Dusky Dolphins of Continental Shelves and Deep Canyons

Abstract

Dusky dolphins (Lagenorhynchus obscurus) exhibit highly flexible foraging and social strategies. Studies in three distinct environments offer a natural experiment for understanding influences shaping dusky dolphin societies. In shallow bays off Patagonia, Argentina, dusky dolphins form small traveling groups during the day in search of small, schooling fish, but fission-fusion of large groups enhances predator detection/avoidance and mating opportunities. Predation risk is also minimized by resting in small groups near shore at night. In the deep open waters off Kaikoura, New Zealand, large mixed age and sex groups and satellite mating and nursery groups occur. Loosely coordinated subgroups forage nocturnally on the deep scattering layer. Large group formation is again an anti-predation strategy. In the shallow wintertime habitat of Admiralty Bay, New Zealand, coordinated bait-ball foraging occurs but in smaller groups than off Patagonia. Outside of the breeding season and in the absence of predation risk, Admiralty Bay grouping patterns are driven by opportunities to secure prey and social partners. Compared to many other delphinids, dusky dolphins are more gregarious yet more loosely bonded. The social brain hypothesis helps to explain the evolution of large relative brain size and complex sociality in dusky dolphins. Bycatch, habitat loss, climate change, and whale-watching are current threats to the species. Application of new technology and research on female behavior, culture, and lesser-studied populations will help to fill knowledge gaps and advance conservation strategies.