Why big-bodied animal species cannot evolve a waste-to-hurry strategy

Abstract

Reserve capacity quantifies the ability of an animal to smooth out fluctuations in food availability. It is defined as the maximum reserve density, and can be quantified through the application of the Dynamic Energy Budget (DEB) theory. In this study, we analyze inter-specific patterns in DEB parameters of 1041 animal species, focusing on those that control reserve capacity (maximum specific assimilation and energy conductance) and maintenance.The co-variation rules of DEB theory expect that specific somatic maintenance and energy conductance are independent of maximum structural length, with the implication that reserve capacity is proportional to maximum structural length among species and independent of somatic maintenance. We found however that the reserve density increases with somatic maintenance among all large animal taxa in the collection: invertebrates, fish and amphibians, sauropsids and mammals.The waste-to-hurry phenomenon implies that small-bodied species frequently have both a higher specific maintenance and higher specific assimilation (with respect to larger species) allowing them to boost growth and reproduction. If waste-to-hurry strategists would increase both parameters in proportion, maximum structural length would not be affected, reserve capacity would become proportional to somatic maintenance and its relationship with maximum structural length would depend on quantitative details. We did find a positive relationship between reserve capacity and specific somatic maintenance, but the increase (as an average over all taxa) is less than proportional. The reason is that specific assimilation is less than proportional to specific somatic maintenance, the scaling parameter roughly being 0.8, while energy conductance hardly depends on somatic maintenance. The implication is that maximum assimilation is proportional to structural length to the power − 0.7, which explains why the waste-to-hurry strategy is not a viable route for big-bodied species: they would need more assimilates than small-bodied ones, not less.We discuss applications of these findings in the context of parameter estimation. As a side-result we also found that birds have a higher specific somatic maintenance and also a higher (mean) energy conductance, compared to other sauropsids at the same body temperature. The first is to be expected because they are demand species, the latter might be an adaptation to flight: reserve density increases for decreasing energy conductance, while it contributes to weight.