Why do some animals live so much longer than others?

Across mammals, lifespan correlates broadly with body size and inversely with metabolic rate, a pattern captured in the old rate-of-living idea that animals burning energy faster wear out sooner. A shrew lives about a year and a bowhead whale can exceed two centuries. Predation pressure is the deeper explanation: species that are likely to be eaten young gain little evolutionary benefit from investing in long-term maintenance and instead reproduce fast, while species that are difficult to kill can profit from durability. Heart rate follows a related pattern, with many mammals living roughly a billion heartbeats regardless of species.

The exceptions are where the research interest lies. Bats live far longer than their size and metabolic rate predict, with some small species exceeding thirty years, and the leading explanation involves flight, which removed predation pressure much as it did for birds, alongside unusual DNA repair and inflammation control. Naked mole-rats live over thirty years against a mouse's three, show almost no increase in mortality rate with age, and are strikingly resistant to cancer. Greenland sharks are estimated to live several centuries, and some clams and corals far longer still.

Two organisms complicate the concept entirely. Certain hydra show no measurable senescence under laboratory conditions, maintaining stem cell populations indefinitely. And the jellyfish Turritopsis dohrnii can revert from its adult medusa stage back to an earlier polyp stage under stress, effectively restarting its life cycle, which has earned it the label immortal though individuals are still routinely eaten or killed by disease. The reason this research is funded generously is transferability: the mechanisms these species use, including enhanced DNA repair, protein quality control, and telomere maintenance, are the same ones implicated in human aging.