An eight-kilogram rhesus macaque can live up to forty years. A cat of the same weight rarely sees its twentieth birthday. This difference is no accident: across all primates, from tiny marmosets to gorillas, lifespan is markedly longer than in other mammals of similar body mass. Standard models linking lifespan to metabolic rate fall short here.
The authors suggest viewing aging as a heat bill. Every heartbeat, every breath, every cell division is a small release of energy, or entropy. The body is born with a limited number of such cycles. When the limit is reached, life ends. In primates, this counter ticks more slowly because a significant share of energy goes to fueling the brain — up to a quarter of all calories in humans. Nervous tissue doesn't just consume fuel; it invests it in three tasks: fine-tuning the internal environment (the liver and kidneys get commands in advance, not after the fact), speeding up cellular repairs, and avoiding danger without unnecessary running around. Together, this reduces internal chaos — less useless friction, less wear.
This gives rise to a neuro-metabolic multiplier. If a cat uses, say, five percent of its energy on brain work, and a macaque uses fifteen, then the remaining 85 percent serves the body, which wears out noticeably slower. Mathematically, this resembles a house with a smart thermostat: the boiler runs less often, and the radiators last longer. The authors don't just speculate: they derive a formula linking the share of neural expenditure, entropy production, and ultimate lifespan. It can be tested by measuring heat flows in different species and comparing them with age.
The idea brings us back to the original picture. The macaque and the cat weigh the same, but the macaque carries in its head a tool that turns calories into years. The same principle may explain why humans live so long even compared to other primates — our brains are even more voracious and even more careful in managing the household.
In humans, the brain consumes about 20% of all energy while making up only 2% of body mass, as if a small lightbulb used a fifth of the electricity in a large house.