Question 22M.2.HL.TZ1.i

In winter when temperatures drop, brown bears (Ursus arctos) enter a cave and hibernate. The graph shows the mean values for the body temperature, heart rate and activity of 14 brown bears throughout the year. The grey shaded areas indicate the periods when the bears are in transition between hibernation and normal activity.

[Source: Adapted from Evans, A.L., Singh, N.J., Friebe, A., Arnemo, J.M., Laske, T.G., Fröbert, O., Swenson, J.E. and Blanc,
S., 2016. Drivers of hibernation in the brown bear. Frontiers in Zoology, 13(7). This article is distributed under the terms of the
Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/).]

It was observed that during hibernation bears are not susceptible to osteoporosis, a condition characterized by a decrease in the density of bone, resulting in porous and fragile bones. This condition may develop in humans during long periods of inactivity and with increasing age. The graph shows the porosity of the tibia bones of black bears (Ursus americanus) and humans at different ages. Age is expressed as the proportion of normal life span.

[Source: Adapted from Journal of Biomechanics, 39(8), Donahue, S.W., McGee, M.E., Harvey, K.B.,
Vaughan, M.R. and Robbins, T., Hibernating bears as a model for preventing disuse osteoporosis,
pp. 1480–1488. Copyright (2006), with permission from Elsevier.]

Healthy bone is constantly being broken down (bone resorption) and at the same time being rebuilt (bone formation). Bone mass should therefore not normally change, nor bone diseases occur. To test whether these processes occurred during hibernation in black bears, blood serum was tested for the markers ICTP (indicating bone resorption) and PICP (indicating bone formation).

[Source: Republished with permission of Company of Biologists Ltd, from Parathyroid hormone may maintain bone formation
in hibernating black bears (Ursus americanus) to prevent disuse osteoporosis. Donahue, Seth W; Galley, Sarah A; Vaughan,
Michael R; Patterson-Buckendahl, Patricia; Demers, Laurence M; Vance, Josef L; McGee, Meghan E, Journal of experimental
biology, 01 May 2006, Vol. 209, Issue Pt 9, pages 1630–1638; permission conveyed through Copyright Clearance Center, Inc.]

Osteocalcin is a peptide hormone that causes calcium to bind in the bones, so is involved in bone formation and regeneration. Research shows that changes in the mean concentration of osteocalcin in blood serum occur before and after hibernation in bears. In this research, concentration of both osteocalcin and parathyroid hormone were measured in the blood serum of bears. Results are shown in the bar chart and graph.

[Source: Republished with permission of Company of Biologists Ltd, from Parathyroid hormone may maintain bone formation
in hibernating black bears (Ursus americanus) to prevent disuse osteoporosis. Donahue, Seth W; Galley, Sarah A; Vaughan,
Michael R; Patterson-Buckendahl, Patricia; Demers, Laurence M; Vance, Josef L;McGee, Meghan E, Journal of experimental
biology, 01 May 2006, Vol. 209, Issue Pt 9, pages 1630–1638; permission conveyed through Copyright Clearance Center, Inc.]