Geographic variation in the response to thermal acclimation in rufous-collared sparrows: are physiological flexibility and environmental heterogeneity correlated?

1. It has been proposed that habitats with low productivity, and hence low resource availability, may favour individuals with lower rates of resource use and energy expenditure. In birds, some studies have shown that basal metabolic rate (BMR) and total evaporative water loss (TEWL) are reduced in desert bird species compared to those that live in more mesic areas. However, the contribution of the phenotypic plasticity of birds to this pattern has been poorly studied. 2. We evaluated the phenotypic flexibility of BMR, TEWL and organ size in sparrows (Zonotrichia capensis) from three populations along an aridity gradient that differ in environmental temperature, rainfall and seasonality. We also tested the hypothesis that populations from variable environments exhibit more physiological flexibility than populations from stable environments. 3. BMR and TEWL in Z. capensis exhibited a positive association with latitude. Additionally, the population from the more stable environment was unaffected by thermal acclimation, but birds from seasonal environments were affected. 4. Our results support the hypothesis of the existence of a correspondence between environmental variability and phenotypic flexibility; that is, birds from localities with a drastic climatic seasonality have a greater response in BMR to thermal acclimation than birds from a more stable environment.