Larval aggression is independent of food limitation in nurseries of a poison frog

Abstract

Aggression between nurserymates is common in animals and often hypothesized to result from proximate resource limitation. In numerous terrestrial frogs, larvae develop in phytotelmata, tiny water bodies where resources are scarce and competition, aggression, and cannibalism are all common between individuals sharing these nurseries. In some species, mothers provision phytotelm-bound young with trophic eggs, a strategy that compensates for low nutrient availability and could allow mothers to reduce costly aggression and cannibalism among nurserymates. We tested this hypothesis using strawberry poison frog (Oophaga pumilio) tadpoles, staging secondary depositions in arenas occupied by residents that had either been food deprived or fed ad libitum. Resident tadpoles were nearly all aggressive and most killed intruders, but aggression was unrelated to resident food deprivation. Unlike most related frogs studied, O. pumilio residents did not cannibalize their victims. This result supports the hypothesis that proximate food limitation and aggression can be independent. Aggression and cannibalism are common in the resource-limited nurseries in which many young animals develop. An intuitive hypothesis for this is that individuals are aggressive because they are hungry. Although this hypothesis has usually been supported in birds, we found no support for it in a test we conducted in a terrestrial frog that feeds its young with trophic eggs. Resident tadpoles fed ad libitum were just as aggressive to staged intruders as those that were food-deprived, and were just as likely to kill intruders. Residents did not cannibalize their victims. These results suggest that hunger-independent aggression in young animals is taxonomically widespread, and demonstrates an important ecological constraint on parents: they cannot prevent fatal aggression, they cannot “feed” tadpoles to older siblings, and thus, they cannot use occupied rearing sites.