Divergent water requirements partition exposure risk to parasites in wild equids

For grazing herbivores, dung density in feeding areas is an important
determinant of exposure risk to faecal-orally transmitted parasites. When
host species share the same parasite species, a non-random distribution of
their cumulative dung density and/or non-random ranging and feeding
behaviour may skew exposure risk and the relative selection pressure
parasites impose on each host. The arid-adapted Grevy’s zebra (Equus
grevyi) can range more widely than the water-dependent plains zebra (Equus
quagga), with which it shares the same species of gastrointestinal
nematodes. We studied how the spatial distribution of zebra dung relates
to ranging and feeding behaviour to assess parasite exposure risk in
Grevy’s and plains zebras at a site inhabited by both zebra species. We
found that zebra dung density declined with distance from water, Grevy’s
zebra home ranges (excluding those of territorial males) were farther from
water than those of plains zebras, and plains zebra grazing areas had
higher dung density than random points while Grevy’s zebra grazing areas
did not, suggesting a greater exposure risk in plains zebras associated
with their water dependence. Faecal egg counts increased with home range
proximity to water for both species, but the response was stronger in
plains zebras, indicating that this host species may be particularly
vulnerable to the elevated exposure risk close to water. We further ran
experiments on microclimatic effects on dung infectivity and showed that
fewer nematode eggs embryonated in dung in the sun than in the shade.
However, only 5% of the zebra dung on the landscape was in shade,
indicating that the microclimatic effects of shade on the density of
infective larvae is not a major influence on exposure risk dynamics.
Ranging constraints based on water requirements appear to be key mediators
of nematode parasite exposure in free-ranging equids.