Evolution of group size in the dolphins and porpoises: interspecific consistency of intraspecific patterns

I investigated group size variability in dolphins and porpoises using intraspecific comparisons. Explanatory factors considered in the analysis were variables of the physical environment, the diet, and the life history of the species. Open habitat and small body size were viewed as increasing predation risk. This pattern was apparent in Risso's dolphins (Grampus griseus) and weakly apparent in bottlenose dolphins (Tursiops spp.). Group size was negatively correlated with body size in pilot whales (Globicephala spp.) and positively correlated with the openness of habitat in killer whales (Orcinus orca), striped dolphins (Stenella coeruleoalba), and common dolphins (Delphinus spp.). No such relationship was found for harbor and Dall's porpoises (Phocoena phocoena, P. dalli). Group size also seemed to vary depending on other physical measures of the habitat, which may indirectly reflect diet; group size showed U-shaped patterns if related to temperature. The predictive power of variables comprising detailed prey information on group size was variable. For example, pilot whales had smaller group sizes when they fed more on mesopelagic fish and less on mesopelagic cephalopods, and common dolphins had larger groups if they fed on varying types of fish. In most Delphinoidea species, group size could be described by the variables considered in this study. But each species showed its own pattern of correlations between group size and a specific set of explanatory variables. Thus, no general and consistent relation between group size and the other variables was found. It remains unknown whether these species-specific patterns result from a historical process or whether they are specialized adaptations.