DIET SELECTION BY A RUMINANT GENERALIST BROWSER IN RELATION TO PLANT CHEMISTRY

Captive mule deer (Odocoileus hemionus hemionus) and black-tailed deer (O, h. sitkensis) were used in cafeteria-type, two choice feeding tri als to test the hypothesis that digestible dry matter (energy) and non tannin phenolics of tree, shrub, and forb leaves are major determinant s of diet preference. Deer selected plants in relation to a trade-off between the benefit derived from digestible dry matter and the cost of nontannin phenolics presumably associated with toxicity when absorbed . When one of the forages contained both the highest digestible dry ma tter and lowest nontannin phenolics, the deer always preferred that pl ant. When one forage had the highest digestible dry matter but the oth er plant had the lowest nontannin phenolics, the deer selected the hig h-energy plant when the difference in nontannin phenolics was relative ly small, but they preferred the low-energy plant when the other forag e had much higher levels of nontannin phenolics. Tannins influenced di et. choice only as one of the factors reducing digestible dry matter i n these forages. Apparently total dry-matter intake was constrained by the nontannin phenolic fraction but not by tannins. Tannins and nonta nnin phenolics both contribute to defending plants against browsers.