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Becky Ripley and Emily Knight investigate physical fitness in the animal kingdom, and ask why animals never seem to have to go to the gym.

Consider the Barnacle Goose, getting ready for one of the most phenomenal physical challenges of the animal world: the annual migration. They leave their sedentary summer life, floating about eating reeds, and take off to fly 2,700 miles. And what do they do to prepare for this incredible feat? Absolutely nothing. They just sit around, eating as much as they can.

The physical fitness of so many animals is hard-wired into their biology. But not ours. If we want to gain muscle, we don't just wait for the seasons to change, we have to work for it. No pain no gain! And if we slack off and laze about, our muscles melt away. Why are we so different? And do I really have to go to the gym?

Produced by Becky Ripley and Emily Knight. Featuring Professor Lewis Halsey from the University of Roehampton, and Professor Dan Lieberman at Harvard University.