The gain function, which describes the amount of food consumed by mammalian herbivores in a plant patch as a function of their residence time in the patch, is the central part of the foraging theory. In this study, the foraging behavior of voles (Microtus fortis) was observed in the patches of fresh clover (Trifolium repens) leaves, the models for their energy gain functions were developed, and the mechanism for decelerating energy gains in mammalian herbivores was validated. The results showed an asymptotic gain function for the voles foraging in the large leaf patches, whereas a linear gain function for the voles foraging in the small and medium leaf patches. No piecewise linear or sigmoid gain functions were detected in the voles foraging in the patches. Behavioral parameters regulating instantaneous intake rates changed dynamically with the vole′s residence time in the patches: in the medium and large leaf patches, bite sizes linearly or exponentially decreased, but processing time linearly or exponentially increased with increasing residence time, although the cropping time, interval time and chewing frequency remained constant. Instantaneous intake rates also declined with increasing residence time of the voles in the patches. In small leaf patches, the behavioral parameters including bite sizes, instantaneous intake rates, cropping time, interval time, processing time and chewing frequency remained constant regardless of the voles′ residence time in the patches. Validation of these models provided a strong support to the hypothesis that deceleration in the energy gain in mammalian herbivores resulted from a decrease in instantaneous intake rates due to reduced availability of large plants, decreased bite sizes and increased processing time as the patches were depleted.