|The Mammals of Texas -
Cetacea : Family Delphinidae : Grampus
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 Rissos 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.
Distribution in Texas. 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 Rissos
dolphins was seen in the Gulf waters off the South Texas
coast in November, 1980.
Habits. Rissos 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 Rissos dolphin called
"Pelorus Jack" was sighted in a New Zealand
harbor for over 20 years.
Pelagic sightings of Rissos
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.
Illustration credit: Pieter