Common Name: Risso’s Dolphin
General Description: The body of this dolphin is deep and massive for the front two thirds, and tapers behind the dorsal fin to the narrow tail stock. There is no beak, and the head is large and blunt. The slight melon is marked by a deep crease that may be observed only from close quarters.
The long, pointed flippers are broad based. The tall, slender dorsal fin is very prominent.
Risso’s Dolphins are dark to light grey above, older individuals being paler. The head is sometimes completely white, with a dark area around each eye. The dorsal fin and an area of the back adjacent to its base, the flippers and flukes remain dark. Extensive white scarring covers the body of adult animals, making them readily recognisable. The scars seem to be the marks made by the teeth of mates or rivals. There is an oval or anchor-shaped white patch on the chest and chin
Size: Adults, Risso’s Dolphins may attain lengths of 3.8 m. Females are a little shorter than males. The weight is 400 to 500 kg, even 680 kg. Calves at birth, 1.5 m long.
Appearance At Sea: Named after a French naturalist, Risso’s Dolphins usually travels in groups of 12 to 50, but may be seen as single animals or in herds of several hundreds. These are active dolphins, spyhopping, breaching and slapping their tail flukes.
The normal intervals between blows is about 15 to 20 seconds which is followed by a deep dive after 3-4 minutes at the surface.
One Risso’s Dolphin named Pelorus Jack that lived in the Cook Strait between North and South Islands of New Zealand is famous. He used to accompany steamships and leap about in front of the bows, as though guiding them through the strait. He did this daily for many years, and became a tourist attraction.
Found In: These are deep-water animals. However, herds may be seen in depths as low as 100 m. They feed mainly on squid.
Records from India: Some Risso’s Dolphins were observed by the Tulip off India. A single specimen was captured off Madras in 1986.
Found In: This species has been observed near Sri Lanka, the Maldives, the Seychelles and the Horn of Africa. It is known to be accidentally caught by Sri Lankan fishermen, and there are records of specimens from the Gulf of Oman and the Red Sea.
Found In: The existence of a Tamil name would suggest that the species is better known in the region than the paucity of records suggest. However, the name cited is not different from that of the Common Dolphin Delphinus delphis. So, ‘panavai meen’ may be a general Tamil name for all dolphins.
World Distribution: Risso’s Dolphin is found in most tropical and warm temperate oceans and seas with a temperature of 15° – 25° C. It avoids polar seas.
Could Be Confused With: At a distance, there is a possibility of confusion with the Bottlenose Dolphin Tursiops truncatus and False Killer Whale Pseudorca crassidens.
They can be distinguished as follows:
Species : Risso’s Dolphin
Colour of back : Pale grey
Scarring : Extensive
Fin: Tall, pointed fin
Head : Short, blunt face
Species : Bottlenose Dolphin
Colour of back : Dark grey
Scarring : Seldom scarred
Fin : Relatively short, pointed fin
Head : Long and pointed beak
Species : False Killer Whale
Colour of back : Dark
Scarring : Never scarred
Fin: Very tall, rounded fin
Head : Slender, tapered head
Diagnostic Features:At sea, prominent dorsal fin, extensive scarring, lack of beak.
Stranded Specimens: A freshly beached Risso’s Dolphin can be recognized by the scarring and the groove on its forehead. Normally there are no teeth in the upper jaw of the Risso’s Dolphin, and only a few in the lower jaw. These teeth are strong and oval, and located at the front of the jaw. They may number 2 to 7 in each row. This dentition configuration is unique among cetaceans.