SPONGE CARRYING BY DOLPHINS (DELPHINIDAE, TURSIOPS SP) - A FORAGING SPECIALIZATION INVOLVING TOOL USE

During long-term research on bottlenose dolphins (Tursiops sp.) in Sha rk Bay, Western Australia, several individuals were observed carrying sponges, Echinodictyum mesenterinum, on their rostra. Over multiple ye ars, five regularly sighted individuals mere usually carrying sponges when encountered (67-100% of encounters). Four additional regularly si ghted individuals were observed with sponges just one time each. All f ive individuals that routinely carried sponges were female. Two of the anomalous, one-time carriers were female, one was likely female, and one was male. Most observations of sponge carrying occurred within a r estricted area, a relatively deep water channel (8-10 m deep). Surface observations of sponge carrying, including focal animal observations, revealed a stereotyped surfacing and diving pattern, and occasional i ndications of prey consumption. Three hypotheses are considered regard ing the function of sponge carrying: 1. dolphins were playing with the sponges; 2. the sponges contain some compound of use to the dolphins (e.g. for medicinal purposes); and 3. the sponges were used as a cool to aid in foraging. The foraging tool hypothesis is best supported, bu t the exact manner in which sponges are used remains to be discovered. Sponge carrying is a behavioural specialization, probably involving f oraging and regularly engaged in by only a small proportion of female dolphins in Shark Bay.