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.
