Staying Fit

For our muscles to stay in shape, we must exercise them. Rodents face a similar situation. But the need for physical exercise to stay fit is not the norm among animals.

Even wild mice readily run on an exercise wheel. As with humans, rodent workouts are rewarded by a desirable dopamine deposit into their system; what people commonly call a “runner’s high.” Hence, though exercising is taxing, Nature provides an intrinsic incentive.

Migratory geese may fly thousands of kilometers at a stretch. To get themselves fit, geese simply sit on the water and stuff themselves with food. Yet in doing so, they develop stronger hearts and bigger flight muscles.

In a wide variety of animals, seasonal signals prompt various alterations, including those related to fitness. Migratory birds undergo innumerable genetic changes that are stimulated solely by the changing hours of daylight.

Bear muscles do not waste away despite months of inactivity. Before hibernating, bears’ bodies spontaneously release muscle-protecting compounds into their blood.

In needing to exercise to keep fit, rodents and humans are exceptions in the animal kingdom. Evolutionary trade-offs prompted this. Our ancestors lived unpredictable lives, particularly regarding food supplies. Hence our body’s ability to add fat more readily than any other animal.

Muscle mass is energetically expensive. Each kilogram of muscle adds 10–15 kilocalories a day to our resting metabolism. 40% of average human body mass is muscle. “Most of us are spending 20% of our basic energy budget taking care of muscle mass,” said American paleoanthropologist Daniel Lieberman.

Our physiology evolved to let weight and fitness fluctuate depending upon food availability. Rodents face similar environmental conditions, and so have a selfsame set of traits related to musculature.

Cytologists have discovered a cheerleading family of proteins that engender physical fitness: sestrins. “Sestrins are small stress-inducible proteins that are found throughout the animal kingdom,” wrote American physiologist Robert Wessells. “Sestrins coordinate metabolic homeostasis by regulating multiple signaling pathways.” Wholesome levels of sestrins have been found in various physically fit animals, including birds, which don’t need to exercise to stay fit.

Physiologists do not know how sestrin levels are regulated in animals. But then, physiology alone cannot explain how matter may animate itself into the adventure of living.

Muscularity is ultimately a matter of energy, not physicality. That muscles may tone themselves is evidence that our bodies are sustained by an energetic force, species-specific in its dynamic characteristics.


Ishi Nobu, Unraveling Reality: Behind the Veil of Existence, BookBaby (2019).

Myungjin Kim et al, “Sestrins are evolutionarily conserved mediators of exercise benefits,” Nature Communications (13 January 2020).