anteriorly (Sukhdeo and Croll, 1981b). Habitat selection by the infective
larvae ofH. polygyrusis similar to that inT. spiralis, and the worms also
penetrate the gut when triggered by bile. If the entry of bile into the
intestine is surgically relocated to more posterior regions of the intestine,
the worms also move posteriorly to establish at the location of the ‘new’
bile-duct (Sukhdeo and Croll, 1981a). In normal hosts, bile is a reliable
signal that the worms have arrived in the small intestine.
Bile is not the only signal used by parasites in the gut, but, while
several components of the gut, including pepsin, trypsin and pH, have
also been shown to trigger these responses, bile appears to be the most
widely used (Lackie, 1975). Activation responses to bile have been seen in
hundreds of diverse parasites of the small intestine (Lackie, 1975), and it
has been experimentally demonstrated that bile is a critical requirement
for successful infection in numerous species (Sukhdeo and Mettrick,
1984, 1986). Nevertheless, these parasites do not orientate to bile, but
simply use it as a sign stimulus.
Fixed behaviours are also important in the habitat selection of several
cestodes and trematodes (Sukhdeo, 1990; Sukhdeo and Sukhdeo, 2002).
Even in apparently complex migrations, parasitic worms do not appear to
utilize orientation mechanisms, but, rather, take advantage of predictable
body architecture (Sukhdeo and Mettrick, 1986) or the unidirectional
flows of blood in the host. Skin-penetrating nematodes appear to have the
most complex migrations through the host body to get to the gut, but in
reality they need only migrate to the lungs, where they are coughed up
and swallowed. It appears that, for most of their migration through the
body, they are just carried passively through the pipes and tubes of the
circulatory system to the lungs. The migration of live skin-penetrating
larvae ofN. brasiliensisto the lungs was no different from the migration of
killed worms injected into small veins (Croll, 1972a,b), suggesting that
penetrating worms might require only a single behaviour that allows them
to burrow through tissue until they hit a vein, and from there they are
passively carried to the lungs, where they get trapped in the pulmonary
capillary beds (Bone, 1981).Theoretical Models of Habitat Selection
The most widely used theoretical model of habitat selection in animals is
the ideal free distribution (IFD) model, which was developed to explain
bird behaviour (Fretwell and Lucas, 1970). Subsequently, there have been
numerous tweakings and modifications of this basic model (Fretwell,
1972; Gillis and Kramer, 1987; Weber, 1998), but the underlying assump-
tions of optimality have not been altered (van der Steen, 1998; Weber,
1998). Modern behaviourists often claim that the IFD was designed to be
a null model on to which more realistic factors could be added, and
they tacitly and explicitly argue that the assumptions of the model are
too simplistic for all but the most basic or artificial sets of conditions.230 M.V.K. Sukhdeoet al.