Risso’s dolphin

Appearance: Risso’s dolphin is the largest ‘true’ dolphin without a distinct beak. It has a robust body and a large, blunt head. A concave groove runs down the front of the melon from the blowhole to the mouth, which is long and turns upwards towards the eye.

Fey Characteristicsrissos-dolphin

  • Robust body.Adults extensively scarred
  • Large rounded blunt head with cease down the midline
  • Indistinct beak
  • Prominent dorsal fin
  • Long, pointed flippers
  • Dark flippers and flukes
  • Older mammals may be almost white in colour
  • Active at the surface

 Dorsal fin: Tall (up to 50cm) high, falcate, located near the middle of the back.

Flippers: relatively long, curved and pointed almost straight backwards.

Flukes: board and curved, with a deep median notch.

Teeth: peg-like; may become worn with age in males, which use their lower teeth to rip and tear at one another’s bodies in mating contests. Upper jaw 0, lower jaw 4-14.

Colouration: As calves, Risso’s dolphins are a uniform light grey without scars, the colour darkening to a dun shade in juveniles. Due to the extensive scarring typical of the species, particularly in males, adults turn from dark grey through silver-grey to nearly white except on their flippers and flukes, which remain almost entirely dark throughout their lives. The scars can be linear scratches, blotches or circular marks. Risso’s dolphins also have a white anchor-shaped patch, similar to a pilot whale’s running from the throat to the stomach.

Behavior: Risso’s dolphins are often seen travelling side by side. They surface slowly, at a 400 angle, to breathe. Their behavior tends to be calm and sedate, though they can occasionally be very active. Porpoising, fluke-slapping (in adults) and spyhopping high in the water with the flippers exposed have all been observed. This species seldom bow-rides but may swim alongside a boat or in its wake.

Typical dives last for about two minutes, through Risso’s dolphins can remain under water for up to 30 min. the flukes may appear above the surface when diving.

Associates: forms travelling pods at sea with other dolphin species such as the bottlenose dolphin.

Diet: primarily small, mid-water squid and fish, including lanternfish. Forages mostly at night.

Distribution: resident but may also be migratory. It is pelagic, usually found in deep oceanic and continental slope waters. Present year-round through most of its range, with some evidence of seasonal movement on the continental shelf in areas such as the Gulf of Manner. Widely distributed in the Laccadive Sea, off southern Sri Lanka and the Bay of Bengal.

Sightings: all year round. Sri Lanka: common. Maldives: abundant. India: occasional