Tf. Ginnett et Mw. Demment, THE FUNCTIONAL-RESPONSE OF HERBIVORES - ANALYSIS AND TEST OF A SIMPLEMECHANISTIC MODEL, Functional ecology, 9(3), 1995, pp. 376-384
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.