Disposable Soma Theory

What it proposes

Tom Kirkwood in 1977 proposed that an organism faces an energetic trade-off: resources can go into reproduction or into maintaining and repairing the body (the “soma”). Evolution selects for an optimal balance that maximises fitness, which generally means investing “just enough” in maintenance to remain functional through the typical reproductive window plus some margin.

The result is that organisms are not built to last indefinitely — they are built to reproduce and then become “disposable”.

Evidence

• Animals with low extrinsic mortality (long-lived bats, naked mole-rats, bowhead whales) tend to invest more in maintenance and live longer.
• Reproductive output and lifespan are often inversely correlated within species.
• Caloric restriction may shift energy from reproduction to maintenance, extending lifespan at the cost of fertility — consistent with the theory.

Implications for interventions

If lifespan is set by maintenance-investment balance, then interventions that boost maintenance pathways (DNA repair, autophagy, proteostasis, mitochondrial quality control) should extend it — which is exactly what most longevity-relevant interventions do.

Limitations

• Doesn’t fully explain why aging is often programmed (timed mortality) rather than random.
• Doesn’t address why some species (Hydra, Turritopsis) appear to have negligible senescence.

Related entries

Antagonistic pleiotropy, Programmed aging, Caloric restriction.