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 (PMR(E)) in 45 species of tropical lowland forest birds and compared these data with PMR(E) for three temperate species. We also compared cold-induced PMR (PMR(C)) with PMR(E) in the same individuals of 19 tropical species. Tropical birds had a 39% lower PMR(E) than did the temperate species. In tropical birds, PMR(C) and PMR(E) scaled similarly with body mass (M(b)), but PMR(E) was 47% higher than PMR(C). PMR(E) averaged 6.44 x basal metabolic rate (BMR) and PMR(C) averaged 4.52 x BMR. The slope of the equation relating PMR(E) to M(b) exceeded the slope for the equation for BMR vs. M(b), whereas slopes for the equations of PMR(C) and BMR vs. M(b) did not differ. M(b)-adjusted residuals of PMR(E) were positively correlated with residual BMR, whereas residual PMR(C) and residual BMR were not correlated. PMR(E) and PMR(C) were not correlated after we corrected for M(b). Temperate birds maintained their body temperature at an 8.6 degrees C lower average air temperature than did tropical species. The lower PMR(E) 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.