THE FUNCTIONAL-RESPONSE OF HERBIVORES - ANALYSIS AND TEST OF A SIMPLEMECHANISTIC MODEL

1. In this paper we test a simple model of herbivore functional respon se. The model predicts a foraging animal's average dry-matter intake r ate as a function of the mean bite size obtained within a small patch of forage. The model partitions a herbivore's per bite handling time i nto cropping and chewing parameters that are assumed to be independent of bite size. Cropping and chewing are assumed to be mutually exclusi ve activities. 2. To test the model, we fed captive giraffes hand-cons tructed patches of forage that varied in potential bite size and video taped the giraffe's responses. This allowed us to estimate the mean bi te sizes and intake rates achieved by the animals. We fitted the model to these data and compared parameter estimates from the regression fi tting to observational estimates obtained by analysing the videotapes. We also tested the hypothesis that the parameters of the model were i ndependent of bite size. 3. Although we found good statistical fit bet ween model and data, regression-derived parameter estimates did not ag ree well with those obtained by observation. The model's parameters we re significantly related to bite size. Although we could not test it e xplicitly, cropping and chewing appeared to overlap. We conclude that the model does not faithfully represent the actual foraging mechanism used by our experimental animals. 4. We propose a more flexible explan ation of the functional response mechanism that partitions a herbivore 's per bite handling time into fixed and incremental (per g) time cost s. Both the fixed and incremental time costs may be the result of crop ping and/or chewing.