Final answer:
Animals reared in darkness show shrinkage of the skeletal muscle due to physical inactivity and atrophy.
Step-by-step explanation:
Animals reared in darkness show shrinkage of the skeletal muscle. This is due to physical inactivity and atrophy, which occurs when animals are inactive and experience muscle weakening and decreased muscle cell size and number. This phenomenon is observed in both rodents and humans, as well as small hibernating mammals.