The speed of life

摘要

It's not the most exciting experiment I've ever seen. An ordinary white mouse sits inside a sealed glass jar, breathing. But that's enough for comparative physiologist Tony Hulbert. Each time the mouse exhales, it releases a trillionth of a gram of two gases, ethane and pentane. All animals, you and I included, puff this stuff out 24/7. It is nothing to us. But Hulbert believes that by measuring these two gases he can tell how quickly this candlewick we call life will burn down. Across the spectrum of Noah's ark, from the 2-gram shrew to the 200-tonne blue whale, much about an animal is determined by its size. In general, the larger the beast, the slower its metabolism and the longer its life, and vice versa. But the question of how nature imprints each creature with its assigned metabolic rate, and why some are destined to die sooner than others, is a long-standing mystery. Now Hulbert and Paul Else, his collaborator at the University of Wollongong in New South Wales, Australia, think they have the answer. They say it is our membranes - the millionth-of-a-centimetre-thick envelopes that enclose our cells and the components within them -that determine all of these things. Gunky, watertight membranes saddle elephants and whales with slow metabolic rates, but also give them long lives. Runny, leaky membranes allow mice and hummingbirds to live fast -but also causes them to die young, their bodies ravaged by highly reactive free-radical oxygen molecules. Ethane and pentane are byproducts of the radicals' relentless gnawing. If Hulbert and Else are correct, their work could answer many questions raised by other studies into longevity, from why birds live longer than mammals of a similar size, to the puzzle of how restricting calorie intake can promote longevity. In short, it could be a grand unifying theory of life speed, life size and lifespan.