Following metabolic size allometry, the specific metabolic rate of mammals increases with decreasing body mass, resulting in a steeper metabolic fall-off and a faster exhaustion of energy reserves under hypoxic conditions. However, both mammalian hibernators and fetuses are able to temporarily "switch-off" Kleiber's rule as an adaptation to limited food or oxygen supply. Further exceptions to the usual metabolic size relationship are observed in newborn mammals. For instance, neonatal mouse hearts exhibit slower calorimetric "dying curves" under conditions of ischemia, although their aerobic tissue metabolic rates are higher than in adult samples. This is apparently due to a transient reduction of metabolic rate back to the former feto-maternal level. A continuing deviation from metabolic size allometry is found in newborn marsupials (Monodelphis domestica) where the "inappropriately" low specific metabolic rate is a precondition of efficient growth and tissue aerobiosis in spite of extreme immaturity. Obviously, adaptive suppression of elevated metabolism in organisms of small size results in a dramatic improvement of oxygen supply. Vice-versa, the overall increase in specific metabolic rate with decreasing body size might be regarded as one of several phylogenetic adaptations to protect tissues from hyperoxygenation.
