Modifications to skeletal muscle morphology correlate with increased thermogenic capacity during cold-acclimation in Dark-eyed Juncos (Junco hyemalis)

Many small endotherms increase their capacity to produce heat in response to cold temperatures. This organismal response is underlain by modifications to lower levels of biological organization. Here we investigated potential changes to muscle morphology in cold-acclimated Dark-eyed Juncos (Junco hyemalis), a widespread North American sparrow. We quantified pectoralis fiber density, fiber cross-sectional area, capillary density, capillary-to-fiber ratio, and fiber type for individuals exposed to cold (−8 °C) or control (18 °C) temperature treatments lasting one to six weeks in duration. We then related pectoralis morphological traits to metabolic traits for these same individuals. We show that pectoralis fiber density quickly increased in the cold, and positively correlated with summit metabolic rate (a proxy for maximal thermogenic capacity) and resting metabolic rate (an index of maintenance cost) in the cold. Fiber density was not, however, related to the duration of the treatment. Moreover, juncos did not exhibit changes in capillarity or fiber type. Our results suggest one physiological avenue by which songbirds may rapidly alter organismal performance in response to variation in ambient temperature.