101 Facts About Ageing #51: Body Mass And Average Lifespan

As Daniel Patrick Moynihan, an American sociologist, politician, and diplomat once said: “Everyone is entitled to his own opinion, but not his own facts”. And we wholeheartedly agree. A shared set of facts is the first step to building a better world with longevity for all. In that spirit, we are creating a series that covers 101 indisputable facts about ageing, health and longevity.

When comparing different species, there is a correlation between body mass and average lifespan. Small species like rodents tend to live short lives, while the likes of elephants and whales tend to have relatively long average lifespans. There are of course many other factors involved, resulting in a large degree of variation and some extreme outliers. Humans are one example, living about twice as long as elephants despite being around 100 times lighter. Other species like the naked mole-rat live remarkably long lifespans (around 30 years) despite being about half the weight of their close genetic cousin, the guinea pig, which lives 4-8 years on average.

While body mass generally correlates with lifespan when comparing different species, the reverse is true within species: smaller animals of a given species tend to live longer than their larger cousins.

We don’t fully understand the reasons for these relationships, but there are some good guesses. Smaller animals tend to have more environmental causes of death – for example, they may be less protected from predators and harsh conditions like extremes of temperature. The kinds of cellular processes we believe are essential for slow ageing (like repairing DNA or clearing up cellular waste products) require a lot of energy, which is a scarce resource in most environments. If you are a mouse that is likely to be eaten by a cat within a few years of being born, it is more evolutionarily advantageous to dedicate most of that energy towards producing as many offspring as possible before you bump into that cat.

Larger animals that store more energy and are more resistant to environmental dangers are much more likely to die from old age, and so genetic traits that slow ageing are more likely to allow for the production and rearing of more offspring. This might be one of the reasons males of a given species tend to have shorter lifespans, as young males are usually more combative and therefore more likely to die before reaching average lifespan. This theory also helps to explain some outliers like naked mole-rats, which are more protected from environmental causes of death (naked mole-rats live in underground colonies).

As for why small animals would live longer than large animals within a species, this may simply be attributed to food intake. Low nutrient intake activates cellular stress responses, triggering pathways that protect cells against damage. This has been shown to protect organisms against diseases like cancer, and the molecular pathways responsible have been highly conserved throughout evolution. Overfeeding, on the other hand, activates growth-promoting pathways with opposite effects to low nutrient intake. Cells dedicate more energy towards dividing faster and less towards damage repair, which accelerates the development of age related disease.