Ontogenetic changes in metabolism may determine diet shifts for a sit-and-wait predator

1. Body size is a central parameter in determining prey selection, and where animals exhibit large increases in size, ontogenetic diet shifts are common. Optimal foraging models have proved to be useful tools in predicting changes in diet, but previous applications have been limited to those associated with changes in external drivers such as habitat use. 2. We conducted laboratory and field experiments to parameterize an optimal foraging model to test whether prey choice can be determined by internal mechanisms such as metabolism. 3. Model predictions were made for Pomatoschistus microps, the common goby (Pisces: Gobiidae), a sit-and-wait predator that exhibits a shift in prey choice independently of external drivers. The model predicted a change in diet at a fish length of 30 mm, which agrees well with existing empirical observations. This is the first time the link between metabolism and prey choice has been considered as a major determinant of diet shifts. 4. P. microps, using a sit-and-wait foraging mode, were expected to act as cost-minimizers but the model indicated that gobies were typically not time-limited and that prey selection maximized energy intake rates both before and after the diet shift. 5. Prey size range and mean prey length were determined primarily by the presence or absence of meiofauna in the diet, supporting the observation that the diet shift occurs between the two ecologically distinct groups of benthic meiofauna and macrofauna.