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 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 PMR_C. PMR_E averaged 6.44 × basal metabolic rate (BMR) and PMR_c averaged 4.52 × 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. Mb-adjusted residuals of PMRe were positively correlated with residual BMR, whereas residual PMR_c and residual BMR were not correlated. PMRe and PMR_C were not correlated after we corrected for M_b. Temperate birds maintained their body temperature at an 8.6℃ 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.
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