The Greenland shark can live more than four hundred years, and the older it gets, the lower its odds of dying. Usually it's the other way around: mortality rises with age. But in some animals that grow throughout their lives, aging runs in reverse. Scientists built a model to figure out why this happens.
An organism is like a household budget that can't be spent twice. Energy can go into reproduction, growth, or body repair. It turned out that when the returns on investment in growth or reproduction diminish (each next portion of energy yields less), evolution favors a strategy of continuous growth. The animal keeps growing even after sexual maturity, and mortality first declines with age. This is negative senescence.
It's now clear that negative senescence is not a rarity but a natural consequence of an evolutionary trade-off. The model helps predict which species will live longer, and it explains why the Greenland shark only gets tougher with age.
The Greenland shark can live more than 400 years, and its mortality apparently does not rise with age. The model predicts that in such species, returns on investment in growth or reproduction should diminish more strongly than in species with classic aging.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.