Gain Functions for Large Herbivores: Tests of Alternative Models

Abstract:

1. The gain function describes the amount of food consumed in a patch as a function of patch residence time. Gain functions play a central role in foraging theory but alternative functional forms portraying dynamics of gain have not been evaluated. We evaluated the strength of evidence in the data for alternative gain functions of mule deer (Odocoileus hemionus, Rafinesque 1817) and blue duikers (Cephalophus monticola, Blythe 1848) feeding in patches composed of different plant species and plant sizes. 2. Gain functions decelerated with patch residence time, but there was considerable variation among individual animals and patch types in the nature of this response. Asymptotic and piecewise-linear models received the greatest support in the data. 3. Deceleration in gain was caused by a composite of effects that retarded instantaneous intake rate, including reductions in bite mass and increases in bite interval (time between successive bites). Bite interval increased as a result of increases in processing time of accumulated forage in the mouth, rather than increases in time allocated to cropping. 4. We demonstrated that unwarranted assumptions about the shape of gain functions can have profound effects on predictions of patch models. Predictions of the classical patch model using purely asymptotic gain functions contrasted sharply with predictions of model-averaged gain functions that were supported by the data.