the dot on a parent bird’s beak releases a fixed begging behaviour from the
hatchling, or the red colour of a male adversary in some fish elicit a fixed
fighting response (Alcock, 1998). Host intestines are extremely predict-
able environments to an invading parasite because, in every individual
of the same host species, intestines are functionally and physically
identical. Hosts have predictable diets, moulded by natural selection, and
the design of the gut is intimately connected to the predictable process of
digesting these foodstuffs. Under these predictable conditions, releaser
responses may provide the optimal solutions.
The infecting worms travel passively in the gut and thus the locations
where the worms penetrate are a function of how fast the gut is flowing
when they are triggered. If intestinal transit is artificially increased, the
worms will establish significantly more posteriorly than normal controls
and, when gut peristalsis is slowed down, the worms establish moreIntestinal Nematode Parasites of Vertebrates 229
Fig. 11.1. Frame-by-frame analysis of the movement patterns of infective larvae of
Trichinella spiralis. The sequence moves from top left to bottom right, the anterior
end of the worm is depicted by an arrowhead and the elapsed time between each
frame is 0.017 s. (a) Movement pattern of a larva in the stomach showing coiling
and uncoiling of the tail. (b) Movement pattern of a larva that has just been triggered
by bile, as it changes from the coiling pattern into a rapid sinusoidal pattern.