A Greenland shark swims through cold water in its fourth century, and with every passing year its odds of dying do not rise but fall. This is not a miracle, but the result of an evolutionary trade-off that has so far eluded theory. An organism is like a family budget that cannot be spent twice: every unit of energy can go into reproduction, growth, or body repair. Classical disposable soma theory held that after sexual maturity it pays to spend everything on offspring, and let the body wear out. But in species that grow throughout life, this calculation does not add up.

A mathematical model shows that if the returns on investment in growth or reproduction diminish, evolution selects a strategy of continuous growth. An individual keeps growing after maturity, and mortality first falls with age. This period of negative senescence arises because growth lowers mortality faster than the accumulation of damage raises it. In numerical examples, mortality drops by 20-30% from its maximum value.

Body repair appears in the optimal strategy only if its returns diminish, or if the returns on growth and reproduction diminish at the same time. Unlike classical disposable soma theory, growth itself can lower mortality if larger individuals have an advantage. This links two previously separate phenomena: unlimited growth and improving life-history indicators with age.

The picture is now clearer: negative senescence is not an exotic exception but a natural outcome of an evolutionary trade-off. The model yields testable predictions for species with unlimited growth, for example for analyzing long-term demographic data. The Greenland shark, which only gets tougher with age, turns out to be not an anomaly but an illustration of a general rule.

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