A comparison of postnatal thermal physiology and energetics in an altricial (Gerbillus perpallidus) and a precocial (Acomys cahirinus) rodent species.

1. Daily weight increase is polyphasic in Gerbillus perpallidus whereas it follows a linear pattern in Acomys cahirinus. 2. Young A. cahirinus achieve full homeothermic capacities much earlier (less than or equal to 12 days old) than G. perpallidus (19-21 days old). 3. The differences in the development of the two species are reflected in the alterations of their metabolic rates. 4. In both species, age-dependent phases of development are obvious, beginning, however, much later in G. perpallidus. 5. It is concluded that, by postponing functional maturity, considerably more energy can be allocated to growth. This may be a precondition for the altricial mode of development.