The functional responses describe the relationship between variability in plants available and the intake rate of mammalian herbivores. Herbivores' vigilance has been assumed to reduce food intake by taking away time from food processing. Recently, however, it has been argued that herbivores might be capable maintaining their rate of food intake despite being vigilant, because of their ability to scan the environment while chewing vegetation. To evaluate the effects of vigilance as a foraging interruption on the patterns of herbivores' functional responses, a new mechanistic model of herbivores' functional responses was developed. The model is based on the hypothesis that herbivores' vigilance takes away time from food intake, and decreases the intake rate and modifies the patterns of functional responses by reducing the time spent on food processing. We conducted behavioral observations to evaluate whether vigilance decreases bite rates and intake rate of voles (Microtus fortis ) foraging in food patches with fresh clover (Trifolium repens) leaves concentrated in space.
We devised the patches 90cm×30cm for voles, in which fresh clover were uniformly spaced 2.0cm apart to scale leaves distribution, and differences in bite size were imposed by varying the size of leaf offered to voles in patches. Because we were interested in instantaneous intake rate achievable at a given bite size, trails were kept brief, typically lasting no more than 2 min. A time series of foraging behavior was obtained by videotaping foraging voles. We estimated visually the proportion of each leaf cropped in each bite, and multiply by leaf size as bite size.
Modeling of foraging process indicated that the proportion of foraging interruption time in medium and large leaf patches (15.42% 26.82%) was higher than that in small leaf patches. Although foraging interruption arising from voles’ vigilance reduced the rate of food intake by 33%, the response of intake rates to the leaf size of clover and the bite size of voleswere still a typeⅡfunctional response. Except for behavioral parameters such as cropping time and foraging bout time remaining constant with the increasing leaf size of clover, processing time and foraging interruption time all linearly increased with the increasing leaf size, and cropping time, processing time and foraging interruption time also linearly increased with the increasing bite size, but the bite rate exponentially decreased with the increasing leaf size and bite size. These results demonstrated that foraging interruption by vigilance was a main variable responsible for the decreased bite rate and intake rate. A significantly linear regressive relationship (P<0.01) was detected between the intake rates observed and those predicted by the model,indicating the model prediction was satisfactory. The foraging costs of the decreased bite rates and intake rates by vigilance were speculated to be compensated by prolonged foraging time. Validation of the models provided a strong support to the following hypotheses: (1) the leaf size could control the intake rate of voles through regulating their bite size; (2) the competition between cropping and chewing and foraging interruption by vigilance might constrain their intake rates; and (3) the response of intake rates to the plant size and bite size still belonged to the type Ⅱ functional response as in voles foraging in food-concentrated patches.