Optimal foraging by predaceous diving beetle larvae on toad tadpoles

The feeding behavior of predaceous beetle larvae (Dytiscus fasciventris) on American toad (Bufo americanus) tadpoles was investigated in the laboratory. The rate of food extraction from an individual tadpole decreases as the prey item is being depleted. Beetles hand fed single tadpoles were shown to have a significantly longer mean handling time than beetles capturing similar size tadpoles from constant prey densities of 5 or 20 tadpoles/l. Handling times were not correlated with mean intercatch intervals but were found to decrease throughout a 10-tadpole catch sequence at all prey densities. Beetles allocated less time per unit weight to large prey items than to smaller tadpoles. However, the marginal ingestion rate (amount consumed/unit of handling time) did not differ with prey size throughout a 10-tadpole catch sequence. These results support the rate expectation hypothesis (Charnov 1976) as an explanation for time allocation to patches.