This is a medium-sized dolphin that averages 3 m in length
and 300 kg in weight. Maximum size is about 4.3 m in length
and 680 kg in weight. Body form is stocky from the dorsal
fin forward but the tailstock is slender. The head is blunt,
beakless, and divided medially by a heavy crease. Coloration
is dark gray with lighter gray patches ventrally. In older
individuals the face and area just forward of the dorsal fin
is also light gray. They are often heavily scarred by parasites
and by wounds inflicted by other Risso’s dolphins. The
blunt, creased head and extensive scarring are noticeable
as the animals ride boat bow waves, or spyhop, and are probably
the best field character for distinguishing these dolphins.
Stranded specimens are distinguished by the unique head shape
and crease, and by the teeth. There are up to seven teeth
in each side of the lower jaw and none in the upper jaw.
Worldwide in warm temperate and tropical waters. They are
uncommon in the Gulf of Mexico and are most frequently observed
in the eastern part of these waters. There is only one stranding
from Texas; a group of nine Risso’s dolphins was seen
in the Gulf waters off the South Texas coast in November,
Risso’s dolphins have been observed in large groups
of several hundred but smaller groups of three to 30 are more
common. They seem to prefer deep offshore waters and in the
Gulf they are probably rare over the shallow waters of the
continental shelf. They eat squid and fishes.
Their reproductive habits are poorly known. They probably
calve in winter. Newborns are about 1.4 m long at birth and
these dolphins may live as long as 24 years. A well-known
Risso’s dolphin called "Pelorus Jack" was
sighted in a New Zealand harbor for over 20 years.
Pelagic sightings of Risso’s dolphins in the Gulf of
Mexico show that these dolphins appear to prefer deep offshore
waters; they have been sighted at ocean depths of 200-1,530
m. They are probably rare near the northern Gulf coast where
the continental shelf is broad and the nearshore waters are
relatively shallow. In deep, offshore waters of the Gulf these
dolphins may be more common than previously thought; however,
additional data are needed to effectively assess their status
in the Gulf.