in the notion of optimality. The idea of optimal behaviour was based on a
1960s concept of the gene as an orderly library of adaptively constructed
genetic information, under which optimal responses could evolve. How-
ever, the modern genome, with its junk, non-coding, repetitive and selfish
DNA, is nothing like the old perception. It is difficult to imagine that these
imperfect genes can lead to optimal behaviour, and yet the idea of optimal
behaviour still permeates our thinking about and our modelling of animal
responses. The assumptions of optimality are rarely questioned. Failure of
the model (or its many modifications) in the real world is explained by the
presumption that animals in the experiments were not perfectly adapted
to their environments or that there was a lack of complete information
about the environment and the additional costs and benefits that compete
with the ones being investigated (see Krebs and Davies, 1978; Begonet al.,
1996). Optimality models are mere caricatures of nature and should not be
promoted for their value as yardsticks against which to compare nature.
These models are simple and easy to program into computers, are relevant
to many classes of ecological problems and are more easily testable
than any other ecological model (Weber, 1998); nevertheless, mere
convenience should not be a justification for sticking with a flawed
paradigm, especially if it precludes fresh new ideas.Resources
In all classical hypotheses on parasite habitat selection (Holmes, 1973;
Hair and Holmes, 1975; Rohde, 1981; Price, 1986; Rohde and Hobbs, 1986;
Brooks and McLennan, 1993), the role of resources is considered to be the
most critical parameter. However, there have been few efforts to identify
these putative resources in parasite systems. Resources are defined
through their use by individuals and their effects on individual fitness
(Wiens, 1984). For gastrointestinal nematodes, we generally consider
three categories of resources: mates, attachment and food. Thus, we would
need to know that changes in mates, attachment or food will influence
their reproductive fitness.
Food resources are probably the most important determinant of
habitat suitability (Partridge, 1978; Dill, 1987). The food resource of
intestinal nematodes may come from one of three compartments: host
ingesta, host tissue or host blood. However, for most parasites, we know
little of what they eat and much of what we think is based on circum-
stantial evidence (Bansemir and Sukhdeo, 1996). For instance, examples
of nematodes that feed on host ingesta have always includedH. polygyrus
(Bawden, 1969) and N. brasiliensis (Croll, 1976). In fact, utilizing
techniques that differentially label the food compartments, it has been
shown that both of these parasites feed exclusively on host epithelial
tissue (Bansemir and Sukhdeo, 1994, 2002). Similarly, the spearlike stylet
in the mouth of the human caecal whipworm Trichuris trichiura
suggested that these worms were blood feeders (Li, 1933; Chitwood and232 M.V.K. Sukhdeoet al.