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Abstract The availability and quality of food resources can alter the intensity of competition and predation pressure within communities. Understanding species capacity to respond to global change‐driven shifts in resource distribution is therefore crucial for biodiversity conservation. Small mammal communities are often structured by competition for food resources, but understanding and monitoring these processes are currently hindered by lack of functional dietary trait information in these hard‐to‐sample systems. In this study, we collected a comprehensive suite of gastrointestinal (GI) measurements from 26 small mammal species (including some never reported), compared them with more traditional craniodental traits in predicting dietary guild, and used them in a novel way to understand how diet structures 22 small mammal communities across the Appalachian Mountains of eastern North America. As predicted, we found GI traits to be effective dietary trait proxies; they were equally or more accurate than craniodental proportions in predicting the dietary guild of individual species. Furthermore, at the community level, we found that both the mean and functional dispersion of GI length were positively correlated with latitude and measures of temperature seasonality. Our results indicate that small mammal communities in more seasonal environments are filtered to include species with longer GI tracts (on average) as well as those that can partition food resources more finely, as expected based on the lower productivity of these regions. Conversely, communities in less seasonal environments display functional redundancy from the addition of species with short to intermediate GI lengths. Proportions of the GI tract represent novel dietary traits that can illuminate community assembly processes across regional environmental gradients and in the face of changing timing and availability of resources. 

Dehnel’s phenomenon describes a seasonal and reversible winter decrease in body size, which is a trait that predicts total energy demand. However, the phenomenon remains less well- studied than common energy-saving or energy-seeking strategies of mammals. Here, we explore the generality of Dehnel’s phenomenon in Sorex shrews on three continents. First, we use new field sampling to document seasonal phenotypic change in masked shrews (Sorex cinereus) in North America at the lowest latitude yet investigated for this species (35.7°). This includes the first documentation of appendicular skeleton remodification in Sorex. Summer-to- winter decreases in S. cinereus body mass, braincase height, and femur length were 13%, 11.5%, and 8.7%, respectively, with subsequent increases of each in second-year individuals. Second, we compile a comprehensive dataset of Dehnel’s-relevant studies to test whether seasonal plasticity in Sorex globally is related to climate, demonstrating that body and braincase plasticity are functions of cold season temperatures. Meta-analytical models for both these traits generalized by a) applying at both inter- and intraspecific scales, and b) predicting the seasonal change newly observed for S. cinereus. Our results support body size plasticity as an environmentally-responsive innovation in these very small, homeothermic mammals.