Why do big creatures live longer?
GEOFFREY WEST: All things scale in a very predictable way and they scale in a way that's non-linear. We developed this very elegant theory that what these scaling laws are reflecting are in fact the generic universal mathematical and physical properties of the multiple networks that make an organism viable and allow it to develop and grow. I think it's one of the more remarkable properties of life actually. Just taking mammals, the largest mammals, the whale, in terms of measurable quantities, is actually a scaled up version of the smallest mammal, which is actually the shrew. They are scaled versions of one another. If you have this theory of scaling laws, you can determine what the parameters are, the knobs that you could conceivably turn to change that lifespan. So it's a fantastic effect, it's a huge effect. Every time you double the size of an organism, you would expect to double the amount of metabolic energy you need to keep that organism alive. Quite the contrary, you don't need twice as much metabolic energy. Systematically you only need roughly speaking 75% as much. So there's this kind of systematic 25% savings. Metabolic rate simply means how much energy or how much food does an animal need to eat each day in order to stay alive. Everybody's familiar with that as sort of roughly 2000 food calories a day for a human being. So here's this extraordinary complex process, yet it scales in a very simple way. Life span also increases following these quarter power scaling laws. The scaling of these quantities is determined by the constraints of flows in networks. Those flows, they are dissipative, which simply means they involve wear and tear. Just as there's a lot of traffic going back and forth on the roads, and those roads wear out, they have to be repaired. And so it is, the traffic through our multiple network systems produce damage. The reason a large animal lives longer than a small one is because the metabolic rate per unit mass or per cell, gets systematically smaller, the bigger the animal corresponding to these quarter power scaling laws. So less damage is done at the cellular level the bigger the animal. When a given fraction of unrepaired damages occur, the system will become non-viable, that is it can no longer be sustained. That gives you a calculation of maximum lifespan. If you were to do the best you possibly could, this is as long as you could possibly live for a given size of mammal. And if you do that, you can understand where roughly speaking this hundred years for a human being comes from. And there's two pieces of that, one is you could decrease, of course, the wear and tear or you could increase the repair. If you think about the damage that is occurring from metabolism, one way we could decrease damage is decrease the amount of food we take in. It may not be so pleasant in terms of your lifestyle but this would predict that you live longer. There have been some controversial experiments on monkeys which have not shown as big an effect, so this is still very much work in progress. There's another way you could also decrease your metabolism and that's the way that it's very difficult for us. But interestingly is very easy, for almost all other organisms on this planet. And that's to do with the fact that we are unique in that we are what's called homeotherms mainly we keep the same temperature. If you look at insects, when they're cold in the morning, they can barely move. They have to wait till the sun comes up to warm themselves and then they can start flying around and moving around and so on, that's true of essentially everything that's around us. We are immune from that and that's been extraordinary powerful for us. It dissociates us from the external temperature, the environmental temperature. Everything else is subject to the ambient temperature in their environment. And here's why it matters, it's because metabolic rate is derived from chemical reactions, and chemical reactions depend exponentially from the temperature at which they're operating. That means a small change in temperature can have a huge effect. So a small change of temperature, small increase in temperature, increases your metabolic rate exponentially. That means that if we were to take drugs that could lower our body temperature, and this has actually been done for mice, you would decrease your metabolic rate and you would decrease therefore the damage and you can live longer. One tangential remark to that, a critical one in our times, and that is to do with global warming. After all where I live the temperature often changes by 40 degrees from night to day. The reason is that things like growth rates and death rates, the whole ecosystem, the whole biosphere is exponentially sensitive to a change in temperature. So one to two degrees change has an exponential effect and some of that is from our viewpoint, highly deleterious and some may actually be advantageous. But I think this is an incredibly important point that I'm afraid my colleagues who work in global warming have not been very good at getting this across.