Metabolic critical temperatures define the range of ambient temperatures where endotherms are able to minimize energy allocation to thermogenesis. Examining the relationship between metabolic critical temperatures and basal metabolic rates (BMR) provides a unique opportunity to gain a better understanding of how animals respond to varying ambient climatic conditions, especially in times of ongoing and projected future climate change. We make use of this opportunity by testing the heat

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... n limit (HDL) theory, which hypothesizes that the maximum amount of heat a species can dissipate constrains its energetics. Specifically, we test the theory's implicit prediction that BMR should be lower under higher metabolic critical temperatures. We analysed the relationship of BMR with upper and lower critical temperatures for a large dataset of 146 endotherm species using regression analyses, carefully accounting for phylogenetic relationships and body mass. We show that metabolic critical temperatures are negatively related with BMR in both birds and mammals. Our results confirm the predictions of the HDL theory, suggesting that metabolic critical temperatures and basal metabolic rates respond in concert to ambient climatic conditions. This implies that heat dissipation capacities of endotherms may be an important factor to take into account in assessments of species' vulnerability to climate change. Manuscript to be reviewed 1 Testing the heat dissipation limitation hypothesis: basal metabolic rates of endotherms 2 decrease with increasing upper and lower critical temperatures 3 Short title: Metabolic critical temperatures and metabolic rates in endotherms 4 5 Imran Abstract 20 Metabolic critical temperatures define the range of ambient temperatures where endotherms are 21 able to minimize energy allocation to thermogenesis. Examining the relationship between 22 metabolic critical temperatures and basal metabolic rates (BMR) provides a unique opportunity 23 to gain a better understanding of how animals respond to varying ambient climatic conditions, 24 especially in times of ongoing and projected future climate change. We make use of this 25 opportunity by testing the heat dissipation limit (HDL) theory, which hypothesizes that the 26 maximum amount of heat a species can dissipate constrains its energetics. Specifically, we test 27 the theory's implicit prediction that BMR should be lower under higher metabolic critical 28 temperatures. We analysed the relationship of BMR with upper and lower critical temperatures 29 for a large dataset of 146 endotherm species using regression analyses, carefully accounting for 30 phylogenetic relationships and body mass. We show that metabolic critical temperatures are 31 negatively related with BMR in both birds and mammals. Our results confirm the predictions of 32 the HDL theory, suggesting that metabolic critical temperatures and basal metabolic rates 33 respond in concert to ambient climatic conditions. This implies that heat dissipation capacities of 34 endotherms may be an important factor to take into account in assessments of species' 35 vulnerability to climate change. 36 37 38 PeerJ reviewing PDF | (Manuscript to be reviewed 42 assessing whether and how species may be able to respond to ongoing and future climate change. 43 Organisms' energy budgets play a key role in this context, and studies on how energy budgets 44 vary in relation to environmental factors as well as to the species-specific characteristics will 45 contribute to our knowledge about species' responses to varying and changing climatic 46 conditions. 47