Aggression is induced by resource limitation in the monarch caterpillar

Food represents a limiting resource for the growth and developmental progression of many animal species. As a consequence, competition over food, space, or other resources can trigger territoriality and aggressive behavior. Throughout their early development stages, insect larvae eat voraciously and limited food availability can potently impact their viability through metamorphosis. In the monarch butterfly, Danaus plexippus, caterpillars feed predominantly on milkweed, raising the possibility that access to milkweed is critical for growth and survival. Here, we characterize the role of food availability on aggression in monarch caterpillars. We find that monarch caterpillars display stereotyped aggressive lunges that increase during development, peaking during the 4th and 5th instar stages. Detailed behavioral analysis reveals that aggressive actions are most likely to occur when the target is feeding and increases the probability that the target will leave the food source. To determine the relationship between food availability and the initiation of an aggressive encounter, we provided groups of caterpillars differing amounts of food availability and measured aggressive behavior. The number of lunges toward a conspecific caterpillar was significantly increased under conditions of low food availability, suggesting resource defense may trigger aggression. We find that aggression occurs independently of light, suggesting the visual system is dispensable for the induction of aggression. These findings establish monarch caterpillars as a model for investigating interactions between resource availability and aggressive behavior under ecologically relevant conditions and set the stage for future investigations into the neuroethology of aggression in this system.