A predator's dilemma: prey choice and parasite susceptibility in three-spined sticklebacks.

The acanthocephalan parasite Pomphorhynchus laevis is transmitted by a suitable intermediate host such as the amphipod Gammarus pulex to its definitive fish host. Parasite-induced alterations in both appearance and behaviour concur to render infected gammarids more vulnerable to predation, thus promoting parasite transmission. Experimental infection of laboratory bred full- and half-sib groups of three-spined sticklebacks (Gasterosteus aculeatus) provided evidence that the parasite imposes a survival cost proportional to the severity of infection on its final host. Variation among sibships in the susceptibility to infection was consistent. When given a choice, fish consumed significantly more infected than uninfected prey. Overall, more resistant fish did not prey upon infected gammarids more often than did relatively susceptible fish. Only fish with a relatively high physical condition properly adjusted prey selection to the extent of their parasite susceptibility, thus exploiting the enhanced profitability of infected prey.