Endothermy and Parental Care in the Terrestrial Vertebrates

Publisher Website
Google Scholar
Abstract
Hypotheses regarding the historical evolution and adaptive significance of parental care and the altricial condition in birds and mammals are discussed. Except at the extreme upper and lower limits of body size, there is no consistent correlation between body size and the degree of maturity at birth. While these adaptations are not neccessitated by very small size, they apparently are sufficient to explain its empirical occurrence. Arguments are presented to support the notion that among extant animals each species'' reproductive features are shaped more by attributes of its niche and habitat than by its body size. The relative time and expense of finding and capturing prey compared with eating and digesting it, and the relative intensity of predation on the young compared with that on the adult appears to be most important in determining the optimum egg (or neonatus) size and its maturity at birth. Based on existing reptiles, reasons for expecting rudimentary postantal parental care to evolve with greater likelihood in larger as opposed to smaller therapsids is examined. The radiation of extended postnatal parental care awaited the evolution of endothermy, which resulted in a situation where the young became relatively more reliant on parental care for survival while their parents evolved attributes which better equipped them to extend it. The crucial step in the evolution of altriciality probably occurred in very small mammals because the relatively large amount of nonthermogenic tissue formed during egg production would have imposed a serious energetic burden on the incipient endothermic parents. By producing much smaller and altricial young this problem would be alleviated. Reptiles produce relatively smaller eggs (or neonates) than altricial birds (or eutherian mammals), and these hatch to produce fully functional precocial young. Why can no existing endotherm produce precocial young from such relatively small eggs? Sacher''s (1966) investigations relating the speed and energetic cost of embryogenesis to the size and complexity of the central nervous system are discussed.