Behavioural Processes 68 (2005) 215–217 Commentary Trophic transmission of parasites and host behavior modification Armand M. Kuris * Department of Ecology, Evolution and Marine Biology and Marine Science Institute, University of California, Santa Barbara, CA 93106, USA Received 18 August 2004; accepted 20 August 2004 Thomas et al. (2005) provide a wide-ranging and in- sightful review of issues surrounding the evolution and ecology of parasite manipulation of host behavior. To support their goal seeking future directions I offer three comments. Firstly, several very different types of interactions are combined under the vague label “parasite”. This ob- scures important differences with respect to host behav- ior modification. These host–parasite interactions are pathogens, parasitoids, parasitic castrators and trophi- cally transmitted parasites (TTPs). All these infectious agents may modify host behavior, but under very differ- ent selective environments (Kuris, 1997). Applying a study of one of these relationships to another without an evaluation of its applicability can often be misleading or irrelevant. For example, behavior modification by parasitoids such as nematomorphs, and parasitic cas- trators such as trematode parthenitae (Thomas et al., 2002; Curtis, 1990) cannot be confused with host de- fensive behaviors. As O’Brien has succinctly put it, a parasitic castrator has a parasite genotype, but a host phenotype (O’Brien and Van Wyk, 1985). The para- sitically castrated “host” is not being manipulated; in an evolutionary sense it is no longer present. Inves- tigations of these relationships have scant bearing on * Tel.: +1 805 893-3998; fax: +1 805 893 4724. E-mail address: kuris@lifesci.ucsb.edu. the issue of adaptation since there is no countervailing host interest. However, they are of great interest as to mechanisms of host control with respect to the time and place of death (parasitoids), and longevity and risk aversive behavior (parasitic castrators). Pathogens may well modify host behavior, often to disseminate dispersal stages. For these interactions, issues of non-adaptive pathology and host defensive responses are highly relevant. Further, these behavior modifications are usually not complex, so sophisticated molecular mechanisms seem unlikely. Hence, informa- tion from systems such as rabies must be critically ex- amined before being applied to, say, TTPs. Host behavior modifications by TTPs often meet the criteria for adaptation set out by Poulin (1995). They are often complex and usually seem “designed” to deliver prey to an appropriate predator (Kuris, 1997, 2003). Independent origin is a less relevant criterion (as long as selection can maintain the trait), and the likelihood that parasite fitness is enhanced is evident. The ability of some TTPs to mask the behavior modifi- cation from the predator is an important variant on the TTP strategy since it enables a relatively virulent para- site to enhance the probability of transmission (Kuris, 2003). Behaviour modification of ants by the lancet fluke, Dicrocoelium dendriticum, is a good example of masked behavior presumably enhancing access to grazing sheep. 0376-6357/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.beproc.2004.08.012