How do herbivores trade-off the positive and negative consequences of diet selection decisions?

Herbivory is central to ecosystem function, and herbivores, through diet selection, exert a major influence on vegetation composition with consequent effects for associated fauna and human livelihoods. Understanding diet selection behaviour is therefore important to allow effective prediction of herbivore impacts on vegetation under different scenarios. Herbivores learn about foods by associating them with the positive postingestive effects of nutrients and the negative postingestive effects of toxins. However, the extent to which trade-offs are made between positive and negative effects, occurring simultaneously, and how this trade-off might be altered by the underlying nutritional status of herbivores are unclear. We investigated these effects using 36 goats, Capra hircus, half of which were on a low nutritional regime and half were moderately well fed. We offered three conifer species to the goats, on 3 consecutive days per week for 3 weeks. While the goats were consuming the conifers, they received one of three levels of a positive nutritional stimulus, simulating nutritional rewards, and one of three levels of a negative stimulus, simulating the effects of plant secondary compounds. Goats preferred conifers paired with positive effects and avoided those paired with negative effects in a dose-dependent way. Underlying nutritional status did not influence the response to the positive and negative postingestive effects. We conclude that diet choice in response to learning about postingestive effects represents a simple integration of the positive and negative consequences of consuming particular plants and that this balance is not greatly influenced by minor differences in nutritional status.