MCNAB, BK; Univ. Florida, Gainesville: The energetics of reproduction in endotherms and its implications for their conservation

The energy expenditure of endotherms, through its impact on rate of reproduction, affects their ability to withstand competition and to tolerate environmental disturbances. For example, the extinction-prone attributes of living terrestrial mammals in China increase with body size and with a decrease in fecundity (Liu and Li 2005). The fecundity of endotherms is influenced both by body size and the level of energy expenditure: small mammals have higher fecundities than larger species and species with low rates of metabolism have lower fecundities than species of the same mass with high rates of metabolism (McNab 1980). Litter size and the rate of post-natal growth increase and gestation period decreases with an increase in basal rate of metabolism. As a consequence, species that are noteworthy for high population fluctuations, such as arvicolid rodents and hares, have high rates of metabolism and reproduction. Furthermore, species with high rates of metabolism appear to out-compete species with low rates when using resources that permit consumers to have high rates of metabolism, which explains why eutherian carnivores out-competed marsupial carnivores, none of which have high basal rates, a replacement that occurred in the Neotropics and Australia (McNab 2005). The dependence of reproduction on basal rate may contribute to our understanding of why there was such a huge die-off of birds endemic to oceanic islands upon the invasion by humans: island endemics appear to be characterized by low rates of metabolism (McNab 2002). The few ungulates that survived from the Pleistocene megafauna in North America have high rates of metabolism, whereas no species with presumptively low rates of metabolism and reproduction, such as ground-sloths and glyptodonts, survived.