Hibernation by echidnas in mild climates: Hints about the evolution of endothermy?

Short-beaked echidnas undergo both torpor and hibernation, expressed to dif ferent extents in different climates. We propose that when well fed animals hibernate in comparatively mild climates, with food available, they are us ing the winter cold as a resource and "putting themselves on ice'' until th e next breeding season. That is, echidnas are hibernating in mild climates for energy advantage, not from energetic necessity. We interpret the use of hibernation by echidnas in the more severe climates, where thc re is a foo d shortage, as a specialisation of a more general, ancestral capacity. Ther e are also avian, marsupial and eutherian species in which torpor in mild c limates appears to be practised for energetic advantage rather than from ne cessity. The similarity or patterns of hibernation in echidnas, mountain pi gmy possums and arctic ground squirrels emphasises the likelihood that torp or and hibernation are plesiomorphic across all three Sub-classes of Mammal ia. Attention is drawn to parallels between the daily/seasonal cycles in th e body temperatures of torpidators/hibernators acid those seen commonly in reptiles, and to the extent to which thermoregulatory mechanisms in reptile s foreshadow those in mammals and birds. The entry of mammals into torpor/h ibernation may involve a reversal of the same physiological mechanisms whic h accompanied the evolution of endothermy from ectothermy, and eohidnas may provide a useful model. A stepwise scenario for the evolution of endotherm y is presented, with torpor/hibernation as a central theme. In presenting t his scenario, we take care to distinguish between pattern and mechanism, re cognising that the terms torpor and hibernation, like poikilothermy and hom eothermy, are descriptive of patterns, not mechanisms, and that this limita tion of the current terminology must be recognised.