FORAGING BEHAVIOUR OF THE INTERTIDAL BEETLE THINOPINUS PICTUS (STAPHYLINIDAE)

Optimal foraging models generally assume that predators are capable of making appropriate foraging decisions and that these decisions affect fitness. These assumptions were tested in a study of the intertidal beetle Thinopinus pictus LeConte (Staphlyinidae). Adult beetles live on sand beaches in temporary burrows from which they emerge at night to prey on amphipods Orchestoidea californiana (Brandt). Some data were also presented for isopods Alloniscus perconvexus Dana, a less important prey species. I measured amphipod activity patterns by pitfall trapping, and beetle activity patterns by direct counts of the number of beetles active on the beach in 1-h searches. There was a good correspondence between beetle and amphipod temporal and spatial activity patterns. However, by manipulating the spatial distribution of prey, I showed that beetles arrived at foraging sites independently of prey availability. Prey capture rate was low, with a mean of 75 min between captures, so that beetles were not always successful in obtaining food during a night. Food deprivation for up to 4-d intervals did not affect beetle survival or oviposition rates in laboratory experiments.;An optimal diet model was presented for two prey types, based on the expected foraging time required for a predator to reach satiation. Predictions differed in some cases from a model based on maximization of the rate of energy intake. Foraging time was minimized by a predator which began as a specialist and then expanded its diet to include lower value prey when it was near satiation. Laboratory experiments on beetles supported these predictions.;Models of amphipod size selection by beetles were constructed using the size distributions of amphipods measured on the beach, and the results of laboratory experiments on capture success, reaction distance and feeding rates. Capture success decreased and the probability that an amphipod was detected increased with increasing amphipod size. Beetles observed during beach searches selected larger sizes of amphipods than predicted by availability of different sizes or by an optimal foraging model.