Chitwood, 1937), but, in fact, their mouth-parts are never near any blood
source (Lee and Wright, 1978). Clearly, we need to be more rigorous in our
assumptions about how parasites feed, especially since this behaviour can
have significant impacts on their habitat selection.
Altering food availability in these parasite can be difficult. When the
host is fasted, there is a significant reduction in the individual fecundities
of the female worms ofH. polygyrus(Sukhdeo and Bansemir, 1996) and
there is also a significant change in their intestinal distribution patterns
(Bansemir and Sukhdeo, 1996). These worms use intestinal villi both for
attachment (wrapping around) and as food, and the length of the villi
is the most important determinant of their habitat selection (Bansemir
and Sukhdeo, 1996). Thus, it is difficult to separate the effects of food
reduction and attachment effort in these worms, because starving the host
causes shortening of the villi and the worms respond by migrating to the
longest villi with the better holdfasts. Nevertheless, attachment resources
appear to be more important than mates in this parasite because, when
males and females are surgically transplanted to the posterior small
intestine (short villi), the smaller males leave the larger females behind as
they scramble towards the long villi in the duodenum (Sukhdeo and
Bansemir, 1996). Eventually the females do catch up, but this response
suggests that, in these long-lived worms, secure holdfasts take precedence
over mates. In short-lived worms, these decisions may be different. For
example, in bothT. spiralisandN. brasiliensis, mates are very important,
and the male worms will actively search for their females, regardless of
other conditions related to attachment or food (Sukhdeo and Croll, 1981a;
Bansemir and Sukhdeo, 2002).
Clearly, future models of habitat selection in intestinal nematodes
will have to recognize that each species has distinct priorities and that
their behavioural strategies must be considered in light of the constraints
imposed by their life histories.Life-history Constraints
It is not always intuitive how life history may shape habitat selection
behaviour in intestinal nematodes. At a gross level, it seems clear that the
manner in which the parasites transmit themselves to the next host can
have a significant effect on habitat selection. For example,H. polygyrus’s
transmission strategy involves shedding infective stages into the faecal
stream; thus a luminal habitat may be more appropriate than a tissue
habitat, such as that ofT. spiralis, which is transmitted by carnivory. In
fact, life-history constraints may exert effects at even finer scales. In these
nematodes, habitats are very narrow, and significant differences in female
fitness can occur over very short distances within the gut (Sukhdeo, 1991;
Bansemir and Sukhdeo, 1996). Several theoretical models suggest that a
trade-off might exist between a parasite’s virulence (a direct effect of using
host resources for parasite reproduction) and its transmission successIntestinal Nematode Parasites of Vertebrates 233