Physiological Constraints on Life History Phenomena: The Example of Small Bear Cubs at Birth

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Abstract
Bears give birth to tiny, immature neonates and, for eutherian mammals, display the most extreme deviations from the regression of litter weight on mother weight. This may reflect physiological constraints on the ability of a pregnant female to meet the requirements of fetal metabolism while in a state of winter dormancy and without access to food or water. Although all postnatal mammals can use body-fat stores to supply energy during fasting, the mammalian fetus does not appear to have the capacity for catabolizing significant quantities of free fatty acids. Instead, it requires glucose as the principal oxidative substrate. However, a fasting female could continue to supply glucose to a developing fetus through gluconeogenesis only by using her body protein, eventually jeopardizing her own survival. By shortening the period of gestation and giving birth to very small, immature neonates, a female would shift from transplacental to mammary nourishment of her offspring. Fatty acids from maternal lipid stores could then be incorporated into milk and used directly for nourishing the offspring, thereby sparing maternal body protein. At the same time, the female would avoid elevated levels of ketone bodies, which might interfere with the maintenance of winter dormancy. The virtual absence of fatty acid catabolism by mammalian fetuses, even in species with an appreciable level of placental transfer of free fatty acids, is in striking contrast to the predominant use of lipids as an energy source by neonates. This notable absence of an energetic pathway in fetuses has never been adequately explained.