Popko Wiersma, Mark A. Chappell, Joseph B. Williams (2007). Cold- and exercise-induced peak metabolic rates in tropical birds. In press, Proceedings of the National Academy of Sicences USA.

ABSTRACT -- Compared with temperate birds, tropical birds have low reproductive rates, slow development as nestlings, and long life spans.  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 (Mb), but PMRE was 47% higher than PMRC.   PMRE averaged 6.44 * BMR and PMRC 4.52 * BMR.   The slope of the equation relating PMRE to Mb exceeded the slope for the equation for BMR vs Mb, whereas slopes for the equations of PMRC and BMR vs Mb did not differ.   Mb 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 °C lower average air temperature than did tropical species.  The lower PMRE values in tropical species suggest that their suit of life history traits on the slow end of the life history continuum are associated with reduced metabolic rates.