Energy is presumed to be limited in supply, body warmth is presumed to be a benefit of endothermy, and fractal distribution networks are presumed to constrain the metabolic design of organisms. However, a new Journal of Animal Ecology paper by John Speakman and Elzbieta Król from the University of Aberdeen challenges all three premises by proposing that limits to heat dissipation explain the scaling of endotherm metabolism and constrain metabolic performance during periods of intense sustained energy expenditure, such as reproduction.
Speakman and Król review a long series of investigations published in The Journal of Experimental Biology seeking to identify factors limiting milk and offspring production in lactating mice. They describe how lactating mice housed in the cold increased food intake, produced more milk and raised faster growing pups than mice held at warmer temperatures. The possibility that cold environments are advantageous to lactating mice because they can produce more milk when it is easier to dissipate heat could explain the experimental evidence but is heretical in the context of prevailing ideas about the warm-body benefits of endothermy. However, a later paper in this series clinched support for this heresy. Shaving fur from the dorsal surface of lactating mice, which increases their capacity to dissipate heat without altering the thermal environment experienced by their offspring, substantially improves their reproductive performance. Body heat is a parental care problem rather than a parental care solution, at least for lactating mice in captivity.
Having identified heat dissipation as a limiting factor, Speakman and Król proceed by developing a mechanistic model of endotherm heat dissipation, which predicts that the maximum capacity to dissipate heat increases with body size with an exponent of about 0.63. This provides a much better empirical match to the scaling of field metabolic rate than the 0.75 exponent predicted by the metabolic theory of ecology. Often the residuals around these allometric patterns are as informative as the slope of the overall relationship, and Speakman and Król speculate that marine mammals have higher rates of energy expenditure than other similar-sized mammals because they occupy aquatic environments that are highly conducive to heat dissipation. The authors invert the observation that many animals increase insulation in winter to ask why animals reduce insulation in summer if heat and energy conservation is a year-round priority. Speakman and Król also suggest that latitudinal patterns of body size variation – first described by Carl Bergmann in 1847 – are driven by more intense selection for the reproductive advantages of small body size and heat dissipation in warm environments.
Is body warmth generally a cost rather than a benefit of endothermy? Does heat dissipation capacity offer a better explanation for the allometric scaling of endotherm metabolism than fractal distribution networks? Are major trade-offs in endotherm energy allocation governed by constraints associated with energy demand rather than energy supply? These are revolutionary and readily testable hypotheses proposed by Speakman and Król reflecting major uncertainties and research opportunities in endotherm energetics.
