Herbivore diet selection in response to simulated variation in nutrient rewards and plant secondary compounds

The interaction between the sensory properties of food plants (i.e. taste and smell) and their postingestive consequences is thought to be an important mechanism by which browsing herbivores learn about the toxic and nutritional properties of foods. The extent to which this mechanism is important when multiple food options are available, each differing in concentrations of both nutrients and plant secondary metabolites, is little known and was tested using goats as a model browsing herbivore. Twenty-four juvenile female goats, Capra hircus, were offered branches of four conifer species, with one species being offered per day on 4 successive conditioning days per week for 5 weeks. On conditioning days, animals were dosed orally with a mixture of two stimuli according to their treatment allocations. Treatments consisted of combinations of a positive (sodium propionate) and a negative (lithium chloride) postingestive stimulus, both imposed at one of four levels. Preference was assessed on the fifth day by offering all species simultaneously for 20 min and measuring relative consumption. From the first week, goats responded in a dose-dependent fashion to lithium chloride by reducing their relative preference of associated conifer species as the dose increased. However, they did not modify their choices with the variation in sodium propionate intensity. Comparisons with other experiments which did not use the conditioning method suggest that in complex situations with many stimuli, animals may need additional preingestive cues to perceive the whole value of the food, particularly for assessing nutrient rewards of food plants