Physiological variation in amethyst sunbirds (Chalcomitra amethystina) over an altitudinal gradient in winter

Abstract

SUMMARYFlexibility of metabolic rates within a species allows for the colonization of different habitats along an altitudinal and thus temperature gradient. The distribution range of amethyst sunbirds (Chalcomitra amethystina) within southern Africa includes an altitudinal gradient from the Drakensberg to the coast of KwaZulu-Natal. We expected that over this altitudinal gradient amethyst sunbirds would exhibit variation in winter metabolic rates, particularly basal metabolic rates (BMR) and resting metabolic rate (RMR) pre-and post-acclimation. Sunbirds from three locations, Underberg (1553 m above sea level; asl), Howick (1075 m asl) and Oribi Gorge (541 m asl) were used for this study. Upon capture, metabolic rate was measured indirectly by quantifying oxygen consumption (V O2 ) using flow-through respirometry, at 5 and 25°C. Birds were then acclimated at 25°C for 6 weeks on a 12 h:12 h L:D cycle. V O2 was measured post-acclimation at eight different temperatures (15, 5, 10, 20, 30, 28, 25 and 33°C). We found little variation in winter V O2 between individuals from the same locality, whereas significant variation was observed in V O2 at the same temperatures between individuals from the different localities and thus between altitudes. In particular, winter BMR decreased significantly with decreasing altitude post-acclimation. This study emphasizes the need to understand plasticity/flexibility in metabolic rates and to acknowledge altitudinal differences within a species, to make accurate predictions about the thermal physiology of a species and its responses to changes in ambient temperatures.

Elevated performance: the unique physiology of birds that fly at high altitudes

“…with larger wing stroke amplitudes) at elevation than lowland birds at sea level (Altshuler and Dudley, 2003). Highland birds may also have a higher capacity for increasing their resting metabolism as a means of generating heat in the cold (Lindsay et al, 2009); however, it is unclear whether thermogenic adaptations are even necessary for dealing with cold during flight, because a lot of heat is already being generated by the active flight muscles (TorreBueno, 1976;Ward et al, 1999). Water loss during flight is high enough to constrain flight duration at sea level (Engel et al, 2006) and is therefore expected to be a major issue at elevation, but it is unclear whether unique water-saving strategies have evolved in highflying birds.…”

Seasonal variation in the metabolism-temperature relation of House Sparrows (Passer domesticus) in KwaZulu-Natal, South Africa

“…The flow rate was adjusted to ensure that there was less than 1% O 2 depletion in the respirometry chambers and was between 0.500 and 0.600 l min À 1 . Details followed were as in Lindsay et al (2009a). VO 2 was recorded digitally every 6 min, corrected for standard temperature and pressure, and expressed as a mass specific value as described in Lindsay et al (2009a).…”

“…Details followed were as in Lindsay et al (2009a). VO 2 was recorded digitally every 6 min, corrected for standard temperature and pressure, and expressed as a mass specific value as described in Lindsay et al (2009a). Ten readings per individual were recorded on an hourly basis and the lowest hourly rate was taken as the minimum RMR.…”

“…As seven metabolic measurements were performed on each individual bird in a particular season Generalised Linear Model (GLM) Repeated Measures Analysis of Variance (RMANO-VA) was used to compare lowest hourly mean metabolic (that was determined for each bird) and body mass within each season and also between the two seasons. Post-hoc Tukey HSD tests were used to determine the width of TNZ, in summer and winter as described in Lindsay et al (2009a).…”

Seasonal variation in hematocrit levels of Amethyst Sunbirds (Chalcomitra amethystina) over an altitudinal gradient

“…In winter there was no significant effect of combined preand post-acclimation and site on body mass of the altitudinal subpopulations of Amethyst Sunbirds (RMANOVA, F 2,22 = 0.345, P = 0.712) (Lindsay et al 2009a). Similarly in summer there was no significant difference between pre-and post-acclimation body mass (g) for each subpopulation, nor between altitudinal subpopulations of Amethyst Sunbirds (RMANOVA, F 2,18 = 0.935, P = 0.411) (Lindsay et al 2009b).…”