Body temperature variation in free-living and food-deprived yellow-necked mice sustains an adaptive framework for endothermic thermoregulation (original) (raw)
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Journal of Experimental Biology, 2019
Theoretical modelling predicts that the thermoregulatory strategies of endothermic animals range from those represented by thermal generalists to those characteristic for thermal specialists. While the generalists tolerate wide variations in body temperature (Tb), the specialists maintain Tb at a more constant level. The model has gained support from inter-specific comparisons relating to species and population levels. However, little is known about consistent among-individual variation within populations that could be shaped by natural selection. We studied the consistency of individual heterothermic responses to environmental challenges in a single population of yellow-necked mice (Apodemus flavicollis), by verifying the hypothesis that Tb variation is a repeatable trait. To induce the heterothermic response, the same individuals were repeatedly food deprived for 24 h. We measured Tb with implanted miniaturised data loggers. Before each fasting experiment, we measured basal metabo...
2001
Locomotor activity has been an important factor during the evolution of terrestrial vertebrates (e.g. Bennett, 1991). For example, according to the aerobic capacity model, endothermy in birds and mammals evolved as a correlated response to selection for high levels of aerobically supported locomotor activity (Bennett and Ruben, 1979; for reveiews, see Hayes and Garland, 1995; Ruben, 1995). So far, theoretical discussions and empirical tests of the aerobic capacity model have focused primarily on the relationship between basal metabolic rate (BMR) and maximum aerobic metabolic rate (for a review, see Hayes and Garland, 1995). Although high basal and maximal metabolic rates (the latter measured during exercise and/or cold-exposure) are key features of endotherm physiology, equally important is their high level of total daily energy expenditure and correspondingly high capacity for energy assimilation (e.g. Karasov and Diamond, 1985; Nagy et al., 1999; Koteja, 2000), in addition to their ability to sustain activity when exposed to low ambient temperatures. Could selection for high locomotor activity lead to an increase in overall energy budgets and increased capacity for energy assimilation under a 'stressful' condition other than locomotor activity, such as cold-exposure? Over the last decade, the problem of what sets the upper limits to energy budgets has been the subject of vivid discussion (for reviews, see Weiner, 1989; Weiner, 1992; Peterson et al., 1990; Hammond and Diamond, 1997). The discussion has focused on whether the long-term, sustainable energy budgets are limited 'centrally,' by the capacity of visceral organs (intestines, liver, kidney; Koteja, 1996b), or 'peripherally,' by the capacity of organs expending energy (e.g. muscles, mammary glands; Hammond et al., 1994; McDevitt and Speakman, 1994). It has been shown that individuals acclimated to a condition requiring increased energy expenditures (e.g. cold-exposure, lactation) are able to 1177
Thermoregulation by an Australian murine rodent, the ash-grey mouse (Pseudomys albocinereus)
Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology, 2012
We examine here the thermal physiology of the ash-grey mouse, as there is a paucity of data to explain how Australian rodents meet thermoregulatory demands. Most ash-grey mice remained normothermic over a range of ambient temperatures (10°C to 30°C), although they became hyperthermic at high ambient temperatures. One individual entered torpor at ambient temperatures of 20°C and 25°C, with minimal body temperatures of 24.5°C and 28.4°C respectively, before spontaneously arousing. This is the first evidence of torpor use by an Australian murine rodent. Our data suggest that although ash-grey mice have the physiological ability to use torpor, it is used rarely, presumably due to other behavioural and physiological adaptations. Their higher-than-expected basal metabolic rate (1.56± 0.25 mL O 2 g −1 h −1) indicates that ash-grey mice do not have a frugal approach to energy expenditure. Other standard physiological variables were typical of a generalised rodent. A readily-available omnivorous diet, nocturnal activity, semi-fossorial habit and social behaviour presumably allow a high energy lifestyle. A reluctance to use torpor, despite an apparent physiological ability to do so, supports the idea that the use of torpor reflects a net balance between the costs and benefits of a heterothermic thermoregulatory strategy.
Daily torpor in mice: high foraging costs trigger energy-saving hypothermia
Biology Letters, 2010
Many animal species employ natural hypothermia in seasonal (hibernation) and daily (torpor) strategies to save energy. Facultative daily torpor is a typical response to fluctuations in food availability, but the relationship between environmental quality, foraging behaviour and torpor responses is poorly understood. We studied body temperature responses of outbred ICR (CD-1) mice exposed to different food reward schedules, simulating variation in habitat quality. Our main comparison was between female mice exposed to low foragingcost environments and high-cost environments. As controls, we pair-fed a group of inactive animals (no-cost treatment) the same amount of pellets as high-cost animals. Mice faced with high foraging costs were more likely to employ torpor than mice exposed to low foraging costs, or no-cost controls (100% versus 40% and 33% of animals, respectively). While resting-phase temperature showed a non-significant decrease in high-cost animals, torpor was not associated with depressions in active-phase body temperature. These results demonstrate (i) that mice show daily torpor in response to poor foraging conditions; (ii) that torpor incidence is not attributable to food restriction alone; and (iii) that high levels of nocturnal activity do not preclude the use of daily torpor as an energy-saving strategy. The finding that daily torpor is not restricted to conditions of severe starvation puts torpor in mice in a more fundamental ecological context.
Journal of Comparative Physiology B, 2008
According to the concept of the "minimal boundary curve for endothermy", mammals and birds with a basal metabolic rate (BMR) that falls below the curve are obligate heterotherms and must enter torpor. We examined the reliability of the boundary curve (on a double log plot transformed to a line) for predicting torpor as a function of body mass and BMR for birds and several groups of mammals. The boundary line correctly predicted heterothermy in 87.5% of marsupials (n = 64), 94% of bats (n = 85) and 82.3% of rodents (n = 157). Our analysis shows that the boundary line is not a reliable predictor for use of torpor. A discriminate analysis using body mass and BMR had a similar predictive power as the boundary line. However, there are suYcient exceptions to both methods of analysis to suggest that the relationship between body mass, BMR and heterothermy is not a causal one. Some homeothermic birds (e.g. silvereyes) and rodents (e.g. hopping mice) fall below the boundary line, and there are many examples of heterothermic species that fall above the boundary line. For marsupials and bats, but not for rodents, there was a highly signiWcant phylogenetic pattern for heterothermy, suggesting that taxonomic aYliation is the biggest determinant of heterothermy for these mammalian groups. For rodents, heterothermic species had lower BMRs than homeothermic species. Low BMR and use of torpor both contribute to reducing energy expenditure and both physiological traits appear to be a response to the same selective pressure of Xuctuating food supply, increasing Wtness in endothermic species that are constrained by limited energy availability. Both the minimal boundary line and discriminate analysis were of little value for predicting the use of daily torpor or hibernation in heterotherms, presumably as both daily torpor and hibernation are precisely controlled processes, not an inability to thermoregulate.
Frontiers in physiology, 2016
Food availability varies substantially throughout animals' lifespans, thus the ability to profit from high food levels may directly influence animal fitness. Studies exploring the link between basal metabolic rate (BMR), growth, reproduction, and other fitness traits have shown varying relationships in terms of both magnitude and direction. The diversity of results has led to the hypothesis that these relationships are modulated by environmental conditions (e.g., food availability), suggesting that the fitness consequences of a given BMR may be context-dependent. In turn, there is indirect evidence that individuals with an increased capacity for aerobic work also have a high capacity for acquiring energy from food. Surprisingly, very few studies have explored the correlation between maximum rates of energy acquisition and BMR in endotherms, and to the best of our knowledge, none have attempted to elucidate relationships between the former and aerobic capacity [e.g., maximum meta...