Bone strength is maintained after 8 months of inactivity in hibernating ground squirrels, Spermophilus lateralis

Abstract

Prolonged inactivity leads to disuse atrophy, a loss of muscle and bone mass. Hibernating mammals are inactive for 6–9 months a year but must return to full activity immediately after completing hibernation. This necessity for immediate recovery presents an intriguing conundrum as most mammals require 2–3 times the period of inactivity to recover full strength. To address this problem, we dissected femur and tibia bones from squirrels in various stages of the annual hibernation cycle and measured the amount of force required to fracture these bones. Three groups were investigated; summer active animals were captured during the summer and immediately sacrificed, animals in the 1‐month detraining group were captured in the summer and sacrificed following a 1‐month period of restricted mobility, hibernating animals were sacrificed following 8 months of inactivity. A 3‐point bend test was employed to measure the force required to break the bones. Additionally, flexural strength and flexural modulus (material stiffness) were calculated for femurs. There were no significant differences between groups for femur break force, femur flexural strength, or tibia break force. Femur flexural modulus for the 1‐month detraining group was significantly different from both the summer active and hibernating groups (ANOVA, p < 0.05). Thus, hibernators seem resistant to the deleterious effects of prolonged inactivity.