Ratios of hatchling and adult mass-independent metabolism: a physiological index to the altricial-precocial continuum.

Measures of metabolism which are theoretically mass-independent (MIM values = BMR divided by M0.67) are calculated for non-passerine species from data in the existing literature. These MIM values are, in fact as the acronym implies, essentially mass independent, with only 8.2% of the variation in MIM values attributable to log mass. In 10 of the 12 orders for which data were analysed, the slopes of the regressions of MIM values on log body mass do not differ significantly from zero. A dimensionless index of the intensity of the energy metabolism of hatchlings relative to that of conspecific adults is derived based upon the mass-independent measures of metabolism for adults and hatchlings. This ranking of metabolic maturity at hatching, a physiological index, is compared with the most commonly used classification of the avian altricial-precocial continuum which is based upon morphological and behavioral characteristics. Several taxonomic groups appear more precocial metabolically than indicated by their ranking based upon morphological and behavioral criteria.