Costs and benefits of group size for sperm whales of the open ocean

Individual feeding success can change with the size of a social group through competition, cooperation, recruitment and other processes. Female sperm whales (Physeter macrocephalus) live in long-term social units that themselves can form larger, temporary groups, but the individual-level impacts of group formation are unknown. We measured feeding success on 88 days following groups off the Galápagos Islands and Chile using defecation rates. Feeding success increased about fourfold as group sizes rose from one social unit (about 10 animals) to 4-5 units. Our data did not suggest recruitment into successful groups, and the whales' prey are generally relatively small and slow, arguing against cooperative capture. Instead, it seems most plausible that individuals receive valuable information about patchy prey distributions from foraging group members over scales of 100 to 1000 m. Feeding success fell for the largest groups, suggesting a foraging optimum of about 40-50 animals off the Galápagos Islands and 30-40 animals off Chile. These are close to the mean typical group sizes in these areas, which indicates that efficient foraging on patchy prey may largely drive group size. Considerable differences between oceans in the group sizes of female sperm whales are probably linked to variation in the patchiness of resource availability.