dolphin joins the sperm whale as denizens of deep water. They
are mostly seen where water is several 100 meters deep and so
they are not often seen near shore (Wilson and Ruff 1999)..
The life history of this species is incomplete as a result. It
prefers warm waters and enters temperate regions only in
summer. It seems to live in groups that travel together among
large herds numbering in the hundres or thousands (Wilson and
Ruff 1999). Calves, juveniles and adults appear to segregate
in these groupings. Risso’s dolphins reach sexual maturity
when they are more than 2.5 m long and off California probably
calve in winter. Calves measure about 1.5 meters long and
blue gray in colour. The colour turns a dark brownish-grey
with age (Wilson and Ruff 1999). This species is highly
gragrious and interative. They leap and porpoise and make high
speed zig-zags. Risso’s dolphins are thought to eat squid.
They lack teetch in the upper jaw and only a few in the lower
jaw, likely as an adaptation to eating soft bodied squid.
Risso's dolphins utter seven vocalisation types: broadband clicks,
barks, buzzes, grunts, chirps, whistles, and simultaneous
whistle and burst-pulse sounds (Corkeron and Van Parijs
and S. Ruff. 1999. Smithsonian book of North American mammals.
UBC Press, Vancouver.
Corkeron, P. and S. Van Parijs. 2001.
Vocalizations of eastern Australian Risso's dolphins,
Grampus griseus, Canadian Journal of Zoology 79: 160-164