Mammalian design and rate of living

Terrestrial placental mammals are physiologically and biochemically similar. However, though some of their design and life history characteristics vary in a uniform way with body mass across the mammalian class (e.g. basal metabolic rate, body length, liver or adrenal mass, etc.), others follow less uniform "laws" with apparent differences among the various orders (e.g. brain mass, life span, etc.). Based on data from 38 species, it is shown that despite the absence of a uniform law describing body mass, my, of young at birth, or litter size, n, their product, i.e. total body mass of litter at birth, m lambda, does follow such a law: It appears that, like basal metabolic rate, m lambda is proportional to the 3/4-power of body mass of the mother; no inter-order differences are apparent. Nevertheless, despite their remarkably similar metabolic apparatus, mammals cannot be functional scale models of each other, i.e. the metabolic machinery has to be set at a lower level as body size increases, leading to reduced heart rate, developmental rate, and "rate of living"--and thus prolonged life span.