Fróðskaparrit - 01.01.2002, Page 140

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ECOLOGY OF ENDOPARASITES OFTHE ATLANTIC PUFFIN
these two studies are approximately the
same with respect to both prevalence and
intensity of infestation.
Host fitness
There is no indication that the fitness of the
birds is negatively associated with parasite
infestation. As the intensity is relatively
low, it is possible that the parasite burden is
below the threshold value necessary to have
an influence (Hoberg and Ryan, 1989).
There have even been speculations that
trematodes in frnal hosts may not be para-
sitic at all. Lafferty (1997) claimed that
adult trematodes have no negative effect on
the final host (piscivorous birds). He even
argued that frnal hosts may take advantage
of trematodes, as the metacercariae change
the behavioural pattern of the intermediate
host so rendering it more accessible to the
birds. This type of symbiosis might be cat-
egorized as commensalism, rather than par-
asitism.
There is a higher mean intensity of ne-
matode infestation in puffins with the high-
est fitness. As many nematode infested
birds had damaged tissue in the anterior
part of the oesophagus, evidently caused by
the worms, it seems unlikely that the high-
er fítness is a direct effect of the parasites.
The same pattern has been reported for her-
ring infested with Contrcicaecum oscula-
tum larvae (Podolska et al., 1997). This
trend is probably the result of a higher feed-
ing rate. It seems that the nematodes, at low
intensities of infestation, are harmless to
their hosts.
The low fitness of a great black-backed
gull (Larus marinus), with a high intensity
of the pentastome Reighardia sternae
(Bockeler and Vauk-Hentzelt, 1979), was
not confirmed in the puffin with a high in-
tensity in the present study, and the mean
fitness was not reduced in pentastome- in-
fested birds, in agreement with Bockeler
(1984). The hypothesis that cestode infes-
tation leads to natural immunity to pentas-
tome infestation (Riley, 1976; Bockelerand
Vauk-Hentzelt, 1979) is supported by the
present study, as none of the cestode-infest-
ed puffins hosted pentastomes.
Host gender
The sexual bias in the relative intensity of
infestation with trematodes (signiilcant)
and cestodes (non-significant), with males
more heavily infested than females is a puz-
zle. There is no evidence that the higher rel-
ative intensity of infestation with these
helminths is caused by a differentiation in
behaviour in relation to reproduction, as the
incubation and the feeding of the chick is
split equally between the parents (Harris,
1984; Nettleship and Birkhead, 1985), and
there does not appear to be evidence of a
foraging differentiation in the literature.
When birds of both sexes move and feed to-
gether they are probably exposed equally to
larval parasites. Consequently, if other
things are equal, differences in parasite bur-
dens should be rare. However, there are
studies showing that males and females do
differ in susceptibility to parasites, males
being the more susceptible (Alexander and
Stimson, 1989; Zuk, 1990; Poulin, 1996).
One possible explanation is that the effi-
ciency of the immune system is determined
by migration behaviour in the pre-repro-