Intraspecific Correlations of Basal and Maximal Metabolic Rates in Birds and the Aerobic Capacity Model for the Evolution of Endothermy

The underlying assumption of the aerobic capacity model for the evolution of endothermy is that basal (BMR) and maximal aerobic metabolic rates are phenotypically linked. However, because BMR is largely a function of central organs whereas maximal metabolic output is largely a function of skeletal muscles, the mechanistic underpinnings for their linkage are not obvious. Interspecific studies in birds generally support a phenotypic correlation between BMR and maximal metabolic output. If the aerobic capacity model is valid, these phenotypic correlations should also extend to intraspecific comparisons. We measured BMR, Msum (maximum thermoregulatory metabolic rate) and MMR (maximum exercise metabolic rate in a hop-flutter chamber) in winter for dark-eyed juncos (Junco hyemalis), American goldfinches (Carduelis tristis; Msum and MMR only), and black-capped chickadees (Poecile atricapillus; BMR and Msum only) and examined correlations among these variables. We also measured BMR and Msum in individual house sparrows (Passer domesticus) in both summer, winter and spring. For both raw metabolic rates and residuals from allometric regressions, BMR was not significantly correlated with either Msum or MMR in juncos. Moreover, no significant correlation between Msum and MMR or their mass-independent residuals occurred for juncos or goldfinches. Raw BMR and Msum were significantly positively correlated for black-capped chickadees and house sparrows, but mass-independent residuals of BMR and Msum were not. These data suggest that central organ and exercise organ metabolic levels are not inextricably linked and that muscular capacities for exercise and shivering do not necessarily vary in tandem in individual birds. Why intraspecific and interspecific avian studies show differing results and the significance of these differences to the aerobic capacity model are unknown, and resolution of these questions will require additional studies of potential mechanistic links between minimal and maximal metabolic output

Differences in Migratory Timing and Energetic Condition among Sex/Age Classes in Migrant Ruby-Crowned Kinglets

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Allometric relationships for MMR in temperate-zone birds

<p>. Least squares allometric regression for log MMR (measured in a hop-flutter wheel) against log M_b for five species of temperate-zone birds for which MMR has been recorded (solid line). For comparison, the allometric regression equation for MMR for tropical birds from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0034271#pone.0034271-Wiersma1" target="_blank">[5]</a> is included as the dashed line. MMR values for other temperate-zone bird species include satin bowerbird <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0034271#pone.0034271-Chappell3" target="_blank">[49]</a>, red-eyed vireo <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0034271#pone.0034271-Pierce1" target="_blank">[42]</a> and house sparrow <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0034271#pone.0034271-Chappell2" target="_blank">[41]</a>.</p

Correlations between BMR vs. M_sum for house sparrows

<p>. Raw BMR and M_sum (upper panel) were significantly positively correlated, but mass-independent residuals (lower panel) were not (<i>R²</i> = 0.060, <i>P</i> = 0.134), indicating that the correlation between raw metabolic values was driven by body mass.</p

Correlations between BMR and M_sum for black-capped chickadees

<p>. Raw BMR and M_sum (upper panel) were significantly positively correlated, but mass-independent residuals (lower panel) were not (<i>R²</i> = 0.048, <i>P</i> = 0.470), indicating that the correlation between raw metabolic values was driven by body mass.</p

Allometric least squares regression equations for log metabolic rates (ml O₂ min⁻¹) against log body mass (Mb, g) for the four study species.

<p>Sample sizes (n) for the different metabolic measurements are provided in parenthesis.</p

Correlation between mass-independent residuals of thermogenic (M_sum) and exercise (MMR) metabolic rates for American goldfinches.

<p>The correlation was not significant (<i>R²</i> = 0.065, <i>P</i> = 0.279).</p