Abstract:
Avian basal metabolic rate (BMR) and summit metabolism (Msum) vary within individuals in response to seasonal
acclimatization in free-ranging birds, and thermal acclimation under laboratory conditions. We examined
relationships between acclimation air temperature (Tacc) and body mass (Mb), BMR and Msum in
female southern red bishops (Euplectes orix) from a relatively mild coastal site and a seasonally colder, inland
site. Following acclimation for 21 days to Tacc = 10, 22 or 35 °C, Mb, BMR and Msum were all significantly and
negatively related to Tacc. The significant relationship between BMR and Tacc did not remain after Mb was included
as a covariate, whereas that between Msum and Tacc did. A subsequent reverse acclimation protocol,
where bishops were acclimated to a second Tacc value and then re-acclimated to the first, revealed that
short-term changes in Mb and BMR were completely reversible, but changes in Msum were only partially reversible.
Following the reverse-acclimation protocol, metabolic expansibility (Msum/BMR) varied significantly
with air temperature, being greatest at Tacc = 35 °C. Our data suggest that the intraspecific variation in seasonal
metabolic variation previously reported for this species is at least partly driven by factors other than
temperature.
