Image courtesy of http://www.healthsci.utas.edu.au/physiol/mono/Mainpage.html

An early theory of mammalian evolution proposed that mammals evolved homeothermy in two distinct evolutionary steps (Dawson et al., 1979): First step: Monotremes.  These "primitive" mammals have low body temperatures and use "reptilian" energetics to exploit a nocturnal niche.  Second step: Marsupials and eutherians.  These mammals perfected homeothermy by increasing body temperature and metabolic rate well above that of the monotremes.  This theory mostly relied on data from echidnas, which were assumed to be representative of all monotremes.  An examination of data from both platypi and echidnas and several other mammals of similar size reveals a slightly different picture.

These data suggest two things.

First, the assumption that echidnas were representative of all monotremes was incorrect.  The platypus has a much higher BMR than the echidna.  The elevated BMR can be attributed to (1) the phylogenetic distance between the echindas and the platypus (30 to 50 million years) and (2) the aquatic lifestyle of the platypus, which requires a higher metabolism to cope with greater thermal stress (Dawson et al., 1979).

Second, when platypi are considered, the difference between monotreme and marsupial BMR does not seem as great.  Although the eutherian still has a much greater BMR than the monotremes, there does not seem to be such a dichotomy between monotremes and other mammals.  The distinct steps of the early theory are blurred.

It now appears that the evolution of homeothermy may have been gradual rather than stepwise.  Monotremes have many ancestral traits, but they can regulate a constant body temperature over a wide range of metabolic rates.  They are competent homeotherms, despite their lack of high eutherian-like body temperature and BMR (Dawson et al., 1979).