A shrew hammers away at eight hundred beats per minute; an elephant manages twenty-eight. Yet over a lifetime, both hearts contract about a billion times. Why does it work out that way?
Every heartbeat spends energy, and that energy dissipates as heat. It turns out that per kilogram of body weight and per single beat, all warm-blooded animals spend almost the same amount of energy. A mouse burns through it faster, an elephant more slowly, but the total lifetime budget is the same. That is why the number of beats matches too.
Some animals break the general rule. Bats live longer because they often enter torpor and save energy. Birds burn fuel more efficiently. Whales slow their pulse when diving, and once you factor that in, their budget turns out to be larger than usual.
If you measure life not in years but in heartbeats, then every species has its own biological time. It runs faster for a mouse and slower for an elephant. This helps estimate biological age: the less energy spent per beat, the more slowly an animal ages.
A shrew's heart beats 800 times per minute, while an elephant's beats 28, yet over a lifetime both perform roughly the same number of beats. In humans, the brain consumes 20% of the body's energy despite making up only 2% of its mass.
The idea of biological proper time echoes Greg Egan's novel 'Permutation City', where virtual personalities live in accelerated or decelerated subjective time, and their 'lived life' is measured by internal events rather than external seconds.