On the Evolution and Adaptive Significance of Postnatal Growth Rates in the Terrestrial Vertebrates

Even among similar-sized organisms a disparity often exists between different species' relative postnatal growth rates. Some authors have interpreted these differences in an adaptive light. That is, some organisms may grow faster than others because it is adaptive to do so (e.g., Lack, 1968). Other investigators have suggested that much of the interspecific variability in growth rates is due to differences in physiological constraints upon growth. That is, all organisms may be growing as fast as possible, but the physiological limits of growth are set at different levels in different organisms (e.g., Ricklefs, 1969, 1973). In the present study, the postnatal growth rates of a variety of reptiles, birds, mammals, and a few fish are compared, in an effort to find evolutionary differences between these vertebrate classes. This analysis suggests that the evolution of endothermy was a key factor in lifting physiological constraints upon growth rates. The maximum observed growth rates of endotherms (except for some marsupials and anthropoid primates) are at least an order of magnitude greater than the maximum growth rate of any ectotherm. The growth rate of altricial land birds is about twice that of most eutherian mammals, whether altricial or precocial. Most nidifugous birds grow at about the same rate as eutherians. Among mammals, some of the seals display the fastest growth rates of all. Other relatively fast-growing mammals include the artiodactyls, perissodactyls, cetaceans, canids, and lagomorphs. Rodents, prosimians, bats, and other seal species, and most other fissiped carnivores grow at more moderate rates. The slowest growing mammals are the marsupials and anthropoid primates. Elephants, bears, and monotremes also display relatively slow growth. Next, interordinal and interspecific differences in postnatal growth rates within the mammals and birds are explored for any evidence of adaptive "fine tuning" of the growth rate to an animal's environment. The results of this analysis, although not without exception, support the notion that growth rate is adapted to certain features of an animal's environment. In particular, the feeding requirements of an animal, its infant mortality rate, and the availability of food to its parents are sufficient to account for much of the interspecific variation in growth rates among birds and mammals. Ricklefs' (1969) mathematical model of growth is extended by allowing infant mortality rates to be "growth-rate"-dependent. With this adjustment, an optimum intermediate growth rate may exist, the magnitude of which will be altered in an empirically appropriate direction with changes in these environmental parameters. The statistical relatioships between postnatal growth rate and litter size, gestation period, relative brain size, relative birth weight, and milk protein levels are examined for mammals as a whole and for particular subgroups. Birth weight is the independent variable which accounts for most of the residual variance in growth rate (after accounting for adult weight) in most of these comparisons. Altricial mammals grow at statistically the same rate as precocial mammals. The selective factors influencing postnatal growth rates appear therefore to be somewhat different than those that influence the maturity of the young at birth.