Foraging on Prey That Are Modified by Parasites

-A model that weighs the energetic cost of parasitism for a predator against the energetic value of prey items that transmit the parasite to the predator suggests that there is often no selective pressure to avoid parasitized prey. This offers an explanation for why parasites so frequently exploit predators and prey as definitive and intermediate hosts, respectively. Furthermore, predators may actually benefit from their parasites if energetic costs of parasitism are moderate and prey capture is facilitated by parasites. Parasite species that benefit predators through modification of prey are not mutualistic, however. Although they are often ignored, parasites can affect predator-prey interactions. Many parasites exploit trophic transmission (whereby infective stages are ingested by the host). Frequently, the definitive host is a predator that preys on the intermediate host. Some protozoans, a few nematodes, many trematodes, most cestodes, and all acanthocephalans are transmitted this way. Although these parasites extract a cost from their definitive hosts, and many other costs of foraging have been suggested (Stephens and Krebs 1986), the risk of acquiring parasites is not often considered for foragers (Moore 1983). Why should predators continue to support trophic transmission? Avoiding parasitized prey would appear to be a convenient solution. Perhaps it is difficult for predators to recognize parasitized prey; alternatively, there may be no fitness advantages for predators that avoid parasitized prey. In this article, I present a foraging model that considers trophically transmitted parasites. The model compares the rate of energy gained for a predator if some prey are parasitized and the predator avoids parasitized prey, if some prey are parasitized and the predator ingests parasitized prey, and if no prey are parasitized. This model suggests that predators should not avoid parasitized prey and that they may actually benefit from the presence of parasites. Predators often take odd or unusual prey individuals (Temple 1987). Parasitized prey can be odd and are often found more frequently than expected in the diet of definitive host predators (Dobson and Keymer 1985). In fact, there is increasing evidence that larval parasites modify the behavior or appearance of intermediate hosts (see reviews in Holmes and Bethel 1972, Moore 1984, and Dobson and Keymer 1985). For example, parasitized prey may be more conspicuous, disoriented, less able to flee, or less likely to show an escape response (Holmes and Bethel 1972). Sometimes, it may be possible to quantify the effect of a parasite on its intermeAm. Nat. 1992. Vol. 140, pp. 854-867. C 1992 by The University of Chicago. 0003-0147/92/4005-0008$02.00. All rights reserved. This content downloaded from 128.111.205.74 on Fri, 05 Jun 2015 20:27:47 UTC All use subject to JSTOR Terms and Conditions FORAGING ON PARASITIZED PREY 855 diate host. Dobson and Keymer (1985) define the degree of parasite-induced behavior modification, a, as the increased rate at which prey are eaten if parasitized. For parasitized prey with no behavior modification, c = 1. With c = 2, parasitized prey are captured twice as often as unparasitized prey. I have interpreted cx as the forage ratio (Ivlev 1961) for parasitized prey divided by the forage ratio for unparasitized prey. Assuming that each parasitized prey carries one parasite (Dobson and Keymer [1985] allow multiple infections), define cx as