ZELDITCH, M.L.*; LUNDRIGAN, B.L.: Developmental causes and costs of precocial development.

The adaptive value of precocial development may depend on its developmental basis. When precociality arises by an increase in developmental rate, later growth may be constrained because terminally differentiated cells do not divide. This trade-off has been postulated for precocial birds. In mammals, precociality might be achieved by increasing gestational length, which should not generate constraints on growth. We compare the growth and development of a precocial rodent, Sigmodon fulviventer, and an altricial rodent, Mus domesticus, the later a widely-used model for mammalian development. The dynamics of growth and development generally support the hypothesis that Sigmodon are older than Mus and birth. Growth and developmental rates are maximal at equivalent gestational ages, and, in both species, local morphogenetic processes shaping the palate occur at similar gestational ages. However, tooth development, while initiated at the same gestational age in both species, proceeds at a lower rate in Sigmodon, which may explain its older gestational age at weaning. In contrast, allometric patterns stabilize at the same chronological rather than gestational age. Sigmodon grows at a higher rate postnatally than Mus, even taking into account its larger neonate; this argues against a hypothesis of a trade-off between early development and later growth.