BEHAVIORAL RESPONSES OF ROUGH‐TOOTHED DOLPHINS TO A DEAD NEWBORN CALF

Abstract

A rare and interesting event in which rough-toothed dolphins (Steno bredanensis) supported a dead newborn calf was observed between 25 and 30 April 2001 off the island of La Gomera in the western Canary Islands. The six sightings described here were made as part of an ongoing research project focusing on human-cetacean interactions off La Gomera. The observation platform was an 11-m wooden whale watching boat based in Vueltas, Valle Gran Rey, operating year-round off the south and southwest coast of the island. Rough-toothed dolphins are resident in the study area (Ritter 2001, Mayr and Ritter 2005). Data were recorded continuously in an ad libitum manner (Altmann 1974, Martin and Bateson 1993), spoken to a voice recorder. Photographs were taken opportunistically of as many members of the encountered groups, using a single lens camera equipped with a 300-mm telephoto lens. However, photo-ID efforts were limited due to the fact that observations were made during regular whale watching trips. Rough-toothed dolphins occur throughout the world oceans, preferring deep waters and sea surface temperatures of 25◦C (Leatherwood and Reeves 1983, Miyazaki and Perrin 1994). Group sizes range from several individuals to fifty or more. Little is known about the behavior of this species, but there is increasing evidence for a complex social organization (e.g., Mayr 2004). Behaviors observed off La Gomera during past years included travel, surface feeding, dive, rest, and milling (Ritter 2002). Site fidelity as well as strong social bonds between individuals of different age classes where documented. The dynamic nature of group/subgroup composition indicates a fission–fusion type of social organization (Mayr and Ritter 2005). The first sighting involving the observation of adult dolphins together with a dead calf was made on 25 April 2001 at 1030. A group of 15–20 rough-toothed dolphins was spotted 1.15 km away from the coast (28◦03′31′′N, 17◦19′83′′W). The animals were widely dispersed and apparently surface feeding, as indicated by repeated changes of the swimming direction of the whole group and a high surface activity with fluke-up dives, skimming, and frequent leaps. Cory’s shearwaters were present and one dolphin was seen pursuing a flying fish. An adult dolphin in the group was observed with a dead newborn calf (Fig. 1). The animal was continuously pushing the carcass toward the surface either with its beak, melon, and front part of the back or taking it into its mouth. It repeatedly performed arched or lateral surfacings and showed other erratic movements close to the surface. From time to