In black bears the last 6–8 weeks of gestation and the first 10–12 weeks of lactation occur in winter while the mother is in a dormant state, and reportedly does not eat, drink, urinate or defaecate. Measurements were made of the body composition and organ weights of cubs, of the composition of milk, and of milk intake (by dilution of 2H2O), in the first 3 months after birth. Additional milk samples were collected until 10 months postpartum. Bear cubs were small at birth, only 3·7 g/kg maternal weight, and chemically immature, as indicated by the high concentration of water (840 g/kg) in their bodies. Organ weights at birth were similar to those of puppies. In the first month after birth cubs gained 22 g/d or 0·23 g/g milk consumed; the milk was high in fat (220 g/kg) and low in water (670 g/kg). About 30% of the ingested energy and 51% of the ingested N were retained in the body. Over the entire 12-week period bear cubs required about 11 kg milk, containing (kg) water 7, fat 2·5, protein 0·8 and total sugar 0·25, to achieve a 2·5 kg weight gain. The birth of immature young and the production of high-fat, low-carbohydrate milk seem to be maternal adaptations to limit the utilization of glucogenic substrates during a long fast. Isotope recycling indicates that mothers may also recover most of the water (and perhaps much of the N) exported in milk by ingesting the excreta of the cubs. Lactation represents another aspect of the profound metabolic economy of the fasting bear in its winter den.
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