White-tailed deer capture records

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.