The herbivores' dilemma: trade-offs between nutrition and parasitism in foraging decisions.

An experiment was carried out using a trade-off framework to determine the rules of sward selection, in relation to gastrointestinal parasite dispersion, used by mammalian herbivores, and the effect of level of feeding motivation and parasitic status on these rules. Twenty-four sheep divided into four animal treatment groups resulting from two levels of feeding motivation (high and moderate) and two parasitic states (parasitised with Ostertagia circumcincta and non-parasitised) were presented with pairs of experimental swards which varied in N content (high and low), sward height (tall and short) and level of contamination with faeces and thus parasites (contaminated and non-contaminated). The selection for tall swards outweighed both the selection for N-rich swards and the avoidance of faecal contaminated swards. The selection for N-rich swards could not completely overcome faecal avoidance. Parasitism in animals with a moderate feeding motivation reduced their bite rates and grazing depths, thereby probably reducing the rate of ingestion of parasitic larvae. In contrast, highly feeding-motivated animals (including those parasitised) increased their bite rates and grazing depths, thereby increasing the rate of ingestion of parasites. The inclusion of parasite distributions, both in the environment and within herbivore host populations, is likely to advance optimal foraging theory by enhancing its predictive power.