more evident than in the exploitation of hosts by their parasites. For para-
sites, the habitat, for at least some part of its life cycle, is another organ-
ism. Hosts are highly discrete habitats, which will eventually deteriorate
in quality. This presents parasites with not only the need to develop trans-
mission stages capable of surviving in another environment outside the
host (perhaps even in another host species), but also the need to respond
to the changing and variable quality of its host by adjusting its resources
between growth and maintenance vs. the production of transmission
stages. This can be extended to free-living organisms, where conditions
becoming increasingly deleterious for growth and replication induce
dispersal-stage production. Thus, although the mechanisms used are vari-
able, the general underlying principles are the same – that is, organisms
use strategies (complex adaptations) to optimize the exploitation of their
current habitat in order to maximize their colonizing potential (or, in
epidemiological jargon, theirR 0 , the reproductive rate).
In the majority of sexually reproducing parasites, transmission and
sex are coupled. Parasites mature within the host and sexual reproduction
occurs either within the host or upon exit from the host prior to dispersal.
Parasites are therefore faced with the need both to assure transmission to
new hosts and to maximize reproduction. The way in which parasites
employ behavioural strategies to optimize host exploitation for repro-
ductive gain concerns specific behaviours, such as those involved in the
decision-making process of how many daughters vs. sons to produce
(dioecious spp.) or how much female vs. male investment to make
(hermaphrodites). This chapter will address the way in which parasites
utilize behaviours that influence the sex ratio to optimize reproduc-
tive success and transmission. Why parasites (or, indeed, free-living
species) should burden themselves with sexual reproduction is beyond
the scope of this chapter, but does warrant a brief appraisal to provide
perspective.Why Sexual Reproduction in Parasites?
The association between sexual reproduction and dispersal to colonize
new hosts is seemingly no accident. Theories on the evolution and
maintenance of sex tend to divide into those considering the role of
sex either in the reduction in the accumulation of deleterious mutations
or for accelerating adaptation to changing environments (Williams, 1975;
Maynard-Smith, 1978; Stearns, 1987; Michod and Levin, 1988), although
both the mutational- and environmental-based models may be at work
simultaneously (Westet al., 1999). The most popular current environ-
mental model is the ‘red queen’ hypothesis, which states that sex provides
an advantage in biotic interactions, most notably host–parasite systems
(Bell, 1982) and for which there is some evidence (Lively, 1987). The ‘red
queen’ hypothesis assumes that there is genetic variation in host–parasite200 R.E.L. Paul