the decision to feed on a particular host. However, the parasite may
also influence the behaviour of the vector to increase the chances of
transmission (Hamilton and Hurd, Chapter 13).Parasite Behaviours within the Host
The proximate behaviours of parasites within their hosts remain one of
the least understood, but most intriguing, aspects of their biology. For
example, consider the tortuous migrations through the host that many
parasites make to get to specific tissues and organs. The adaptive value
of these behaviours often appears to be related to some aspect of the
parasite’s transmission. Thus,Schistosoma mansoni, the human blood
fluke, migrates to the gut because of life-cycle requirements to shed its
infective stages in the host’s faecal stream.Dicrocoelium dendriticum,
the sheep fluke, migrates to the brain of its intermediate host to alter
its behaviour and increase its chances of being eaten by the parasite’s
definitive host. Numerous examples of this strategy are presented in
this book (Lafferty, Chapter 8; Poulin, Chapter 12; Hamilton and Hurd,
Chapter 13). However, it is not at all clear what mechanisms of parasite
locomotion and navigation are used in these journeys through the host. In
addition, there are several other equally mysterious parasite behaviours
that occur within the host. For example, it is not obvious how parasitoid
larvae might recognize and then actively seek and destroy competitors in
their insect hosts, eggs or seed (Messina, Chapter 4; Strand, Chapter 7).
In almost all cases, the host is treated as a black box, and parasite
behaviours are usually defined and interpreted only within an ultimate
context. However, the lack of proximate details illustrates the challenges
to understanding the behaviour of parasites in their hosts. The major
obstacles relate to the technical difficulty of observing parasitesin situ.
Almost all of our understanding of parasite proximate behaviours comes
from observations made after the host has been opened and the parasites
removed. These actions create dramatic changes in the parasites’ environ-
ment, which render classical behavioural studies virtually impossible.
Additionally, parasite responses may be state-dependent, and their
proximate strategies will change with changing conditions in the host. For
example, tapeworms alter their reproductive strategies in response to
food deprivation in the host (Sukhdeo and Bansemir, 1996), and blood
flukes reduce their blood feeding and egg production when in hosts with
immune systems weakened by bacterial or viral infections (Davieset al.,
2001).
An implicit assumption of most investigators is that parasite
behaviours within the host are not significantly different from parasite
behaviours outside the host. As several chapters in this book demonstrate,
parasite behaviours in the free-living environment are exquisitely linked
in ecological time and space to the behaviours of their hosts. Many of
these behaviours are similar to the behaviours of free-living organisms342 E.E. Lewiset al.