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

We tested whether Svalbard reindeer (Rangifer tarandus platyrhynchus) minim ise the risk of gastrointestinal nematode infection by avoiding patches wit h a high density of faeces. This experiment was performed in preferred summ er foraging habitat. The possibility that reindeer assess infection risk on the basis of faecal contamination levels across plant communities was dete rmined by measuring the distribution of faeces in seven plant communities, and nematode developmental success in two plant communities with contrastin g soil moisture content. We explored whether variation within individual re indeer in the levels of infection by gastro-intestinal nematodes was relate d to their diet. Reindeer avoided pastures where faecal contamination was i ncreased, and thereby potentially reduced the risk of becoming infected by Trichostrongyle nematodes. Dung density was inversely related to soil moist ure 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 w ould tend to feed in wetter areas where nematodes thrive. Therefore, dung d ensity may be an unreliable predictor of the risk of infection. The absence of a strong relationship between an individual's infection level and its d iet might be due to the unpredictability of pasture infection level.