Relative prey size consumption in toothed whales: implications for prey selection and level of specialisation

We investigated whether toothed whales consume prey in relation to their availability in the local environment based on the fact that availability of potential prey is likely to decrease exponentially with increasing size, reflecting the usual size-abundance relationships found in marine communities. We calculated relative prey size frequency spectra for 13 species of toothed whale from the northeast Atlantic. These differed considerably from an exponential distribution, suggesting that toothed whales preferentially consume larger, less abundant organisms over smaller, more abundant ones. The prey size spectra of the various cetacean species could be separated into 3 distinct groups based on the strength of the mode, maximum value and inter-quartile range. Group 1 species, such as the common dolphin, consume a wide range of relatively large organisms. In contrast, Group 2 and 3 species, such as the northern bottlenose whale and the sperm whale respectively, specialise on narrow ranges of relatively small organisms. We hypothesise that these differences are related to the mode of prey capture. Group 1 species can capture prey using pincer-like movement of jaws containing a large number of small, homodont teeth, as well as suction-feeding, allowing them to be relatively generalist in terms of relative prey size. In contrast, Group 2 and 3 species have a greatly reduced dentition and specialise on using suction to capture prey. The morphological adaptations that make suction-feeding more efficient restrict the size of prey that can be ingested, so that suction-feeders are limited to relatively small prey.