NUTRITION AND GROWTH OF SUCKLING BLACK BEARS (URSUS-AMERICANUS) DURING THEIR MOTHERS WINTER FAST

In black bears the last 6-8 weeks of gestation and the first 10-12 wee ks of lactation occur in winter while the mother is in a dormant state , and reportedly does not eat, drink, urinate or defaecate. Measuremen ts were made of the body composition and organ weights of cubs, of the composition of milk, and of milk intake (by dilution of (H2O)-H-2), i n the first 3 months after birth. Additional milk samples were collect ed until 10 months postpartum. Bear cubs were small at birth, only 3.7 g/kg maternal weight, and chemically immature, as indicated by the hi gh concentration of water (840 g/kg) in their bodies. Organ weights at birth were similar to those of puppies. In the first month after birt h cubs gained 22 g/d or 0.23 g/g milk consumed; the milk was high in f at (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 lim it the utilization of glucogenic substrates during a long fast. Isotop e 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 metab olic economy of the fasting bear in its winter den.