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  The Mammals of Texas - Online Edition

Risso's Dolphin
Order Cetacea : Family Delphinidae : Grampus griseus (Cuvier)

Risso's Dolphin (Grampus griseus).  Illustration by Pieter A. Folkens. 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.

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 Risso’s dolphins was seen in the Gulf waters off the South Texas coast in November, 1980.

Habits. 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.

Illustration credit: Pieter A. Folkens.