Abstract. According to the aerobic capacity model, endothermy in birds and mammals evolved as a correlated response to selection for an ability of sustained locomotor activity, rather than in a response to direct selection for thermoregulatory capabilities. A key assumption of the model is that aerobic capacity is functionally linked to basal metabolic rate (BMR). The assumption has been tested in several studies at the level of phenotypic variation among individuals or species, but none has provided a clear answer whether the traits are genetically correlated. Here we present results of a genetic analysis based on measurements of the basal and the maximum swim-and cold-induced oxygen consumption in about 1000 bank voles from six generations of a laboratory colony, reared from animals captured in the field. Narrow sense heritability (h 2 ) was about 0.5 for body mass, about 0.4 for mass-independent basal and maximum metabolic rates, and about 0.3 for factorial aerobic scopes. Dominance genetic and common environmental (ϭ maternal) effects were not significant. Additive genetic correlation between BMR and the swim-induced aerobic capacity was high and positive, whereas correlation resulting from specific-environmental effects was negative. However, BMR was not genetically correlated with the cold-induced aerobic capacity. The results are consistent with the aerobic capacity model of the evolution of endothermy in birds and mammals. Birds and mammals employ an extravagant economy of energy usage. They spend energy at a rate about 20 times higher than do reptiles (Nagy et al. 1999) and dissipate 20-50% of the energy as basal metabolism for maintenance of basic function (McNab 2002). How such a wasteful strategy of energy use evolved from an energetically frugal strategy is puzzling. This puzzle is the core of a nearly half-centurylong debate on selection forces behind the evolution of endothermy (i.e., the ability to maintain an elevated body temperature by means of metabolic heat production) in birds and mammals (see reviews: Hayes and Garland 1995;Ruben 1995;Farmer 2000Farmer , 2003 Koteja 2000Koteja , 2004McNab 2002;Angilletta and Sears 2003;Gomes et al. 2004).Although several hypotheses concerning the evolution of endothermy have been proposed, only one-the aerobic capacity model-has been subject to extensive empirical testing. According to the model, high basal metabolic rate (BMR) and consequently endothermic thermoregulation, evolved as a correlated response to selection for an ability of sustained locomotor activity, supported by aerobic metabolism (Bennett and Ruben 1979), rather than as a response to direct selection for thermoregulatory capability. A fundamental assumption of the hypothesis is that the aerobic capacity is functionally linked with BMR, which translates into a statistically tractable hypothesis of a positive correlation between BMR and the maximum rate of oxygen consumption (V O2max ), and therefore can be tested against empirical results obtained in extant species. The assumption has been...

Basal metabolic rate (BMR) is a fundamental energetic trait and has been measured in hundreds of birds and mammals. Nevertheless, little is known about the consistency of the population-average BMR or its repeatability at the level of individual variation. Here, we report that average mass-independent BMR did not differ between two generations of bank voles or between two trials separated by one month. Individual differences in BMR were highly repeatable across the one month interval: the coefficient of intraclass correlation was 0.70 for absolute log-transformed values and 0.56 for mass-independent values. Thus, BMR can be a meaningful measure of an individual physiological characteristic and can be used to test hypotheses concerning relationships between BMR and other traits. On the other hand, mass-independent BMR did not differ significantly across families, and the coefficient of intraclass correlation for full sibs did not differ from zero, which suggests that heritability of BMR in voles is not high.

According to the aerobic capacity model, endothermy in birds and mammals evolved as a correlated response to selection for an ability of sustained locomotor activity, rather than in a response to direct selection for thermoregulatory capabilities. A key assumption of the model is that aerobic capacity is functionally linked to basal metabolic rate (BMR). The assumption has been tested in several studies at the level of phenotypic variation among individuals or species, but none has provided a clear answer whether the traits are genetically correlated. Here we present results of a genetic analysis based on measurements of the basal and the maximum swim- and cold-induced oxygen consumption in about 1000 bank voles from six generations of a laboratory colony, reared from animals captured in the field. Narrow sense heritability (h2) was about 0.5 for body mass, about 0.4 for mass-independent basal and maximum metabolic rates, and about 0.3 for factorial aerobic scopes. Dominance genetic and common environmental (= maternal) effects were not significant. Additive genetic correlation between BMR and the swim-induced aerobic capacity was high and positive, whereas correlation resulting from specific-environmental effects was negative. However, BMR was not genetically correlated with the cold-induced aerobic capacity. The results are consistent with the aerobic capacity model of the evolution of endothermy in birds and mammals.

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