4 ideas
At 41, biologist Bernd Heinrich entered the 1981 US national 100-kilometre championship in Chicago after months of self-designed training in rural Maine. He modelled that training and his race-day fuelling on what he knew of animal endurance physiology. He won in about 6 hours 38 minutes, setting an American record, which made his own body a test case for the biological principles he describes.
Humans cannot outsprint antelope, but they can outlast them over long distances in heat. This is because humans shed heat through abundant sweat glands and largely hairless skin, while furred quadrupeds must stop and pant to cool down. Persistence hunting exploits this gap: the hunter keeps a steady pace until the animal overheats and collapses, which suggests distance running is an evolved human specialty rather than a modern hobby.
Endurance often fails because an animal cannot get rid of heat fast enough, not because its muscles give out. Different species solve this in different ways. Camels let their body temperature rise during the day to save the water that sweating would cost, and bees and moths manage heat produced by their flight muscles. Seen this way, a runner's pace, clothing, hydration and choice of race conditions are all heat-budget decisions.
Migrating birds such as tiny songbirds fatten heavily before departure because fat stores far more energy per gram than carbohydrate. This lets them fly non-stop for many hours. The same logic applies to ultradistance runners: glycogen stores are too small for a 100-km effort, so performance depends on a metabolism trained to burn fat and spare limited carbohydrate.