A new look at energy conversion in ectothermic and endothermic animals

In this review I offer a solution to the problem why endotherm populations appear to be so inefficient in converting food energy into body substance despite the fact that individual endotherms are just as 'efficient' in this respect as individual ectotherms. Calculated for individuals of half the adult mass both ectotherms and endotherms convert about the same proportion of food energy into somatic growth although for a given body mass the latter expend about 10 times more aerobic power than the former. On the other hand, early in life, during the period of maximum growth, ectotherms channel a 2-3 times greater percentage of metabolic energy into growth than endotherms. Even greater becomes the difference between these two groups if we consider the relative cost of reproduction. It can be shown that, weight by weight, nematodes, fish, birds and mammals require almost the same amount of energy for the production of offspring-, roughly 250 kJ per day and kg of eggs, hatchlings or litter. However, whereas the cost of producing offspring represents only 2%-6% of the total metabolizable energy of an endotherm, a fish has to spend 35%, a nematode nearly everything it has for this purpose. This may explain the finding by Humphreys (1979) and others that in nature the production, 'efficiencies' of endotherm populations appear to be at least one order of magnitude lower than those of ectotherm populations. However, rather than calling endotherms less 'efficient' energy converters, I suggest that by increasing total metabolic power more than ten-fold but keeping the energy cost of reproduction constant, this group of animals achieved emancipation from the burden of reproduction. Conversely, ectotherms have to channel a much greater proportion of metabolic power into reproduction because only in this way are they able to fit their low-rate life cycle schedules into the ecological schedules of the environment.