As an old north of England saying puts it, "There's nowt so queer as folks." Or, interpreting, there is nothing so strange as people. I want to point out one way in which humans are indeed peculiar, and different from all our animal cousins.

You never see a large animal with the delicate, fragile build of a daddy longlegs. The reason for this was pointed out more than a century ago. If a plant or animal doubles in size, its volume and hence its weight goes up by a factor of eight. Its support system of stem or legs, however, goes up in area only by a factor of four. Hence, the load per unit area doubles. Increase the size by another factor of two, and the load per unit area on each leg doubles again. Keep going, and finally the loading will reach a breaking point. Elephants and rhinoceros have thick, sturdy legs because they absolutely need them, and this weight-to-area rule, sometimes called the cube/square law, imposes a limit on the size of land animals (whales and other sea creatures don't have this problem, because the water around them buoys up their body).

There is nothing at all new in what I have just said. J.B.S. Haldane, writing 75 years ago about the importance of the weight-to-area ratio, offered a memorable image: "You can drop a mouse down a thousand-yard mine shaft; and, on arriving at the bottom, it gets a slight shock and walks away. A rat is killed, a man is broken, a horse splashes."

However, if we focus our attention on warm-blooded animals a totally new factor emerges. All such animals, when they are resting, have the same body temperature to within ten degrees, and lose about the same amount of heat per square inch of skin. In what follows I am going to restrict the discussion to mammals, because this includes humans and also because the range of sizes we must consider is far bigger for mammals than for the other class of warm-blooded animals, the birds. A pygmy shrew can weigh as little as one twentieth of an ounce, a blue whale as much as 190 tons - a weight variation factor of 120 million. A human being sits around the middle of the range. We weigh a couple of thousand times as much as a mouse, while a whale is a couple of thousand times as heavy as a human.

Now let us examine the consequences of being warm-blooded. Again, notice that the cube/square law applies. Animals come in a huge variety of shapes, but the total surface area increases as the square of the animal's size, while its volume goes up as the cube. Although animals try various sneaky ways to get around this problem, so that humans, for example, have about 100 square yards of total lung area, the basic law still applies. This is particularly true when we look at the related problems of keeping warm on the one hand, and getting rid of generated body heat on the other.

Every animal, from rat (one pound) to dog (50 pounds) to human (150 pounds) to horse (1000 pounds) to elephant (six tons), is made up of cells. These cells are all about the same size, regardless of which species they come from. They are all capable of generating energy, and therefore heat, at about the same rate. However, if they did so it would be disastrous. A shrew or a mouse, with a very small weight compared to its surface area, loses heat at a huge rate. In order to survive it must live life in the fast lane, burning energy quickly. It is no accident that there are no small land mammals in extreme arctic regions. They would die rapidly of hypothermia. Conversely, if an elephant generated energy at the same rate per unit volume as a mouse, it would cook itself. It simply does not have enough surface area to get rid of all the heat that would be generated.

The answer, adopted throughout the animal kingdom, is a simple one. The rate at which energy is generated per unit volume goes down with the animal's size. A shrew spends most of its time eating, in order to feed its energy generators. An elephant, or a whale, leads a far more leisurely life.

We can see this, very directly, by examining the pulse rates of animals. As mammals increase in size, their pulse rates go down. If you hold a mouse so that you can feel its working pulse, it seems incredibly fast, while a whale's heartbeat is a slow and thunderous event. Not surprisingly, an animal's life span depends how "fast" it lives. The life expectancy of a mammal is usually around a billion heartbeats.

There is a rule relating weight to length of life: an animal's weight varies like the fourth power of its life expectancy. Double the life span, and the weight is 16 times as big. Triple the life span, and the weight will be 81 times as big. Apply this to a mouse, weight one ounce, and a whale, weight 50 tons, and you find that a whale ought to live 36 times as long as a mouse - 70 years compared to two years.

This simple rule applies rather well over the whole range of mammal sizes, with one glaring exception. A human weighing 150 pounds should live around 15 years. By the standard of every other mammal, human life expectancy, both in its average and in its maximum values, is six times as much as it ought to be. Chimps, our closest living relatives in the animal kingdom, exhibit the same phenomenon on a lesser scale, living up to three times what we would expect based on body weight.

In the whole field of mammals, with its huge range in sizes, weights, and body types, humans stand out as a perplexing oddity. We live far longer than we should, and no one can offer a good reason why.

On the other hand, I'm certainly not going to the one to complain about it.

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