Ecological characteristics contribute to sponge distribution and tool use in bottlenose dolphins Tursiops sp.

Abstract

In Shark Bay, Western Australia, bottlenose dolphins Tursiops sp. carry conical sponges Echinodictyum mesenterinum on their rostra in the only documented cetacean foraging behaviour using a tool ('sponging'). In this study, we examined the influence of various ecological factors on live sponge distribution and the occurrence of sponging in parts of the western gulf of Shark Bay. We assessed sponge distribution and seagrass cover along 12 transects of approximately 11 km length, by recording sponges and seagrass in a total of 1380 quadrats (1 x 1 m), of which 56 quadrats contained conical sponges. The occurrence of sponging dolphins ('spongers') was documented along 10 of these 12 transects. The distribution of conical sponges was negatively correlated with the distribution of seagrass: no conical sponges were observed in water depths of 12 m. A digital elevation model, created from the sample depth data, identified channels in the region. Binary logistic and Poisson log-linear generalised linear models showed that water depth and bathymetric features including channel, substrate and slope were significant in predicting the occurrence and the mean number of conical sponges, as well as that of seagrass. Conical sponge distribution was positively correlated with the distribution of sponging, indicating that ecological factors influence where sponging occurs. The greater number of spongers found in this region may be explained by the larger area of habitat suitable for conical sponges in the western than the eastern gulf of Shark Bay.