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

I investigated group size variability in dolphins and porpoisesusing intraspecific comparisons. Explanatory factors consideredin the analysis were variables of the physical environment,the diet, and the life history of the species. Open habitatand 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 pilotwhales (Globicephala spp.) and positively correlated with theopenness of habitat in killer whales (Orcinus orca), stripeddolphins (Stenella coeruleoalba), and common dolphins (Delphinus spp.). No such relationship was found for harbor and Dall'sporpoises (Phocoena phocoena, P. dalli). Group size also seemedto vary depending on other physical measures of the habitat,which may indirectly reflect diet; group size showed U-shapedpatterns if related to temperature. The predictive power ofvariables comprising detailed prey information on group sizewas variable. For example, pilot whales had smaller group sizes when they fed more on mesopelagic fish and less on mesopelagiccephalopods, and common dolphins had larger groups if theyfed on varying types of fish. In most Delphinoidea species,group size could be described by the variables considered inthis study. But each species showed its own pattern of correlationsbetween group size and a specific set of explanatory variables. Thus, no general and consistent relation between group sizeand the other variables was found. It remains unknown whetherthese species-specific patterns result from a historical processor whether they are specialized adaptations.