Due to its offshore habits, relatively little is known about the biology of Risso’s dolphin (5). However, in common with most other dolphins, this species is highly sociable, and typically travels in groups of 10 to 50, averaging around 30, although huge gatherings of up to 4,000 animals have been seen on occasions where food is in abundance (4) (5) (9). Many groups consist of same sex and similar aged individuals, and Risso’s dolphin often mixes with groups of other cetaceans while foraging (5) (9). It feeds almost exclusively on squid at night time, when its prey migrates towards the water’s surface (2) (5) (10).
The reproduction of Risso’s dolphin is poorly understood. However, due to its almost cosmopolitan distribution, it is likely that breeding and calving occurs year-round, although it at least peaks between December and April off South Africa (2) (4). A single calf is born after a gestation period of some 13 to 14 months, and it remains with the female and continues to feed on energy-rich milk for several months (2) (8). The seasonal movements of this species are equally unclear, but it appears as though some populations in more stable environments stay in the same place all year, while others show northern movements during summer and southern movements during winter (9).
While at the water’s surface, Risso’s dolphin displays an impressive variety of acrobatic acts, including high speed jumps out off the water, known as breaching, violent tail slapping and bobbing the head. Although these behaviours are not fully understood, they may be used in communication (2). Similarly, this species produces a number of sounds, including characteristic signature ‘whistles’, and many of these vocalisations are important for detecting prey through echolocation (8) (11).