We studied the potential for response to selection in typical physiological-thermoregulatory traits of mammals such as maximum metabolic rate (MMR), nonshivering thermogenesis (NST) and basal metabolic rate (BMR) on cold-acclimated animals. We used an animal model approach to estimate both narrow-sense heritabilities (h 2 ) and genetic correlations between physiological and growth-related traits. Univariate analyses showed that MMR presented high, significant heritability (h 2 ϭ 0.69 Ϯ 0.35, asymptotic standard error), suggesting the potential for microevolution in this variable. However, NST and BMR presented low, nonsignificant h 2 , and NST showed large maternal/common environmental/nonadditive effects (c 2 ϭ 0.34 Ϯ 0.17). Heritabilities were large and significant (h 2 Ͼ 0.5) for all growth-related traits (birth mass, growth rate, weaning mass). The only significant genetic correlations we found between a physiological trait and a growth-related trait was between NST and birth mass (r ϭ Ϫ0.74; P Ͻ 0.05). Overall, these results suggest that additive genetic variance is present in several bioenergetic traits, and that genetic correlations could be present between those different kinds of traits.

Abstract. We explored how morphological and physiological traits associated with energy expenditure over long periods of cold exposure would be integrated in a potential response to natural selection in a wild mammal, Phyllotis darwini. In particular, we studied sustained energy expenditure (SusMR), the rate of expenditure fueled by concurrent energy intake, basal metabolic rate (BMR), and sustained metabolic scope (SusMS ϭ SusMR/BMR), a measure of the reserve for sustained work. We included the masses of different central processing organs as an underlying factor that could have a mechanistic link with whole animal traits. Only the liver had heritability statistically different from zero (0.73). Physiological and morphological traits had high levels of specific environmental variance (average 70%) and postnatal common environmental variance (average 30%) which could explain the low heritabilities estimates. Our results, (1) are in accordance with previous studies in mammals that report low heritabilities for metabolic traits (SusMR, BMR, SusMS), (2) but not completely with previous ones that report high heritabilities for morphological traits (masses of central organs), and (3) provide important evidence of the relevance of postnatal common environmental variance to sustained energy expenditure.Key words. Energy budget, heritability, mammal, sustained metabolic rate.Received June 25, 2003. Accepted September 22, 2003 In recent years the evolutionary role of physiological variation within natural populations has been a subject of intensive research (Garland and Carter 1994;Bradley and Zamer 1999;Feder et al. 2000). A key parameter focused in many of the studies is the narrow-sense heritability (h 2 ) or the proportion of population phenotypic variance of a trait that is additive genetic (Roff 1997). This is because the extent of the evolutionary response to natural or sexual selection is dependent on the magnitude of h 2 . However, organisms comprise far more than just one trait, and thus the evolutionary response to selection could involve simultaneous modifications of several traits at the same time, and at different levels of biological organization . The estimation of genetic covariances among several different traits are indispensable in predicting the evolutionary response to selection (Arnold 1994). These covariances could show if multivariate evolution could be facilitated or constrained among a particular set of traits (Arnold 1994).In seasonal environments, daily and seasonal variations in ambient temperature represent an important challenge to small nonhibernating eutherian mammals (McNab 2002), and therefore mechanisms that control thermoregulatory homeostasis are paramount (Lynch 1994; Nespolo et al. 2003a,b). In addition, it is well known that acclimatization to cold in small mammals involves aspects of physiology, behavior, and morphology (Geiser et al. 1996). However, it is less clear whether an evolutionary response could take place in traits related to thermogenic capacity and whether such a re...

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We explored how morphological and physiological traits associated with energy expenditure over long periods of cold exposure would be integrated in a potential response to natural selection in a wild mammal, Phyllotis danwini. In particular, we studied sustained energy expenditure (SusMR), the rate of expenditure fueled by concurrent energy intake, basal metabolic rate (BMR), and sustained metabolic scope (SusMS = SusMR/BMR), a measure of the reserve for sustained work. We included the masses of different central processing organs as an underlying factor that could have a mechanistic link with whole animal traits. Only the liver had heritability statistically different from zero (0.73). Physiological and morphological traits had high levels of specific environmental variance (average 70%) and postnatal common environmental variance (average 30%) which could explain the low heritabilities estimates. Our results, (1) are in accordance with previous studies in mammals that report low heritabilities for metabolic traits (SusMR, BMR, SusMS), (2) but not completely with previous ones that report high heritabilities for morphological traits (masses of central organs), and (3) provide important evidence of the relevance of postnatal common environmental variance to sustained energy expenditure.

A major area of interest in comparative physiology has been to understand how animals cope with changing environmental demands in time and space. The digestive system has been identified as one of the more sensitive systems to changes in environmental conditions. However, most research on this topic has evaluated these effects during peak energetic demands, which do not allow for evaluation of the dynamics of the digestive response along a more natural continuous gradient of environmental conditions. We examined phenotypic flexibility in digestive responses of the leaf-eared mouse Phyllotis darwini to increments in total energy demands (via sequential exposure to 26, 12 and 0 degrees C). Additionally, we evaluated the effect of a moderate energy demand (12 degrees C) over three different time periods (7, 17 and 27 days) on digestive traits. Moderate increases in energy demand were associated with changes in the distribution of digesta in the gut, whereas higher increases in energy demand involved increases in the tissue mass of digestive organs. Time-course analysis showed that at 12 degrees C practically all digestive variables reached stable values within 7 days, which is in agreement with empirical data and theoretical deductions from cellular turnover rates. We conclude that although the input of energy and nutrients into the digestive tract is typically periodic, many aspects of digestive physiology are likely to be flexible in response to environmental variability over both short-term (daily) and long-term (seasonal) time scales.