Faecal avoidance and the risk of infection by nematodes in a natural population of reindeer.

We tested whether Svalbard reindeer (Rangifer tarandus platyrhynchus) minimise the risk of gastro- intestinal nematode infection by avoiding patches with a high density of faeces. This experiment was performed in preferred summer foraging habitat. The possibility that reindeer assess infection risk on the basis of faecal contamination levels across plant communities was determined by measuring the distribution of faeces in seven plant communities, and nematode developmental success in two plant communities with contrasting soil moisture content. We explored whether variation within individual reindeer in the levels of infection by gastro-intestinal nematodes was related to their diet. Reindeer avoided pastures where faecal contamination was increased, and thereby potentially reduced the risk of becoming infected by Trichostrongyle nematodes. Dung density was inversely related to soil moisture content, with high densities of faeces in dry plant communities and low densities in wet communities. However, nematode developmental success was positively related to soil moisture content, and was highest in the wetter sites. Thus, by avoiding dry areas with high dropping densities, reindeer would tend to feed in wetter areas where nematodes thrive. Therefore, dung density may be an unreliable predictor of the risk of infection. The absence of a strong relationship between an individual's infection level and its diet might be due to the unpredictability of pasture infection level.