Humans age very slowly, but the older we get, the more likely death gets. Not every animal follows that pattern though - some species live for only very short periods, but their age doesn't actually seem to affect when they die. Figuring out why these animals age the way they do could help us understand why humans age...maybe even how to stop it.
There's two different ways to look at the aging process - the pace of aging and the shape. The pace is simpler to understand - it's just how long an animal lives. Humans have a very slow pace of aging, which means we live for a long time, while little birds like the robins have a very fast pace, which means they only live for a couple of years. But the shape of aging determines how the risk of death changes as animals get older. Expert on aging Dr. Annette Baudisch of Germany's Max Planck Institute explains:
"At the age of 15, only 2 out of 100,000 girls in Sweden die, but one out of every two women aged 110 will die. This large difference in mortality at the beginning and end of adult life means that for humans the shape of ageing is steep, whereas in other species like the common swift it is shallow. And in some species the risk of death can even fall with age, with older individuals having the least risk of dying. This seems to be the case for the desert tortoise, and for alligators or crocodiles.
Comparing robins with Swedish women, humans have a slow pace of ageing whereas the robin's is fast, so in terms of length of life the humans are doing best. But if we look at the impact ageing has on death rate the robin wins. Its shape of ageing is fairly flat whereas the humans' is steep, indicating that death rates increase markedly with age."
The shape of aging determines a species's aging factor. For the short-lived robin, its mortality never changes all that much, so its aging factor is just two. But for humans, who only relatively rarely die young but become more and more likely to die the older they get, the aging factor is a whopping 2,132.
Now, what's interesting is that there's a lot of different ways to split this information. For instance, it might seem readily apparent that short-lived species like robins will have small aging factors - after all, they don't live long enough in general for there to be much distinction between "middle-aged" and "elderly" birds. Indeed, because their aging factor is so small, those terms don't even properly apply to robins.
They don't experience the deleterious effects of aging that would make death significantly more likely as they get older. They basically remain young their entire lives, but they die that way too. You might say robins and other species with their particular pace and shape of aging are the embodiment of the old cliché "live fast, die young."
But it's still not quite that simple. The Dall mountain sheep, for example, doesn't live much longer - only about 4.2 years - but it has the relatively high aging factor of seven. That's obviously nowhere near as extreme as it is in an extremely long-lived species like humans, but it still suggests sheep do age over their 4.2 years and are much more likely to die the older they get. Baudisch explains:
"Not all species with short lives live fast and die young. Robins do, but mountain sheep do things differently. They also live pretty fast but die older. From the data I have, it seems that live fast die young is only one option; you can also live fast and die older, or live slower and die young, or live slow and die old. There might be every combination in nature. That's something we need to find out in the future with better data."
Just to run through the examples, humans are an example of living slow and dying old, whereas tortoises - which don't show any signs of age-related infirmity - live slow and die young. And it's not just a matter of age-centric bookkeeping - Baudisch believes looking at these factors could tease out unsuspected connections between animals:
"Separating pace from shape of ageing gives a clearer picture of the characteristics of ageing. It could reveal that certain species are very similar to each other in terms of their shape of ageing, species that we would maybe never have grouped together. Ultimately, this should help us identify the determinants of ageing – the characteristics that determine whether death rates goes up or down with age and reveal species that can successfully avoid ageing."