Risso's dolphin
click to enlarge

Grampus griseus

Names

Azores : Moleiro - POR : Grampo - ITA : Grampo - SPAIN : Delfin de Risso - GB : Risso’s dolphin - GER : Rundkopfdelphin - NEDER : Grijze dolfijn - SWEEDS : Rissos delfin - NOORS : Grampus - DEENS : Halvgrindehval - JAP : Hana gondo kujira

Biology

Adult Risso's dolphins average size is 3 m. The females and the males can reach a maximum length of 3.5 and 4.3 m, respectively. Adults normally weigh 300 to 500 kg, but they can reach a maximum of 700 kg. The dentition of this species is reduced; generally they have 2 to 7 pairs (usually 3 to 4 pairs) of teeth in the lower jaw. More rarely, the upper jaw presents 1 to 2 pairs of vestigial teeth. This type of dentition is characteristic of animals that feed mainly on cephalopods, however, fish are also part of the Risso's dolphins diet. Their large melon head is bisected by a deep crease. They do not have a distinctive beak. Newborns measure 1.2 to 1.7 m and they are a light gray colour changing to an even chocolate brown early in life. Gestation lasts for 12 months. As they age, the adults' bodies gain very deep, white scarring on the flanks probably as a result of fights among themselves or with squid. Some animals, usually the oldest, are completely white, and this is the reason for their common name " moleiro " in the Azores. They can live up to 30 years. The Risso's dolphins emit various sounds with a frequency range of 0.1 to 8 kHz, mainly echolocation clicks and, more rarely, small bursts similar to a horn sound.

Observation

In the Azores, the Risso's dolphins are not very active, and therefore, it is very difficult to observe them more than a few miles away from the lookouts (" vigias ") on land. The sound they make is equally difficult to measure with a hydrophone. Despite this, we observed them regularly - 24 % of the time of our field trips - and they were usually near the coast (no doubt as a result of the technique we used for land observation). They are very sensitive to the presence of people, and they rarely get close to the vessels but we have very good memories of the few times that we had entire groups calmly riding our bow waves. This large Delphinidae is a very fast swimmer and it can reach maximum speeds of 14 knots. Sometimes they can execute fantastic jumps where they leap clear of the surface. The groups we observed were usually no more than few dozen animals. However, a few times we were able to observe herds with more than 100 animals. This species can be confused with other species such as the bottlenose dolphin, the short-finned pilot whale and the false killer whale. Among the groups with Risso's dolphins, we frequently observed animals with a lighter gray or even a total white coloration. Similar to the sperm whales, Risso's dolphins show their dorsal fin when they initiate a deep dive. A " normal dive " takes about 1 to 2 minutes, but several times we observed dives of more than 25 minutes duration. Risso's dolphins are also part of the group of cetaceans that are able to dive to a couple of hundred meters depth.

Cetaceans in the Azores