Spatial consequences for dolphins specialized in foraging with fishermen

Abstract

According to theory, individuals forage in ways that maximize net energy intake. Distinct foraging strategies may emerge within a population in response to heterogeneous resources, competition and learning, among other drivers. We assessed individual variation in, and ecological consequences of, an unusual, specialized foraging tactic between animals and humans. In southern Brazil, bottlenose dolphins, Tursiops truncatus, herd fish schools towards artisanal fishermen, who cast nets in response to behavioural cues from the dolphins. This apparent cooperative tactic likely involves costs as well as benefits for both interacting parties, but such trade-offs remain poorly understood, especially for dolphins. We show that individual dolphins vary markedly in the frequency with which they interact with fishermen, and that this foraging variation is linked to ranging behaviour. Not all individual dolphins interact with fishermen; those that routinely do so concentrate around the limited interaction sites and have smaller home ranges than independent foragers. This suggests that foraging with fishermen increases foraging success and reduces search costs (i.e. foraging range). Competition for interaction sites may offset such benefits, since some individuals often forage at the high-quality sites while others forage at low-quality sites. Taken together, our findings suggest that two alternative tactics emerge in the population from trade-offs involving food access, foraging area, learning techniques and competition: dolphins either forage by themselves over larger areas on unpredictable resource patches (passing fish schools), or learn to interact with fishermen to access and compete for more predictable resource patches (interaction sites). By revealing some of the ecological drivers of this remarkable human–animal interaction, our study contributes two broader insights. First, specialized foraging can have ranging consequences for individuals and so structure the population spatially; second, interspecific cooperation may be founded upon intraspecific competition.