VARIATION IN GROWTH FORM AND PRECOCITY AT BIRTH IN EUTHERIAN MAMMALS

Using the flexible Chapman-Richards model for describing the growth cu rves from birth to adulthood of 69 species of eutherian mammals, we de monstrate that growth form differs among eutherian mammals. Thereby th e commonly used Gompertz model can no longer be considered as the gene ral model for describing mammalian growth. Precocial mammals have thei r peak growth rate earlier in the growth process than altricial mammal s. However, the position on the altricial-precocial continuum accounts for most growth-form differences only between mammalian lineages. Wit hin mammalian genera differences in growth form are not related to pre cocity at birth. This indicates that growth form may have been associa ted with precocity at birth early in mammalian evolution, when broad p atterns of body development radiated. We discuss four non-exclusive in terpretations to account for the role of precocity at birth on the obs erved variation in growth form among mammals. Precocial and altricial mammals could differ according to (i) the distribution of energy outpu t by the mother, (ii) the ability of the young to assimilate the milk yield, (ii) the allocation of energy by the young between competing fu nctions and (iv) the position of birth between conception and attainme nt of physical maturity.