An adult mayfly lives for a single day. A tiny fish called the pygmy goby cashes in its chips after just eight weeks. Yet some sea urchins can live for 200 years, and one bowhead whale is known to be 211 years old and still going strong. Why the massive variation in longevity between species? If we knew, perhaps we could exploit their secrets to give ourselves a few extra years.
One possible explanation is encapsulated in the phrase 鈥渓ive fast, die young鈥�. Small animals have a higher metabolic rate than big ones, and also tend to live shorter lives. The obvious link is free radicals, as the higher your metabolic rate, the more radicals you produce. The idea that you can only cope with so much damage is bolstered by studies showing that animals of very different sizes use a similar amount of energy during their lifetime for a given mass of tissue: 260 kilocalories per gram of guinea pig compared with 280 per gram of horse, for example. Live fast, die young, might also explain the age-defying effects of caloric restriction, assuming that the metabolism of an animal on a starvation diet becomes more efficient, so producing fewer free radicals.
It鈥檚 a neat idea. Unfortunately, evidence against it is mounting. First there is the fact that caloric restriction, though it increases lifespan in every species it has been tested in, only leads to a temporary slowdown in metabolism. The metabolic rate then rebounds, sometimes becoming faster than ever. Then there are all the exceptions. Some groups of creatures, including birds, bats and porcupines, live far longer than equivalent-sized animals with the same rapid metabolic rate. Another exception is humans: we live four times as long as we should for a species of our size and metabolic rate. A third fly in the ointment is the discovery that taking antioxidant supplements to mop up free radicals does not increase longevity.
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Fly now, die later
If not metabolic rate, why do some species live so much longer than others? Steven Austad from the University of Texas Health Science Center in San Antonio is one of a growing band who think the answer lies in ecology and evolution. He argues that the investment in costly mechanisms that reduce cellular damage, and hence extend lifespan, is only worthwhile once a species has evolved ways to evade predators. He calls this 鈥渇ly now, die later鈥�, and believes it explains the exceptional longevity of birds, bats, gliding opossums and flying squirrels. And it鈥檚 not just about flying: clams, tortoises, spiny animals and naked mole rats also have a longer natural lifespan than you鈥檇 expect from their size and metabolic rate.
That still doesn鈥檛 answer the question of how these species stave off ageing. Perhaps they have raised levels of DNA repair enzymes or a special mechanism to limit damage from free radicals or to ward off the effects of chemicals and radiation. In fact, we have little idea about what actually happens in long-lived animals 鈥� and that鈥檚 a shame for anyone interested in human ageing.
Our hearing, for example, starts to degenerate in early adulthood. Yet Brandt鈥檚 bat can live more than 40 years 鈥� the equivalent of a human reaching around 300 鈥� without losing any of its talent for hearing ultrasonic frequencies. Then there is the age-defying trick that naked mole rats and many birds pull off. Older individuals show no more oxidative damage to their DNA than younger ones, Austad has found. Unfortunately nobody knows why, and adding a further twist to the puzzle is the fact that the DNA of these animals appears to suffer more damage than that of comparable but shorter-lived species such as rats or mice.
Some animals even show virtually no signs of ageing: they seem to have no age-related increase in mortality rate or any decrease in reproduction rate after maturity, and no age-related decline in physiological capacity or disease resistance. Notable among these are several species of rockfish, the longest-lived being the rougheye rockfish, which can reach 205. Their secrets are being investigated by a group called AgelessAnimals.org. Its initial findings suggest, among other things, that rougheye rockfish continue to produce high levels of antioxidant enzymes and the enzyme telomerase 鈥� which prevents chromosomes becoming degraded as they replicate 鈥� and that DNA damage follows a pattern similar to the one in birds.
The ultimate aim of the group is to apply its findings to allow humans to live longer and healthier lives. 鈥淧eople are just starting to realise the potential of comparative biology,鈥� says Austad.
