Mechanics of browsing in dense food patches: effects of plant and animal morphology on intake rate

There appear to be two dominant forms of the functional response of vertebrate herbivores, corresponding to animals foraging in spatially concentrated or spatially dispersed food patches. We examine the factors contributing to the functional response of herbivores feeding on spatially concentrated browses. Specifically, we tested the hypothesis that the instantaneous intake rate of four boreal browsers (moose, Alces alces, woodland caribou, Rangifer tarandus caribou, white-tailed deer, Odocoileus virginianus, and snowshoe hares, Lepus americanus) feeding on red maple (Acer rubrum) browse is an asymptotic function of the size of bite obtainable by the herbivore. Based on this functional response model, we also hypothesized that (i) cropping rate of the herbivores would decline curvilinearly with bite size, (ii) intake rate would decline with increasing fibrousness (measured, as the neutral detergent fiber of the foods consumed), and (iii) maximum intake rate would scale as the 2/3 power of herbivore body mass (BM). Intake rates of all animals increased 2- to 10-fold with increasing bite size of stems and leaves, and conformed to the hypothesized asymptotic relation. Similarly, cropping rate declined curvilinearly with increasing bite size, and intake rate declined with increasing fibrousness of foods consumed. Maximum intake rate scaled with BM0.76, not significantly different from the hypothesized exponent of 0.67. These data support the hypothesis that bite size affects intake rate by controlling the ratio of cropping and chewing, and that the asymptotic intake rate of herbivores is related to the food processing capability of the mouth.