Sexual dimorphism and body scarring in the boto (Amazon River dolphin) Inia geoffrensis

Measurements and quantitative descriptions of a large sample of live adult botos (Inia geoffrensis) were obtained from the Mamiraua Reserve in the central Amazon. Males were on average 16% longer and weighed 55% more than females, demonstrating that this species is one of the most sexually dimorphic of all cetaceans for size. Males were also pinker than females, more heavily scarred by intraspecific tooth rakes, and had more life-threatening injuries. Some larger males had areas of modified skin that may simply be scar tissue, but may also be a heritable characteristic used as a shield or weapon. As in sperm whales, sexual size dimorphism and rnale-male aggression appear to be linked in botos, Suggesting fierce competition for a resource-probably mating opportunities. The boto is unique among river dolphins in that males are larger than females. This distinction implies long evolutionary separation and fundamental differences in social behavior.