Cold- and exercise-induced peak metabolic rates in tropical birds

Compared with temperate birds, tropical birds have low reproductive rates, slow development as nestlings, and long lifespans. These slow life history traits are thought to be associated with reduced energy expenditure, or a slow pace of life. To test predictions from this hypothesis, we measured exercise-induced peak metabolic rates (PMRE) in 45 species of tropical lowland forest birds and compared these data with PMRE for three temperate species. We also compared cold-induced PMR (PMRC) with PMRE in the same individuals of 19 tropical species. Tropical birds had a 39% lower PMRE than did the temperate species. In tropical birds, PMRC and PMRE scaled similarly with body mass (M-b), but PMRE was 47% higher than PMRC. PMRE averaged 6.44 x basal metabolic rate (BMR) and PMRC averaged 4.52 x BMR. The slope of the equation relating PMRE to Mb exceeded the slope for the equation for BMR VS. M-b, whereas slopes for the equations of PMRc and BMR vs. Mb did not differ. M-b-adjusted residuals of PMRE were positively correlated with residual BMR, whereas residual PMRc and residual BMR were not correlated. PMRE and PMRc were not correlated after we corrected for Mb. Temperate birds maintained their body temperature at an 8.6 degrees C lower average air temperature than did tropical species. The lower PMRE values in tropical species suggest that their suite of life history traits on the slow end of the life history continuum are associated with reduced metabolic rates.