Browse selection in response to simulated seasonal changes in diet quality through postingestive effects.

Browse species undergo seasonal changes in nutritional value and secondary plant compound concentrations. The capacity of herbivores to monitor such change through postingestive effects and to modify their food choice appropriately was investigated. Twenty-four goats were offered a different conifer species on four successive learning days per 7-d period for six periods. During conifer consumption on learning days, animals received either a positive or a negative digestive stimulus to simulate the nutritional rewards and toxic consequences of browse consumption. For each animal, a different postingestive stimulus treatment was associated with each conifer species. The treatments consisted of an increasing positive stimulus, a decreasing positive stimulus, an increasing negative stimulus, or a decreasing negative stimulus. The levels of the stimuli were adjusted in 20% increments in successive periods (from 0 to 100% for the increasing treatments and from 100 to 0% in the decreasing treatments) to simulate seasonal changes in browse characteristics. Diet preference was measured on d 5 of each period. Animals adjusted their diet choice in response to the changing intensity of the negative stimulus, but not the positive stimulus. Animals avoided foods associated with the negative stimulus to a greater extent when the stimulus was increasing each period compared to when it was decreasing. The results suggest that herbivores are adept at monitoring and responding to temporal change in secondary compound concentrations through assessing their changing postingestive effects and adjusting their food choice accordingly.