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POLAR BEARS (URSUS MARITIMUS), THE MOST EVOLUTIONARY ADVANCED HIBERNATORS, AVOID SIGNIFICANT BONE LOSS DURING HIBERNATION


Abstract

Some hibernating animals are known to reduce muscle and bone loss associated with mechanical unloading during prolonged immobilisation,compared to humans. However, here we show that wild pregnant polar bears (Ursus maritimus) are the first known animals to avoid significant bone loss altogether, despite six months of continuous hibernation. Using serum biochemical markers of bone turnover, we showed that concentrations for bone resorption are not significantly increased as a consequence of hibernation in wild polar bears. This is in sharp contrast to previous studies on other hibernating species, where for example, black bears (Ursus americanus), show a 3-4 fold increase in serum bone resorption concentrations posthibernation,and must compensate for this loss through rapid bone recovery on remobilisation, to avoid the risk of fracture. In further contrast to black bears, serum concentrations of bone formation markers were highly significantly increased in pregnant female polar bears compared to non-pregnant,thus non-hibernating females both prior to and after hibernation. However, bone formation concentrations in new mothers were significantly reduced compared to pre-hibernation concentrations. The de-coupling of bone turnover in favour of bone formation prior to hibernation, suggests that wild polar bears may posses a unique physiological mechanism for building bone in protective preparation against expected osteopenia associated with disuse,starvation, and hormonal drives to mobilise calcium for reproduction, during hibernation. Understanding this physiological mechanism could have profound implications for a natural solution for the prevention of osteoporosis in animals subjected to captivity with inadequate space for exercise,humans subjected to prolonged bed rest while recovering from illness, or astronauts exposed to antigravity during spaceflight.© 2008 Elsevier Inc. All rights reserved.

... The bear gets its energy from stored fats, and muscle mass is not lost, but renewed ( Lohuis et al., 2007). The products of catabolism, such as urea, are not excreted but recycled ( Nelson et al., 1975;Floyd et al., 1990; Barboza et al., 1997) and the bone tissue does not suffer losses but is actively remodelling (Donahue et al., 2006;Lennox and Goodship, 2008;McGee et al., 2008), although at a lower rate than during the active season. During hibernation the synthesis of proteins is made from the ni- trogen compounds produced thanks to the recycling of the reab- sorbed urea. ...

... In conclusion, we provide both in vivo and in vitro evidence supporting the expression of circadian rhythms in bears during winter dormancy. These findings, along with earlier work in grizzly bears [64] and polar bears add to the mounting evidence that these closely related species may exhibit an evolutionarily advanced form of torpid biology [65, 66]. ...

https://www.researchgate.net/publication/5603402_Polar_bears_Ursus_Maritimus_the_most_evolutionary_advanced_hibernators_avoid_significant_bone_loss_during_hibernation/amp Reply