an increase in the laceration of the mucosa and subsequent blood loss
as the males migrate more actively to find the females (Beaveret al.,
1964). Males and females of several species will find each other, even if
inoculated at separate locations in the gut (Beaver, 1955; Roche, 1966;
Sukhdeo and Meerovitch, 1977). Laboratory studies using specially
designed orientation chambers have conclusively demonstrated that the
sexes are attracted to each other (Bonner and Etgers, 1967; Anya, 1976;
Bone and Shorey, 1977; Belosevic and Dick, 1980).
Clearly, these worms possess the mechanisms for orientation, and
these orientation responses are undoubtedly adaptive in mate finding
over short distances in the gut. (Turbulence from peristalsis would
make gradients unlikely over large distances.) Surprisingly, however,
orientation responses have not been demonstrated in the habitat-selection
strategy of parasites: that is, parasitic worms do not orientate to any signal
within their hosts except those coming from potential mates (Sukhdeo
and Sukhdeo, 1994; Sukhdeo, 1997). This is odd because the classic
mechanistic explanation for why parasite habitat selection is so precise
is that the worms are attracted to specific signals emanating from
their habitats (Bone, 1981). Yet, despite intense efforts to identify
these attractive signals during habitat selection, none have been found
in parasites (Sukhdeo, 1990; Sukhdeo and Sukhdeo, 2002).Releaser Responses
On entering the intestines of their hosts, nematode infective stages are
usually carried passively to their sites of penetration. The encysted stages
ofT. spiralisare digested in the stomach and, stimulated by pepsin, the
worms whip their tails around to help break out of the cysts (Fig. 11.1a).
These tail-whipping worms are then carried passively with the stomach
contents into the small intestine, where bile is secreted into the very first
section (duodenum). Bile triggers an instantaneous change in the worms’
behaviour and they enter into a frenetic sinusoidal behaviour pattern
(Fig. 11.1b). This behaviour is characteristic of genetically fixed behaviour
patterns called releaser responses (Lorenz and Tinbergen, 1957), and it
makes the worms automatically migrate out of the gut contents and pene-
trate into the gut wall (Sukhdeo, 1990). The worms will even penetrate
abnormal habitats in the large intestine when triggered by bile. Once trig-
gered, this response will proceed unabated for hours, until the worms run
out of energy and die (Sukhdeo and Croll, 1981b). At this stage, there is no
need for the worms to conserve energy because, if they do not successfully
penetrate, they will be swept out with the faeces.
Releaser responses are fixed stereotyped activities (occur in the same
way every time) that are triggered by sign stimuli, in this case, bile. These
responses generally evolve in response to environmental or behavioural
conditions that are predictable or constant, and they are triggered by sign
stimuli that are unambiguous (Lorenz and Tinbergen, 1957). For example,228 M.V.K. Sukhdeoet al.