The ideal aspiration of a long life full of children is a dream of happiness for many. However, a new study reveals that the animal kingdom establishes an exclusive relationship between longevity and fertility: animals that live longer often have fewer young; and vice versa.
This association may seem somewhat counterintuitive to us, but scientists from the Department of Evolutionary Theory at the Max Planck Institute for Evolutionary Biology in Plön (Germany) explain that this delicate balance has to do with survival in a study published by the journal The American Naturalist . They have developed a mathematical model that makes it possible to measure the exclusive relationship between fertility and longevity, and they calculated the best combination of survival and fertility for the optimal physical condition of an organism. Fertility and longevity are considered here as two individual components that have a negative effect on each other: if one increases, the other must decrease.
The scientists’ calculations revealed that the average age of reproduction can be used as a conversion factor to calculate the effect that develops from the compromise between fertility and survival: if an organism lives longer, its fertility decreases. According to the researcher Stefano Giaimo, one of the study authors: “An increase in the probability of survival by 2 percent from one period of time to the next means, for example, a percentage of fertility loss at any age equal to twice of the organism’s generation time “.
Therefore, a seemingly insignificant increase in the probability of survival can seriously affect reproduction. Mice, for example, have a generation time of 2.5 months. A 4% increase in survival from month to month can reduce monthly fertility by up to 1%.
Can it be applied to humans?
The effect of longer life on human fertility cannot be determined exactly at this time. However, research on aging based on animals with shorter lives may underestimate the price we would have to pay for longer lives.
The elderly elephants vs. the mayflies the rats
African elephants and desert tortoises can live up to 80 years, making them ideal subjects for research into the secret of longevity. But studying the age of animals with a longer life is almost impossible. The life of a scientist would not be enough to investigate the differences between just two generations of elephants. “Such research projects would be extremely long and would also be very expensive,” as detailed in the journal Phys in relation to this study. So scientists often turn to animals with relatively short lives , such as mice and fruit flies, when they want to investigate aging and longevity.
On the other hand, life-prolonging genetic changes are often easier to detect in such animals, and it is reasonable to assume that similar genes in humans would also develop life-prolonging effects.
But there is another component that is directly related to the lifespan of an organism: its fertility. Animals with shorter lives are usually very fertile, while those that live longer produce only a few offspring. Mice, for example, only live for about two years, but they become sexually mature after a few weeks and give birth to three to eight babies up to eight times a year. Elephants, on the other hand, can live up to 80 years, but throughout their lives, elephants can only give birth to a maximum of ten young. So there seems to be some sort of mutually exclusive relationship between longevity and fertility in evolution: one can only increase at the expense of the other.
Stefano Giaimo et al. Generation Time Measures the Trade-Off between Survival and Reproduction in a Life Cycle, The American Naturalist (2019). DOI: 10.1086/704155
