Body size and digestive system shape resource selection by ungulates: a cross-taxa test of the Forage Maturation Hypothesis

The Forage Maturation Hypothesis (FMH) states that energy intake for ungulates is maximized when forage biomass is at intermediate levels. Nevertheless, metabolic allometry and different digestive systems suggest that resource selection should vary across ungulate species. By combining GPS relocations with remotely-sensed data on forage characteristics and surface water, we quantified the effect of body size and digestive system in determining movements of 30 populations of hindgut fermenters (equids) and ruminants across biomes. Selection for intermediate forage biomass was negatively related to body size, regardless of the digestive system. Selection for proximity to surface water was stronger for equids relative to ruminants, regardless of body size. To be more generalizable, we suggest that the FMH explicitly incorporate contingencies in body size and digestive system, with small-bodied ruminants selecting more strongly for potential energy intake, and hindgut fermenters selecting more strongly for surface water.