Furthermore, maintenance of homeothermy
during this transition period strongly supports the well being of lambs, however, failing of this may lead to increase mortality rate (Dwyer, 2008).
They argued that mammalian homeothermy evolved in two steps.
The ability to maintain a relatively constant body temperature, termed homeothermy, is one potential advantage of endothermy.
In any case, as body size decreased and mass-specific metabolism increased, homeothermy based on thermal inertia was supplanted by endothermic homeothermy.
For example, if it could be shown from fossil indicators that high resting metabolic rate and endothermic homeothermy preceded high maximal aerobic capacities (and associated high stamina), then the aerobic capacity model would be falsified.
Standard metabolism of monotremes and the evolution of homeothermy. Australian Journal of Zoology 27:511-515.
However, Bennett (1991) pointed out that metabolic heat production at most levels intermediate between those of living reptiles and mammals is insufficient to establish endothermic homeothermy, and thus provides little thermoregulatory benefit, while substantially increasing the animal's energy budget.
Metabolic capabilities of monotremes and the evolution of homeothermy. Pp.