Sponge carrying by dolphins (Delphinidae, Tursiops sp.): A foraging specialization involving tool use?

During long-term research on bottlenose dolphins (Tursiops sp.) in Shark Bay, Western Australia, several individuals were observed carrying sponges, Echinodictyum mesenterinum, on their rostra. Over multiple years, five regularly sighted individuals mere usually carrying sponges when encountered (67-100% of encounters). Four additional regularly sighted individuals were observed with sponges just one time each. All five 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 restricted 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 indications of prey consumption. Three hypotheses are considered regarding 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, but the exact manner in which sponges are used remains to be discovered. Sponge carrying is a behavioural specialization, probably involving foraging and regularly engaged in by only a small proportion of female dolphins in Shark Bay.