their insect hosts go to places where the release and dispersal of fungal
spores will be facilitated (Evans, 1988; Maitland, 1994). Mermithid
nematodes can force their terrestrial insect hosts to enter water, where the
parasite and/or its eggs must be released (Maeyamaet al., 1994; Vance,
1996). Malaria and other vector-borne protozoans can modify the flight
and probing behaviour of mosquitoes to enhance their transmission to
new hosts (Rossignolet al., 1984; Rowland and Boersma, 1988). The
majority of examples, however, involve helminth parasites with complex
life cycles, in which transmission from an intermediate host to the next
host occurs via predation. Many parasites with such life cycles are known
to alter the behaviour of their hosts in ways that increase their susceptibil-
ity to predation by the next host in the cycle. The following discussion
bears mainly on these parasites.
Three points need to be made about the available empirical support
for the functional role of host manipulation by parasites. First, the vast
majority of studies made specific predictions about the type of behav-
ioural changes they expected parasites to induce in their hosts, but not
about their magnitude. Without quantitative predictions, it is difficult to
assess how much these changes in host behaviour are the product of
cost-effective natural selection. Secondly, the magnitude of published
estimates of changes in host behaviour induced by parasites has been
steadily decreasing over the years (Poulin, 2000). The existence of the
phenomenon of host manipulation by parasites is not in doubt; however,
changes in host behaviour resulting from infection appear much more
subtle on average than the first, and still widely cited, examples to be pub-
lished. Thirdly, small changes in host behaviour can result in substantial
increases in parasite transmission. For instance, Lafferty and Morris
(1996) showed that larval trematodes that induced a fourfold change in
fish behaviour benefited from a 30-fold increase in their transmission rate
(by predation) to bird definitive hosts. In general, published estimates of
parasite-mediated behavioural changes tend to be rather small, whereas
estimates of how these changes translate into enhanced transmission
success provide more striking results (Fig. 12.1). Whereas most studies
focus on proximate changes in host behaviour induced by parasites, it is
their ultimate effect on parasite transmission that is under selection.
Behavioural ecology provides some powerful conceptual tools, allow-
ing rigorous, quantitative predictions to be made and tested in respect of
when a parasite should manipulate its host and how strongly. Here, I shall
discuss how the three theoretical frameworks forming the cornerstones of
behavioural ecology (optimality theory, game theory and evolutionarily
stable strategies, and kin selection) can be used to study host manipu-
lation by parasites.
Optimality models are based on the premise that natural selection is
an optimizing agent, favouring behavioural strategies that best promote an
individual’s reproductive success. Taking an approach borrowed from
economics, optimality models weight the various strategies available to
an organism in terms of their costs and benefits, and make predictionsParasite Manipulation of Host Behaviour 245