Use of trade-off theory to advance understanding of herbivore-parasite interactions

1. Trade-off theory has been extensively used to further our understanding of animal behaviour. In mammalian herbivores, it has been used to advance our understanding of their reproductive, parental care and foraging strategies. Here, we detail how trade-off theory can be applied to herbivore-parasite interactions, especially in foraging environments. 2. Foraging is a common mode of uptake of parasites that represent the most pervasive challenge to mammalian fitness and survival. Hosts are hypothesized to alter their foraging behaviour in the presence of parasites in three ways: (i) hosts avoid foraging in areas that are contaminated with parasites; (ii) hosts select diets that increase their resistance and resilience to parasites; and (iii) hosts select for foods with direct anti-parasitic properties (self-medication). We concentrate on the mammalian herbivore literature to detail the recent advances made using trade-off frameworks to understand the mechanisms behind host-parasite interactions in relation to these three hypotheses. 3. In natural systems, animals often face complex foraging decisions including nutrient intake vs. predation risk, nutrient intake vs. sheltering and nutrient intake vs. parasite risk trade-offs. A trade-off framework is detailed that can be used to interpret mammal behaviour in complex environments, and may be used to advance the self-medication hypothesis. 4. The use of trade-off theory has advanced our understanding of the contact process between grazing mammalian hosts and their parasites transmitted via the faecal-oral route. Experimental manipulation of the costs and benefits of a nutrient intake vs. parasite risk trade-off has shown that environmental conditions (forage quality and quantity) and the physiological state (parasitic and immune status) of a mammalian host can both affect the behavioural decisions of foraging animals. 5. Naturally occurring trade-offs and the potential to manipulate their costs and benefits enables us to identify the abilities and behavioural rules used by mammals when making decisions in complex environments and thus predict animal behaviour