Metabolic reduction (TMRrel) as a function of body mass. Slight increases of TMRrel among daily heterothems were non-significant (birds: t= 1.12, P=0.275; mammals: t=1.59, P=0.117). Among hibernating mammals there was a significant relationship between TMRrel and body mass (log10TMRrel=0.81+0.20 log10BM, t=5.40, P<0.0001, R2=0.42).