Longevity depends on your species

When a baby is born, there is a tag on its ankle with ‘best before .. etc’ stamped on it.

The nurses remove these tags, lest the dates on them distress the doting parents. In 1944, an Irish male had a life expectancy of 59 years, which means that I am well past my sell-by date. But it’s great to be still around; some of my friends haven’t been so lucky. Indeed, there are people dying now who never died before! But, as Maurya says in Riders to the Sea, “no man at all can be living for ever and we must be satisfied”.

Thanks to science, the tags on today’s babies carry much more ambitious dates; a boy born in Ireland this year has a life expectancy of 78 years and a girl can expect to reach the age of 81. Medical advances will push these limits even further; life-expectancies may soon pass the 100 mark. Jeanne Louise Calment, of Arles in the South of France, lived for 122 years and 164 days, the longest human life on record.

Our animal relatives can’t match us in the longevity stakes but they too have their survivors. Older readers may remember the Tarzan films. Tarzan, created by Edgar Rice Burroughs, was a boy who got lost in an African jungle, where he was raised by apes. Between 1918 and 1989, no fewer than 39 Tarzan films appeared, Johnny Weissmuller, an Olympic swimmer, giving the definitive portrayal of our hero. A chimpanzee, named Cheetah, also featured, providing light relief between the thrill-a-second sequences. Cheetah, at the ripe old age of 75, is living in Palm Springs Zoo. Not only is he the world’s oldest chimp, he is the oldest known non-human primate.

The oldest elephant, according to the Guinness Book of Records, was Lin Wang of Taipei Zoo in Taiwan, who was 86 years old when he died. Whales almost certainly live longer than elephants but we can’t measure the life-spans of these elusive creatures. The experts suggest an upper limit of about 90 years but discoveries by archaeologists suggest otherwise. Stone-tipped harpoon-heads have been found embedded in the skins of bow-head whales. Hunters stopped using stone-heads late in the 19th century, when metal-tipped ones became available. A whale with a stone- head in its skin must, the archaeologists claim, be more than a century old. Analysis of amino acids from whale carcasses appears to support these claims; one bow-head was estimated to have reached the age of 211. This would make it, by far, the oldest known mammal.

But why should there be such differences in life expectancy between species? All animals are made of the same stuff, proteins, and all should, on the face of it, have equal survival prospects. Yet, a pigmy shrew lives for a year while whales and ourselves may last a century. Freud remarked that “reproduction is the beginning of death” and evolutionary theory suggests that he was right; longevity seems to be linked to the time a creature takes to reproduce. From a biological point of view, an animal is just a structure to replicate genes and pass them on to the next generation. An individual must survive long enough to get the job done and it’s not an easy task, given the risks to be faced. However, once its quota of offspring has been raised, a parent no longer has a role and it dies. If it were to live beyond the reproductive years, it might deplete vital food and other resources which its descendants, the bearers of the all-important genes, need for their survival.

Stoats provide an interesting illustration of the reproduction-versus-death equation. These little animals lead short precarious lives. The exceptional individual might last up to 10 years, but the overall life expectancy of stoats is about a year and a half, not a lot of time in which to produce young. However, the stoat has a trick up its sleeve; the males indulge in paedophilia. Female babies, still lactating in the nest, will mate with their mother’s current partner. Given the high death rate of stoats, he is unlikely to be their father, so inbreeding is not a major risk. The babies are pregnant when they leave the nest and give birth the following Spring. The result is that females start having young unusually early in life, increasing the survival prospects of their genes and allowing the parents to die younger.

But, you will object, women can live for up to five decades after the menopause, so the theory must be wrong. Not necessarily. Our species differs from all others in having menopause and in one other crucial respect; we educate our young to a much higher level than is found in any other creature. Humans, with their huge brains, take a long time to grow up and, during the extended childhood, vast amounts of crucial information are transferred from parent to offspring. But it’s not just the parents who do the educating, aunties and grandparents are further sources of wisdom. Without lethal teeth or claws, and slow of foot, we humans depend on our wits for survival and the knowledge provided by elders is invaluable. Their role has been taken over by specialists, libraries and the internet but, until comparatively recently, the advice of grannies could be crucial in facing the trials of life.