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.Please see the README_Tombak_ZebExpRisk.txt file attached.Funding provided by: National Science FoundationCrossref Funder Registry ID: http://dx.doi.org/10.13039/100000001Award Number: IBN-0309233Funding provided by: Princeton UniversityCrossref Funder Registry ID: http://dx.doi.org/10.13039/100006734Award Number: Funding provided by: National Science FoundationCrossref Funder Registry ID: http://dx.doi.org/10.13039/100000001Award Number: IIS-0747369Funding provided by: National Science FoundationCrossref Funder Registry ID: http://dx.doi.org/10.13039/100000001Award Number: IIS-0705822Funding provided by: National Science FoundationCrossref Funder Registry ID: http://dx.doi.org/10.13039/100000001Award Number: IOB-9874523Funding provided by: National Science FoundationCrossref Funder Registry ID: http://dx.doi.org/10.13039/100000001Award Number: CNS-025214Please see the Materials and Methods section of the open access article associated with these data (in press in Ecology and Evolution). Raw data were processed as described in the file descriptions (under 'File List') and as described in the descriptions of the variables in each file (under 'Data Specific Information') provided in the README_Tombak_ZebExpRisk.txt file attached