Herbivore Physiological State Affects Foraging Trade-Off Decisions between Nutrient Intake and Parasite Avoidance

Abstract

The trade-offs between nutrient and parasite intake for herbivores foraging in nutrient-poor systems are poorly understood. We tested whether a trade-off exists between the benefit of increased nutrient intake rate through grazing relatively tall swards and risks of parasitism in a grazing situation for sheep. The effect of level of feeding motivation and parasitic status on the grazing behavior of sheep faced with this trade-off was also investigated. Animals were presented with pairs of experimental swards (36 × 21 cm) that varied in height (12 cm = H+; 6 cm = H−) and level of contamination with feces from sheep infected with Ostertagia circumcincta (20 g feces per sward = F+; no feces = F−) and were allowed to graze for short periods. Experiment 1 presented four choices (H+F+ vs. H−F−; H+F+ vs. H−F+; H+F− vs. H−F−; H+F− vs. H−F+) repeated three times to 24 5-mo-old sheep divided into four animal treatment groups resulting from two levels of feeding motivation (high and moderate) and two parasitic states (parasitized and nonparasitized). Experiment 2 presented the above four choices three times each to 24 12-mo-old sheep in four animal treatment groups created from two parasitic states (parasitized and nonparasitized) and two immune states (immune and naive to O. circumcincta). All animals selected 12-cm swards over 6-cm swards in both experiments. In all choices except the trade-off choice, in both experiments, one sward held a clear height or feces benefit (absence of feces) that was selected for by all treatments. When presented with the trade-off choice (H+F+ vs. H−F−), animals with a high feeding motivation selected the H+F+ sward more than moderately motivated animals did in Experiment 1. Immune animals selected the H+F+ sward more than naive animals did in Experiment 2. Subclinical parasitism in moderately feeding-motivated animals resulted in increased avoidance of feces and reduced grazing depths, thereby reducing further risk of parasitism in Experiment 1. This effect of parasitism was not repeated in the older animals of Experiment 2, where all animals maintained a high-parasite-risk grazing strategy by selecting the H+F+ sward of the trade-off choice. However, parasitized animals in Experiment 2 reduced parasite intake and, therefore, the risks associated with grazing the H+F+ sward by significantly reducing their grazing depth compared to all other treatment groups. Sheep grazing decisions involving forage intake and feces avoidance were affected by their feeding motivation, immune state, and parasitic state. Strong avoidance of feces-contaminated pasture can be overcome by the attraction of herbivores to tall swards. Trade-off frameworks are able to predict diet selection and grazing behavior, are useful when investigating host–parasite interactions, and may enhance the predictive powers of optimal foraging models.