Risso’s Dolphins known to breed in Irish waters
OVERNIGHT, the tide washed in the 3.5m-long remains of a recently-dead Risso’s Dolphin, its tongue indecently protruding from the side of its gaping, lifeless mouth set in an expressionless face. The stranded remains bore poor testimony to the magnificence of these large marine mammals that are resident off our shores.
Records of sightings of these animals at sea collected by the Irish Whale and Dolphin Group suggest that Risso’s Dolphins occur as far out as the edge of the continental shelf. There is evidence of an inshore movement during the summer months and the establishment of localised tight-knit family groups.
When dolphins are mentioned many people think of an animal with a beak; however, Risso’s Dolphin is an example of a dolphin without a noticeable beak and a large blunt forehead. In 1812, this species was named in honour of Antoine Risso, a distinguished French naturalist.
From analysis of the stomach contents of dead animals, Risso’s Dolphins are known to feed exclusively on squid, octopus and cuttlefish. They have peg-like teeth confined to the front of their lower jaws. There are no teeth in the upper jaw. Since the teeth are certainly not used for chewing, it has been suggested that their main function may be in social interaction, and/or courtship and mating.
The heads and backs of young Risso’s Dolphins are uniformly dark grey in colour. As they age, white scratches appear on their skin. It has been suggested that these scratches may be rake marks made by other dolphins’ teeth. Mature adults have so many of these scratches and so much scarring that their originally dark grey skin is often pale grey or even pure white especially around the head area.
It has also been suggested that the teeth may also be used for visual displays of aggression between rival males. Some of the markings on the heads of older adults are more difficult to explain; more than likely, some have been caused by a lifetime of conflict with squid, a major prey item.
The life expectancy of Risso’s Dolphins is unknown, but it is reckoned to be about 20 years. Length of gestation is also unknown, but these animals do breed in Irish waters: records show that a young calf showing colouration typical of a young Risso’s Dolphin stranded at Rosbrin Cove, Co Cork, in 1990 and one was born in Blacksod Bay in Co Mayo in the 1930s.