, we allowed focal flies to fight for control of an attractive food patch over 4 days and then compared their survivorship to that of flies not engaged in conflict. In all three experiments, fly survivorship was lower in the aggression than no‐aggression treatments. Microscopic examination revealed no differences in wing damage between flies of the aggression and no‐aggression treatments. The two most likely, non‐mutually exclusive explanations for lower survivorship post‐fighting are physiological changes due to stress, and metabolic alterations associated with a life‐history strategy optimized for high‐conflict settings.