A penny saved is a penny earned: lean season foraging strategy of an alpine ungulate

Abstract

Lean season foraging strategies are critical for the survival of species inhabiting highly seasonal environments such as alpine regions. However, inferring foraging strategies is often difficult because of challenges associated with empirically estimating energetic costs and gains of foraging in the field. We generated qualitative predictions for the relationship between daily winter foraging time, body size and forage availability for three contrasting foraging strategies including time minimization, energy intake maximization and net energy maximization. Our model predicts that for animals employing a time minimization strategy, daily winter foraging time should not change with body size and should increase with a reduction in forage availability. For energy intake maximization, foraging time should not vary with either body size or forage availability. In contrast, for a net energy maximization strategy, foraging time should decrease with increase in body size and with a reduction in forage availability. We contrasted proportion of daily time spent foraging by bharal, Pseudois nayaur, a dimorphic grazer, across different body size classes in two high-altitude sites differing in forage availability. Our results indicate that bharal behave as net energy maximizers during winter. As predicted by the net energy maximization strategy, daily winter foraging time of bharal declined with increasing body size, and was lower in the site with low forage availability. Furthermore, as predicted by our model, foraging time declined as the winter season progressed. We did not find support for the time minimizing or energy intake maximizing strategies. Our qualitative model uses relative rather than absolute costs and gains of foraging which are often difficult to estimate in the field. It thus offers a simple way to make informed inferences regarding animal foraging strategies by contrasting estimates of daily foraging time across gradients of body size and forage availability.