The Functional Response of Herbivores: Analysis and Test of a Simple Mechanistic Model

In this paper we test a simple model of herbivore functional response. The model predicts a foraging animal's average dry-matter intake rate 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 into cropping and chewing parameters that are assumed to be independent of bite size. Cropping and chewing are assumed to be mutually exclusive activities. To test the model, we fed captive giraffes hand-constructed patches of forage that varied in potential bite size and videotaped the giraffe's responses. This allowed us to estimate the mean bite sizes and intake rates achieved by the animals. We fitted the model to these data and compared parameter estimates from the regression fitting to observational estimates obtained by analysing the videotapes. We also tested the hypothesis that the parameters of the model were independent of bite size. Although we found good statistical fit between model and data, regression-derived parameter estimates did not agree well with those obtained by observation. The model's parameters were significantly related to bite size. Although we could not test it explicitly, cropping and chewing appeared to overlap. We conclude that the model does not faithfully represent the actual foraging mechanism used by our experimental animals. We propose a more flexible explanation of the functional response mechanism that partitions a herbivore's per bite handling time into fixed and incremental (per g) time costs. Both the fixed and incremental time costs may be the result of cropping and/or chewing.