Longevity Briefs: Do Slow-Ageing Animals Actually Age As Slowly As We Think?

Longevity briefs provides a short summary of novel research in biology, medicine, or biotechnology that caught the attention of our researchers in Oxford, due to its potential to improve our health, wellbeing, and longevity.

Why is this research important: As we grow older, the probability of dying from age-related causes increases exponentially – this is known as Gompertz law of mortality. But what if your probability of death barely increased at all, or even decreased as you grew older? This is seemingly the reality for some species, and it’s called negligible senescence (not to be confused with cellular senescence). Classic examples of organisms with negligible senescence include naked mole-rats (who live significantly longer than their closest evolutionary cousins) and hydras (which are functionally immortal). If we can learn why these species age more slowly than others, perhaps we can learn how to delay ageing in our own species.

What did the researchers do: It is difficult, and sometimes impossible, to know the true lifespan of many wild animals. Often, lifespan data is collected by marking animals, releasing them, and recapturing them. The estimated age of the animal when they are captured for the last time is recorded, and it is assumed that they died at an unknown time thereafter.
In this study, researchers used mathematical modelling to compare the reported lifespans using the above method with expected lifespans estimated by accepted models of ageing.

Key takeaway(s) from this research: Since a captured animal is unlikely to die right after being released, it’s not surprising that reports based on capture and release tend to overestimate mortality risk at a given age. However, the more important finding is that mortality risk gets overestimated more in animals that are younger. In addition, the less likely it is for the same animal to be captured more than once, the larger these overestimates become.

Because reports overestimate mortality risk in early life, this gives the illusion that mortality rates are not increasing as rapidly as they are in reality. In other words, many reports have probably inflated the extent of negligible senescence, especially in species with low recapture rates – small animals like rodents, bats and birds. That’s not to say that exceptional organisms don’t exist – naked mole-rats, for example, still have incredibly long lifespans for their size. However, we should perhaps be more cautious when it comes to evidence of negligible senescence in some species.