Testing the ecological relevance of the heat dissipation limit hypothesis in a small mammal

Everything that animals do behaviourally and physiologically requires energy. However, while there are lots of things energy can be expended on, there is only one source of that energy / food intake. If animals could eat process and expend unlimited quantities of energy there would be no problem, but of course they can't, which leads to leading to important trade-offs between competing demands within the total available budget. The maximal rate at which energy can be ingested is called the Sustained energy intake or SusEI. I have been working on the limits to SusEI and its wider ramifications for about 10 years and we have recently made a significant breakthrough in our understanding these limits. The current proposal builds on this important advance. To set this advance in context it is necessary to provide some background to the research area. Two ideas were proposed in the early 1990s concerning how limits to SusEI might be mediated. Limits might be imposed by the capacity of the alimentary tract and associated organs (the central limits hypothesis) or imposed at sites where the energy is utilised (the peripheral limits hypothesis). Over the past two years we have been working on a radically different idea, and our findings were published last year in a series of three papers in the Journal of Experimental Biology. Our hypothesis draws on agricultural observations that the performance of dairy cows is significantly affected by their capacity to dissipate heat generated during lactation. Observations we have made in laboratory mice are consistent with the hypothesis that mice similarly constrained in their capacity to dissipate heat / the heat dissipation limit hypothesis. There is, however, a problem. Laboratory mice have been selected for unusually high offspring productivity. Although the heat dissipation limitation hypothesis explains our observations in this model system, for wild small mammals, which have smaller litters, this limitation may never be reached. Establishing the ecological relevance of our observations is therefore critical before we can explore the wider ramifications of this finding. The current proposal involves work that will establish the ecological relevance of this breakthrough in our understanding of limits on SusEI of laboratory mice, therefore potentially enabling a broad spectrum of future studies. Because of the pivotal importance of SusEI, we have been able in the past to use studies of it to provide significant insights into a range of fundamentally important ecological questions. For example, understanding how climate changes influence animal distributions (Thomas, Blondell and Speakman 2001; Science; Humphries, Thomas and Speakman, 2002 Nature) and why kleptoparasitism has such an important effect on the population biology of African wild-dogs (Gorman et al 2001: Nature). The implications of understanding what limits SusEI also stretch to many other areas, including understanding the evolution of endothermy (Bennett and Ruben, 1989; Science Hayes and Garland,1996; Physiol Zool.) and the regulation of energy balance and obesity (eg Speakman et al 2002: Proc Nut Soc).