Abstract
Bergmann’s rule describes the tendency for endothermic body size to increase with latitude, a pattern often attributed to climatic factors. However, the underlying developmental and evolutionary mechanisms remain debated. Latitudinal gradients in temperature and precipitation are often thought to be responsible for Bergmann’s rule, but climate may generate such geographical patterns via several alternative pathways. We examined geographic patterns in body size of 5,596 burrowing owls (Athene cunicularia) throughout western North America to inform the underlying processes responsible for range-wide phenotypic variation. Burrowing owls followed Begmann’s rule, with larger individuals in northern latitudes. We also detected a longitudinal gradient, with the largest owls in the northwest of the breeding range. We considered several mechanisms to explain geographic gradients in burrowing owl body size: developmental plasticity, reversible plasticity, and local adaptation. Developmental plasticity links body size to early-life environmental conditions, causing permanent changes; reversible plasticity involves temporary physiological shifts; local adaptation reflects evolutionary changes optimizing body size to local conditions. We tested predictions generated by each of these three mechanisms to assess how resource availability and climate shape continental body size patterns in burrowing owls. Spring precipitation explained variation in adult body mass, with contrasting effects in warm versus cold climates, supporting both local adaptation to thermal extremes and reversible plasticity in response to resource availability. Drought explained variation in juvenile body mass, supporting the developmental plasticity mechanism via resource availability. Juvenile mass was more influenced by maximum temperature during the prior breeding season than by natal year conditions, suggesting that extreme heat may affect parental condition or suppress resource availability into the following year, consistent with developmental or reversible plasticity. Extreme temperatures and drought during the prior breeding season explained variation in adult tarsus length, more so than 21-year mean climate conditions or geographic gradients, supporting the developmental plasticity mechanism via thermal or drought stress. Body size of burrowing owls largely conforms to Bergmann’s rule and appears to reflect both acute and evolutionary responses to resource availability and thermoregulatory mechanisms depending on whether a trait was more plastic (body mass) or more fixed (tarsus) during adulthood.