much smaller microhabitats than vagile and populous species of the
same group. There is indeed evidence that microhabitat width increases
in some species when infection intensities increase, but this is very often
not the case (Rohde, 1991). Nevertheless, Adamson and Caira’s (1994)
arguments have to be taken seriously, and only a holistic viewpoint will
lead to an estimate on how widely the mating hypothesis can be applied,
taking all evidence jointly into account, e.g. general saturation of niche
space with species and abundance of populations, in addition to the
points discussed at the beginning of this section. Also, occasional
exceptions, e.g. that in the guinea-worm mating occurs prior to habitat
selection, do not prove that a rule is invalid. It is also important to note
that free-living insects, i.e. trypetid flies, were shown to use certain plants
as a meeting place for mating (Zwölfer, 1974).
Concerning the role of aggregation of parasites in host populations, it
is generally thought to reduce the net deleterious effects of parasites on
host populations (e.g. Jaenike, 1996, and further references therein), but
Jaenike (1996) has used mathematical modelling to show that, under
certain conditions, the opposite may be true. Concerning the role in
mating, May (1977) has demonstrated that, in dioecious species of
parasites, mating probability is increased if both sexes share the same
negative binomial distribution, but it is reduced if they have different
distributions. For schistosomes, the former is indeed the case. Galvani
and Gupta (1998) have examined this further and concluded that mating
probability also depends on whether worms are monogamous or promis-
cuous. In the latter case, mating probability is increased.The Relative Roles of Niche Segregation and Differences of
Copulatory Organs in Reinforcing Reproductive Barriers
To maintain their identity, i.e. to avoid interspecific hybridization,
species need isolating mechanisms, which may be differences in
microhabitats, hosts, geographical area, etc. Examples for site segregation
can be found in the section on microhabitat restriction (see, for example,
Figs 9.1 and 9.2), and an example of geographical isolation is two species
ofGrubea on the gills ofScomber spp. (legend of Fig. 9.6). Further
examples can be found in Rohde (1993). Intraspecific chemoattraction is
another factor that may be involved. The discussion above (section on
mate finding) has shown that trematodes, for example, exhibit stronger
intra- than interspecific attraction, and that selection of appropriate
sperm (i.e. sperm belonging to the same species) may occur. Synxenic
monogeneans (i.e. monogeneans infecting the same host species) are
excellent examples to demonstrate the relative importance of site
segreation and morphological differences in copulatory sclerites for the
maintenance of specific identity. Closely related species (usually belong-
ing to the same genus) that possess identical copulatory organs useNiche Restriction and Mate Finding in Vertebrate Hosts 185