Seasonal metabolic acclimatization varies in direction and magnitude among populations of an afrotropical Passerine bird

Abstract

Avian metabolic responses demonstrate considerable diversity under fluctuating environmental conditions, a well-studied example being the seasonal upregulation of basal metabolic rate (BMR) and summit metabolism (Msum) in temperate species experiencing harsh winters. Fewer studies have examined seasonal metabolic acclimatization in subtropical or tropical species. We investigated seasonal metabolic variation in an Afrotropical ploceid passerine, the white-browed sparrow-weaver (Plocepasser mahali; ∼47 g), at three sites along a climatic gradient of approximately 77C in winter minimum air temperature (Ta). We measured Msum (n ≥ 10 per site per season) in a helox atmosphere, BMR of the same birds at thermoneutrality (Ta ≈ 307C), and resting metabolic rates at 57C ≤ Ta ≤ 207C. Patterns of seasonal adjustments in BMR varied among populations in a manner not solely related to variation in seasonal Ta extremes, ranging from BMR ∼52% higher in winter than in summer to no seasonal difference. Greater cold tolerance was found in a population at a colder desert site, manifested as higher Msum (∼25% higher) and lower helox temperature at cold limit values compared with a milder, mesic site. Our results lend support to the idea that greater variance in the pattern of seasonal metabolic responses occurs in subtropical and tropical species compared with their temperatezone counterparts and that factors other than Ta extremes (e.g., food availability) maybe important in determining the magnitude and direction of seasonal metabolic adjustments in subtropical birds.