“On the interplay among ambient temperature, basal metabolic rate, and body mass”

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Daniel E. Naya, Hugo Naya, and Craig R. White (Oct 2018)

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Researchers demonstrate that the effect of temperature on metabolic rate is highly dependent of the size of the sampled species

Body size distribution and phenotypic evolution

(A) Body size distribution for all extent mammal species. (B) The effect of body mass (mb) and ambient temperature (Tmean) on basal metabolic rate (BMR, represented in color code) for 458 species of mammal species. Note that for species smaller than 2.0 Log10 mb units (i.e., < 100 g) color bands tend to be curved, meaning that BMR is affected by both mb and Tmean; by contrast, for species larger than 2.0 Log10 mb units (i.e., > 100 g), color bands tend to be parallel, indicating that BMR is only affected by mb. (C) The carpincho or capybara (Hydrochoerus hydrochaeris), one of the species analyzed in this study and the largest living rodent in the world.

Even though body mass in mammals ranges from a few grams to several tons, more than one-half of extant species weigh less than 100 g. Clearly, the distribution of mammals’ body mass in nature is far from follow a normal distribution (panel A on figure at right). But could this highly skewed distribution of body size be affecting our overall impression about the role of different climatic and ecological factors that shape physiological evolution?

To answer this question, two researchers from Uruguay (Daniel Naya and Hugo Naya) and one from Australia (Craig White) analyzed if the effect of ambient temperature on basal metabolic rate (BMR) change with the size of the species included in the analysis. And, yes, they found that it does! Specifically, these authors found that for species smaller than about 100 g, BMR changed with both body mass and ambient temperature, while for species larger than about 100 g BMR only changed with body mass (panel B on figure below). In other words, as authors conclude, a study analyzing all the 458 species in the dataset will affirm that the effect of temperature on BMR is markedly and highly significant, while a study analyzing only the 197 species larger than 100 g will affirm that this effect is negligible and not significant.

Asked about the implications of this result, Daniel Naya says, “To us, the work is relevant for at least two reasons. First, the specific result obtained for BMR is valuable by itself given the central role of this variable on current ecological and evolutionary theories. Second, and maybe more important, given that most organismal traits scale allometrically with body mass, it could be possible that our current impression of the effect of external factors shaping the evolution of phenotypic traits is hardly affected by the large number of small species in nature.”


Abstract

One of the most generalized conclusions arising from studies analyzing the ecological variation of energy metabolism in endotherms is the apparent negative correlation between ambient temperature and mass-independent basal metabolic rate (residual BMR). As a consequence, ambient temperature has been considered the most important external factor driving the evolution of residual BMR. It is not clear, however, if this relationship is size dependent, and artifacts such as the biased sampling of body masses in physiological data sets could cause us to overstate the ubiquity of the relationship. Accordingly, here we used published data on body mass (mb), BMR, and annual mean temperature (Tmean) for 458 mammal species (and/or subspecies) to examine the size-dependence of the relationship between temperature and BMR. We found a significant interaction between mb and Tmean as predictors of residual BMR, such that the effect of Tmean on residual BMR decreases as a function of mb. In line with this, the amount of residual variance in BMR explained by Tmean decreased with mb, from 20 – 30% at body sizes < 100 g to almost zero at body size > 1,000 g. These data suggest that our current understanding of the importance of broad-scale variation in ambient temperature as a driver of metabolic evolution in endotherms probably is affected by the large number of small species in both nature and physiological datasets.