C - Michigan Technological University

120-122]. Most small hibernators are also believed to lose bone during hibernation [104-109], though recent evidence suggests that golden-mantled ground squirrels do not[110]. In contrast, bears only drop 1-8 o C below active body temperature, do not excretewastes or eat, and do not lose bone during hibernation [13, 18-21, 34, 133, 161, 174]. Insmall hibernators like the raccoon dog and the marmot, serum leptin decreases steeplyupon winter fast, preceding changes in weight [233, 234, 237], though in the raccoondog leptin levels rise again before the end of winter [234]. The decrease in serum leptinof American black bears took place gradually through hibernation and continued intopost-hibernation (Figure 3.1). It may be an evolutionary advantage for bears to retainraised leptin concentrations longer into hibernation, as the leptin would help themmaintain higher body temperatures and suppress appetite-induced arousal.Furthermore, serum leptin decreases by 67% in hind-limb unloaded rats, and replacingserum leptin during unloading inhibits disuse-induced bone loss [215-217]. Therefore,maintaining high leptin levels longer into hibernation could help bears suppress boneloss during this extended period of disuse.3.4.2 Reduced IGF-1 and increased NPY in the serum of hibernating bears favorsuppression of bone turnoverThe current study found that serum IGF-1 decreased during hibernation in blackbears. IGF-1 also decreases in the serum of hibernating golden-mantled groundsquirrels [238], which do not lose bone during this extended period of disuse [110]. It ispossible that depressed serum levels of IGF-1 could promote the lower rate of boneturnover observed in hibernating bears [18, 20] by slowing the rates of osteoblast [221]and osteoclast [222-224] recruitment and activation. Such a decrease in bone turnovermay, in the unique conditions of hibernation, help preserve bone despite disuse.Bears switch to fat metabolism during hibernation, due to anorectic conditions[111]; therefore they may be compared to animals on high fat diets: rats fed a highfat/low carbohydrate diet experience raised concentrations of leptin and lowered IGF-1 inthe serum [239]. These rats display depressed rates of bone formation and unchangedbone resorption, resulting in reduced bone length and BMD of the tibia [239]. However,these 4-week-old rats were at an age of rapid growth [239, 240] through the duration of53