Parasite Manipulation of Host Behaviour

Parasite Manipulation of Host 12

Behaviour

Robert Poulin

Department of Zoology, University of Otago, PO Box 56, Dunedin,

New Zealand

Introduction

Parasites of all kinds are known to modify the behaviour of their hosts in

ways that appear to enhance the parasite’s chances of completing its life

cycle (Holmes and Bethel, 1972; Moore and Gotelli, 1990; Poulin, 1995,

1998a). This phenomenon can be seen as the extended phenotype (sensu

Dawkins, 1982) of the parasite, i.e. as parasite genes being expressed in

host phenotypes. Because it is the product of natural selection and not

mere pathology, parasite manipulation of host behaviour should be a

good area of focus for behavioural ecologists. A simple definition of

behavioural ecology is that it is the study of the evolution of behaviour in

relation to ecology. Textbooks of behavioural ecology (e.g. Krebs and

Davies, 1997) do not specifically address host manipulation by parasites,

but they present the theoretical framework necessary to approach this

phenomenon with more rigour than it has received in the past.

Consider the following examples. An orb-weaving spider suddenly

begins to build a strange web hours before the parasitic wasp larva it has

harboured for several days emerges from the spider; the strange web

will serve to protect the wasp larva from being swept away by heavy rain

after the spider’s death (Eberhard, 2000). Male mayflies infected by a

mermithid nematode develop female morphological traits and adopt a

female behaviour: they return to water to oviposit, but release only the

eggs of the parasite they carry (Vance, 1996). Rats infected by the proto-

zoan parasiteToxoplasma gondiishow an imprudent attraction to the

odours of cats, the parasite’s definitive host, and not the innate aversion to

such odours typical of uninfected rats (Berdoyet al., 2000). In each case it

has been shown that the parasite causes the behavioural change. We can

then ask the question: why does the parasite alter the behaviour of its

host?

CABInternational2002.The Behavioural Ecology of Parasites
(eds E.E. Lewis, J.F. Campbell and M.V.K. Sukhdeo) 243