LATITUDINAL PATTERNS IN CONSUMPTION OF WOODY GIANTS BY SNOWSHOE HARES IN THE EASTERN UNITED-STATES

Previous studies have documented large-scale spatial variation in palatability of woody plants to snowshoe hares, Lepus americanus, but clear latitudinal gradients have not been demonstrated. We conducted a series of feeding trials designed to compare palatabilities of woody plants that differed either in latitude of collection, geographic distribution, or stage of development. Tests were conducted using three species of birches (Betula) and two species of aspens (Populus) found in the eastern United States within the range of snow/shoe hares. When presented with conspecific juvenile- and mature-stage twigs of species with predominantly boreal distributions, hares generally avoided juvenile-stage twigs, whereas little discrimination was evident between juvenile- and mature-stage twigs with nonboreal distributions. When presented with a choice of congeneric juvenile-stage aspens, hares avoided the twigs of the boreal species. No differences in consumption were noted among birches; however, Bryant et al. have observed an avoidance of boreal birches by hares in Alaska and Connecticut. Finally, when presented with conspecific twigs collected at different latitudes, hares did not discriminate among mature-stage twigs. However, discrimination did occur when hares were offered juvenile-stage twigs; twigs from northern latitudes were eaten less than twigs from southern latitudes, and this trend was more pronounced for species with predominantly boreal distributions. It is doubtful that the observed patterns were generated solely, or even primarily, by latitudinal gradients in temperature affecting either bioenergetics of hares or chemicals associated with cold hardiness in plants. Rather, the available evidence suggests that the elevated levels of avoidance exhibited by hares for juvenile plants with predominantly boreal distributions have resulted from more intense selection for chemical defense against herbivory in winter-dormant plants occupying boreal areas. Colder temperatures, by limiting nutrient availability, also could increase the selection pressure for antiherbivore defenses in boreal areas by magnifying the cost of herbivory.