Hibernation and hemostasis

Abstract

Hibernating mammals have developed many physiological adaptations to accommodate their decreased metabolism, body temperature, heart rate and prolonged immobility without suffering organ injury. During hibernation, the animals must suppress blood clotting to survive prolonged periods of immobility and decreased blood flow that could otherwise lead to the formation of potentially lethal clots. Conversely, upon arousal hibernators must be able to quickly restore normal clotting activity to avoid bleeding. Studies in multiple species of hibernating mammals have shown reversible decreases in circulating platelets, cells involved in hemostasis, as well as in protein coagulation factors during torpor. Hibernator platelets themselves also have adaptations that allow them to survive in the cold, while those from non-hibernating mammals undergo lesions during cold exposure that lead to their rapid clearance from circulation when re-transfused. While platelets lack a nucleus with DNA, they contain RNA and other organelles including mitochondria, in which metabolic adaptations may play a role in hibernator’s platelet resistance to cold induced lesions. Finally, the breakdown of clots, fibrinolysis, is accelerated during torpor. Collectively, these reversible physiological and metabolic adaptations allow hibernating mammals to survive low blood flow, low body temperature, and immobility without the formation of clots during torpor, yet have normal hemostasis when not hibernating. In this review we summarize blood clotting changes and the underlying mechanisms in multiple species of hibernating mammals. We also discuss possible medical applications to improve cold preservation of platelets and antithrombotic therapy.

Hibernating or not hibernating? Brown bears’ response to a mismatch between environmental natural cues and captive management, and its welfare implications

“…Wild bear hibernation response has proved to retain a certain degree of flexibility [ 4 , 12 , 18 , 47 – 49 ] in response to extremely variable environmental conditions experienced in different parts of their wide geographical distribution (ranging from 20°N to the Arctic Ocean [ 81 ]). Brown bear, in fact, has been defined as a shallow hibernator species [ 24 , 82 , 83 ] as opposed to obligate hibernators (e.g., chipmunks, ground squirrel and groundhog [ 24 , 84 ]). In the latter, for example, body temperature reaches values close to freezing, in comparison to the about 31–32°C reported for bears [ 4 ], although mechanisms employed by small and large heterotherms, functional to metabolic rate reduction, are likely to be different due to different surface-to-volume ratios and related energetic challenges [ 4 , 85 ].…”

“…Main environmental triggers of hibernation are reported to be temperature, photoperiod and food availability [ 3 , 4 , 12 , 20 – 23 ]. Captive management of such a species, genetically programmed and physiologically adapted to hibernate (reviewed in [ 24 ]), and whose metabolism is strongly affected by circadian rhythms (e.g., [ 25 ]), might require a specific attention to the best practices accommodating bears needs.…”