Selection and consumption of food patches in mammalian herbivores depend not only on the availability of patches, but also on the latent risks around. Whether predation risk affect the patterns of functional responses of herbivores by influencing their vigilance in foraging is still unknown. To evaluate the effects of predation risks on patterns of functional responses of voles (Microtus fortis), we measured behavioral parameters of the voles' foraging in food concentrated patches consisting of fresh leaves of clover (Tritolium repens), and tested a newly constructed mechanistic model of functional responses.
Modeling of foraging processes indicated that predation risk significantly prolonged foraging decision time of voles. The intake rate in different patches was similar, the patterns of functional responses remained unchanged, and were still typeⅡfunctional responses. Except for cropping time in controls remaining stable with the increasing leaf size of clover, cropping time in treatments, processing time and interrupting time in both controls and treatments increased linearly with increasing bite size. The interruption time of the treatments was obviously longer than that of the controls; bite rates in both controls and treatments decreased nonlinearly, but the bite rates in treatments were slightly lowered when compared with those in controls.
These results demonstrate that, although the variation of these behavioral parameters of voles under the pressure of predation risk are likely to depress intake rates, voles are able to modify the frequency of vigilance actions such as lessening the smelling and listening action and mounting the staring action, so as to effectively buffer the pressure of predation risk, thus can maintain their intake rates. A significantly linear regressive relationship was detected between the intake rate observed and that predicted by the model, indicating the predictability of the model is satisfactory. Validation of the models provided a strong support to the following hypotheses: vigilance in herbivores' foraging is capable to buffer the pressure of prediction risks; the intake rate of herbivores is controlled by plant sizes through regulating their bite sizes, and is also constrained by competition between cropping and chewing and foraging interruption at vigilance; the responses of intake rates to plant sizes and bite sizes are still a type Ⅱ functional response as voles foraging in food-concentrated patches.