Human exceptionalism

It is always a delight to come across a new book and/or author who can capture your interest. I picked up Martin Wells' Civilization and the Limpet. It is full of interesting titbits.

Small mammals always run through their lives at a pace in terms of footfalls if not furlongs - that would leave their larger relatives literally breathless. One can, in fact, be quite precise about it. The formula is: specific fuel consumption (food needed per gram per hour) falls with increasing body wieght as Weight (-0.25) (double the weight, metabloic rate rises by 84 percent, not 100 percent; at ten times the weight, the cost per gram is down to 56 percent). The Weight (-0.25) exponent applies to practically any activity you care to think about: heartbeats or breaths, time to digest a meal or reach sexual maturity, gestation and lactation periods, life expectancy. It means that all of us mammals get through just about the same number of heartbeats or square meals in the course of a lifetime. It is one of the universal laws regulating the life of mammals, and, amazingly, we have no clear idea why it should be.

Whatever the cause, it has some rather curious consequences, because all of us, of whatever size and rate of tick-over, inhabit the same planet, so that, like it or not, a day-night cycle lasts twenty-four hours, a tidal cycle runs over twenty-eight days, and seasons happen yearly. A twelve-hour night is inconveniently long for small mammals and for birds, which have to pack away enough food inside them to see them through what is, for them, a horribly long fast. Small birds in Europe or North America only just about make it in summer, when the days are long and the nights short. In the winter, most clear out and fly south. The very smallest, hummingbirds, couldn't even make it in summer in northern latitudes if they didn't go torpid and drop their body temperature at night, slowing down physiological time so that their reserves last until morning. Bats have the same trouble with days, and the smaller ones adopt the same solution. In winter, bats in temperate climates huddle together in some secluded roost and maintain a body temperature that is just sufficient to stop them from freezing. With a bit of luck their fat reserves will last them until the insects come out again in the spring.

Shrews and small mice just keep going, wake up and ferret around for food every few minutes. Most stash away supplies so that there are snacks available whenever they wake up. At the other end of the scale one might imagine that an elephant or a hippopoamus would appreciate a significant slowing of astronomical time.

Humans are unusual in this as in so many other ways, a difficult animal to fit into the general picture. One of our many outstanding features is that we live too long, by the standards of other mammals. Placed on the shrew-to-elephant weight scale, we fit in nicely in terms of metabolic rate, but ought to die off in our thirties; we've had our ration of heartbeats by then. The fact that we somehow contrive to live for twice as long is, arguably, a matter of brains, and looking after ourselves properly. But even animals in zoos, which typically live for a lot longer than their relatives in the wild, don't manage to beat the system by a factor of two or three as we do, so that the notion that we get by by coddling ourselves is unlikely to be the whole explanation.