By Calvin Jones
Risso’s dolphins are quite large stocky dolphins with a blunt head lacking the familiar “beak” of many other dolphin species. They are seen regularly around the Irish coast, and are quite easy to recognise by their blunt fore-head, pale colour and characteristic scarring on the body.
The scarring is thought to be caused by the teeth of other Risso’s dolphins. The species only has teeth on the front part of the lower jaw, and these are used to scrape the skin of other dolphins during play and occasional fighting. Some scarring is also attributed to one of this dolphin’s main prey items, squid.
Adult Risso’s dolphins range from 2.6 to 3.8 metres ( 8.5 to 12.5 foot) in length and weigh from 300 to 500 kilos (661 to 1102 lb). At close range the blow is visible, but in reality is rarely seen in the field. The head is blunt and bulbous, sloping upwards steeply from the mouth to the forehead. A deep, V-shaped crease bisects the dolphin’s forehead, extending from the blow-hole down to the tip of the upper lip. This crease is unique to Risso’s dolphins and is clearly visible at close range.
About midway along the back is a prominent dorsal fin that reaches up to 50cm (c. 20 inches) tall. The size of this fin and its dark colouration sometimes leads to Risso’s dolphins being confused with the larger bottlenose dolphin or even with the killer whale in the field. The pectoral fins are long, dark and sickle-shaped.