The resource rule hypothesis predicts that geographic differences in body size among populations of organisms are due to the amount, availability, and quality of food resources. For instance, body size of large herbivores is often correlated with soil characteristics because better soils produce better forage. In semi-arid environments, rainfall variation is an important driver of forage availability, especially highly nutritious annual forbs. Thus, in such pulsed-resource environments, it is unclear whether body size of large herbivores is influenced by fixed resources correlated with soil characteristics, irregular resource pulses correlated with rainfall, or both. Furthermore, it is not clear if phenotypic expression is a function of forage quality or quantity. During early autumns of 2011–2018, we captured 4,554 white-tailed deer (Odocoileus virginianus) on 7 rangeland sites in the semiarid climate of South Texas, USA. The sites range from coastal to 140 km inland and represent gradients in both soil texture (percent sand) and annual rainfall. We recorded age- and sex-specific indices of skeletal size, antler size, and body mass. Site-specific soil characteristics explained most variation in skeletal size; percent sand was inversely related to skeletal size. For environmentally sensitive phenotypes (antler size and body mass), both soil characteristics and rainfall were influencers; increases in rainfall reduced the negative effect of sand. Percent sand and rainfall were positively correlated with annual biomass of preferred forbs, yet all phenotypic traits declined with increases in forb quantity. Increases in percent shrub cover increased all phenotype sizes. Our data suggest that phenotypic expression of large herbivores in semi-arid environments is driven by forage quality via edaphic characteristics rather than forage quantity via rainfall. Specifically, less sand in the soil allows for development of woody vegetation communities (i.e., shrubs), which in turn provide a consistent source of forage in a variable, pulsed-rainfall environment. Although forbs are higher quality, they are highly ephemeral. The availability of a consistent source of forage may enable white-tailed deer to extend time invested in body growth, which results in greater phenotype size. Our findings align with the resource rule hypothesis that identifies resource availability as a fundamental element explaining geographical variation in phenotypic expression.

Helicopter based net-gunning of white-tailed deer to collect morphometric measurements.