Dusky Dolphins: Flexibility in Foraging and Social Strategies

Abstract

Dusky dolphins (Lagenorhynchus obscurus) exhibit fission–fusion dynamics as individuals join and split from groups of two to several thousand individuals. During the past three decades, our studies of dusky dolphins in three distinct marine systems have revealed how habitat type, predation risk, and prey availability influence foraging and social strategies. In the large Argentine bay of Golfo San José, fission–fusion dynamics are driven by large group formation during the day to coordinate prey-herding behaviors on southern anchovy (Engraulis anchoita) and nighttime resting in small groups near shore for predator avoidance. In the small New Zealand bay of Admiralty Bay, fission–fusion dynamics are also driven by daytime coordinated prey-herding strategies on small schooling fishes, but changes in group size are relatively muted and there is little predation risk. In the open, deep-water environment off Kaikoura, New Zealand, dusky dolphins rest and socialize near shore during the day, moving offshore at night to feed on the deep scattering layer (DSL). Changes in group size are also relatively muted off Kaikoura, and large groups serve to reduce predation risk. Comparisons between the three sites reveal a pattern of increasing group size with increasing openness of habitat. Response to predation pressure includes formation of large groups or formation of small, inconspicuous groups near shore. In the bay systems, fission–fusion dynamics are driven by coordinated foraging strategies on patchily distributed schooling fishes. In the open ocean system, DSL prey resources are more reliable, and fission–fusion dynamics are instead driven by strategies to obtain mates and avoid predators. Despite these differences, dusky dolphins exhibit polygynandry in all three systems. The presence of a single social-sexual system in spite of variability in fission–fusion dynamics has also been documented in chimpanzees (Pan troglodytes). In both societies, flexibility in social and foraging strategies enables individuals to respond to changing socioecological conditions. However, traits of the marine environment such as few physical refuges, low cost of transport, the need to herd mobile food resources into tight prey balls, and separation of oxygen and prey may be factors contributing toward differences in fission–fusion dynamics between dolphins and primates.

A dusky dolphin (Lagenorhynchus obscurus) performs a noisy leap in Admiralty Bay, New Zealand. (Photo courtesy of Chris Pearson)