What living things have a very short lifespan

Those who live longer have fewer children

The time between two generations determines the price of fertility

A long life and lots of children - this was a widespread wishful thinking until not so long ago. However, a look at the animal kingdom shows that high fertility and a long lifespan are often mutually exclusive: particularly short-lived animals are often very fertile, while long-lived animals, on the other hand, often have fewer offspring. With limited resources, organisms can apparently either live long or be very fertile - but not both at the same time. Researchers from the Max Planck Institute for Evolutionary Biology in Plön have now discovered how the compromise between survival and fertility works. According to this, the average age at reproduction is a measure of the loss of fertility with increasing lifespan.

The African elephant or the desert tortoise can live up to 80 years. They are exposed to many dangers throughout their lives and receive no medical help whatsoever. Such animals would actually be an ideal research subject to uncover the secret of a long life. Age research on long-lived animals can hardly be implemented in reality. A researcher's life would not be enough to investigate the differences between just two generations of elephants. In addition, such extremely long research projects would be very expensive.

Scientists therefore mostly use animals with a relatively short lifespan, such as mice or fruit flies, as models for aging and longevity. Since these animals also reproduce quickly, changes in aging and lifespan between different generations can be easily observed and investigated. Often genetic changes become visible that extend the lifespan. If humans have similar genes, it can be assumed that these also have a life-prolonging effect in humans.

However, there is another component that is directly related to an organism's lifespan: its fertility. Short-lived animals are often very fertile, while long-lived animals have only a few offspring. Mice, for example, only live about two years, but become sexually mature after just a few weeks and then give birth to three to eight young up to eight times a year. Elephants, on the other hand, can live up to 80 years, but in the course of their life a cow elephant only has up to ten offspring. In evolution there seems to be a kind of compromise between longevity and fertility: one can only be increased at the expense of the other.

Stefano Giaimo and Arne Traulsen from the Evolutionary Theory Department at the Max Planck Institute for Evolutionary Biology in Plön have derived from a mathematical model how this compromise can be measured. To do this, they calculated the best combination of survival and fertility for the optimal fitness of an organism. Fertility and longevity are viewed as two components that have a negative impact on each other: If one becomes larger, the other must become smaller.

The researchers' calculations showed that generation time, i.e. the average age at reproduction, is a factor that converts the compromise between fertility and survival: If an organism lives longer, its fertility decreases. "An increase in the probability of survival by two percent from each time period to the next means, for example, a percentage loss of fertility at any age equal to twice the generation time of the organism," says Stefano Giaimo.

A seemingly insignificant increase in the probability of survival can therefore have a massive effect on reproduction. A mouse, for example, has a generation time of 2.5 months. A four percent increase in the chance of survival from one month to the next can decrease monthly fertility by up to ten percent. How a longer lifetime affects human fertility cannot be precisely determined at this point in time. “But it could be that the age research on short-lived animals greatly underestimates the price we would have to pay for a longer life,” explains Arne Traulsen.