A comparative analysis of thermogenic capacity and cold tolerance in small birds

Small birds showing marked seasonal changes in cold tolerance also exhibit winter increases in summit metabolic rate (Msum=maximum cold-induced thermogenesis or thermogenic capacity) relative to summer birds. However, some birds show modest seasonal changes in cold tolerance without winter increases in Msum and others exhibit large seasonal changes in cold tolerance with only minor changes in Msum. Thus, the degree of correlation between cold tolerance and Msum is uncertain and no interspecific study has directly addressed this question. In this study, we measured cold tolerance and Msum in summer- (21 species) and winter- (11 species) acclimatized birds from southeastern South Dakota. Msum was measured as the maximum oxygen consumption attained during exposure of individual birds to a declining series of temperatures in 79% helium/21% oxygen (helox). Cold tolerance was measured as the temperature at cold limit (TCL), which is the helox temperature that induced hypothermia in individual birds. Residuals from allometric regressions of logMsum and logTCL were significantly and negatively related for summer (R2=0.34, P=0.006) and winter (R2=0.40, P=0.037) birds. Data were also subjected to a comparative analyses with phylogenetically independent contrasts to remove potential confounding effects of phylogeny, and results were similar to the non-phylogenetic analyses, with significant negative correlations in both summer (R2=0.47, P<0.001) and winter (R2=0.40, P=0.049). Thus, birds with high Msum tended to show reduced TCL (i.e. high cold tolerance), suggesting that cold tolerance and summit metabolism are phenotypically linked in small birds.