“Generation time measures the trade-off between survival and reproduction in a life cycle”

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Stefano Giaimo and Arne Traulsen (Aug 2019)

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Generation time measures the trade-off between survival and reproduction in a life cycle

Optimization theory shows limitations of model organisms of longevity

A rose chafer (Cetonia aurata) after the rain in Bad Malente (Germany). This beetle, like most organisms, is likely to face a trade-off between survival and reproduction.
(Credit: Stefano Giaimo)

The African elephant or the desert tortoise can live up to 80 years in a dangerous world without medicine: Shouldn’t we do genetics of longevity on them to get their secret? Yes, in principle. But it would very impractical. Looking at differences between only two generations of these animals would exceed the life expectancy of a human researcher and the usual horizon of science funding. For this reason too, animal models of aging and longevity, like the fruit fly and the mice, are usually chosen that are fast-lived in comparison to humans. Mutations that extend lifespan in these animals are isolated in the hope of finding similar human genes. But results from this approach may not transfer smoothly to humans.

Mutants with longer life are often less fertile. Optimization theory shows that the reproductive cost of a slightly increased longevity in a species is measured by the average age at parenthood in the species, the so-called generation time. A 1% increase in survival, for example, imposes a percentage cost in reproduction equal to the generation time. But fruit flies usually give birth at around 11 days, while humans need on average 20 to 30 years. Therefore, studies on animal models may underestimate the fertility price of enhanced longevity in humans. While short-lived animals are likely to remain important and very useful models in the genetics of longevity, this result helps us to better understand their potential limitations.


Survival and fertility are the two most basic components of fitness and they drive the evolution of a life cycle. A trade-off between them is usually present: when survival increases, fertility decreases – and vice versa. Here we show that, at an evolutionary optimum, the generation time is a measure of the strength of the trade-off between overall survival and overall fertility in a life cycle. Our result both helps to explain the known fact that the generation time describes the speed of living in the slow-fast continuum of life cycles and may have implications for the extrapolation from model organisms of longevity to humans.