The Greatest Show on Earth (37 page)

RUNNING TO STAY IN THE SAME PLACE

The five fastest runners among mammal species are the cheetah, the pronghorn (often called ‘antelope’ in America although it is not closely related to the ‘true’ antelopes of Africa), the gnu (or wildebeest, a true antelope although it doesn’t look much like the others), the lion, and the Thomson’s gazelle (another true antelope, which really does look like a standard antelope, a small one). Note that these top-ranked runners are a mixture of hunted and hunters, and my point is that this is no accident.
Cheetahs are said to be capable of accelerating from 0 to 60 mph in three seconds, which is right up there with a Ferrari, a Porsche or a Tesla. Lions, too, have formidable acceleration, even better than gazelles, who have more stamina and the ability to jink. Cats generally are built for sprinting, and springing on prey taken unawares; dogs, such as the Cape hunting dog or the wolf, for endurance, for wearing down their prey. Gazelles and other antelopes have to cope with both types of predator, and they perhaps have to compromise. Their acceleration is not quite so good as a big cat’s, but their endurance is better. By jinking, a Tommy can sometimes throw a cheetah off its stride, thereby postponing matters until the cheetah has gone beyond its maximum acceleration phase into the exhausted phase, where its poor stamina starts to count. Successful cheetah hunts usually end soon after they start, the cheetah relying on surprise and acceleration. Unsuccessful cheetah hunts also end early, with the cheetah giving up to save energy when its initial sprint fails. All cheetah hunts, in other words, are brief!
Never mind the details of top speeds and accelerations, stamina and jinking, surprise and sustained pursuit. The salient fact is that the fastest animals include both those that hunt and those that are hunted. Natural selection drives predator species to become ever better at catching prey, and it simultaneously drives prey species to become ever better at escaping them. Predators and prey are engaged in an evolutionary arms race, run in evolutionary time. The result has been a steady escalation in the quantity of economic resources that animals, on both sides, spend on the arms race, at the expense of other departments of their bodily economy. Hunters and hunted alike get steadily better equipped to outrun (surprise, outwit, etc.) the other side. But improved equipment to outrun doesn’t obviously translate into improved success in outrunning – for the simple reason that the other side in the arms race is upgrading its equipment too: that is the hallmark of an arms race. You could say, as the Red Queen said to Alice, that they have to run as fast as they can just to stay in the same place.
Darwin was well aware of evolutionary arms races, although he didn’t use the phrase. My colleague John Krebs and I published a paper on the subject in 1979, in which we attributed the phrase ‘armament race’ to the British biologist Hugh Cott. Perhaps significantly, Cott published his book, Adaptive Coloration in Animals, in 1940, in the depths of the Second World War:
Before asserting that the deceptive appearance of a grasshopper or butterfly is unnecessarily detailed, we must first ascertain what are the powers of perception and discrimination of the insects’ natural enemies. Not to do so is like asserting that the armour of a battle-cruiser is too heavy, or the range of her guns too great, without inquiring into the nature and effectiveness of the enemy’s armament. The fact is that in the primeval struggle of the jungle, as in the refinements of civilized warfare,* we see in progress a great evolutionary armament race – whose results, for defence, are manifested in such devices as speed, alertness, armour, spinescence, burrowing habits, nocturnal habits, poisonous secretions, nauseous taste, and procryptic, aposematic, and mimetic coloration; and for offence, in such counter-attributes as speed, surprise, ambush, allurement, visual acuity, claws, teeth, stings, poison fangs, and anticryptic and alluring coloration. Just as greater speed in the pursued has developed in relation to increased speed in the pursuer; or defensive armour in relation to aggressive weapons; so the perfection of concealing devices has evolved in response to increased powers of perception.
Note that the arms race is run in evolutionary time. It is not to be confused with the race between an individual cheetah, say, and a gazelle, which is run in real time. The race in evolutionary time is a race to build up equipment for races run in real time. And what that actually means is that genes for making the equipment to outsmart or outrun the other side build up in the gene pools on the two sides. Second – and this is a point that Darwin himself knew well – the equipment for running fast is used to outrun rivals of the same species, who are fleeing from the same predator. The well-known joke, which has an almost Aesopian ring to it, about the running shoes and the bear is apposite.* When a cheetah chases a herd of gazelles, it may be more important for an individual gazelle to outrun the slowest member of the herd than to outrun the cheetah.
Now that I have introduced the terminology of the arms race, you can see that trees in a forest, too, are engaged in one. Individual trees are racing towards the sun, against their immediate neighbours in the forest. This race is particularly keen when an old tree dies and leaves a vacant slot in the canopy. The echoing crash of an old tree falling is the starting gun for a race, in real time (although a slower real time than we animals are accustomed to), between saplings that have been waiting for just such a chance. And the winner is likely to be an individual tree that is well equipped, by genes that prospered through ancestral arms races in evolutionary time, to grow fast and high.
The arms race between species of forest trees is a symmetrical race. Both sides are trying to achieve the same thing: a place in the canopy. The arms race between predators and prey is an asymmetric arms race: an arms race between weapons of attack and weapons of defence. The same is true of the arms race between parasites and hosts. And there are even, though it may seem surprising, arms races between males and females within a species, and between parents and offspring.
One thing about arms races that might worry enthusiasts for intelligent design is the heavy dose of futility that loads them down. If we are going to postulate a designer of the cheetah, he has evidently put every ounce of his designing expertise into the task of perfecting a superlative killer. One look at that magnificent running machine leaves us in no doubt. The cheetah, if we are going to talk design at all, is superbly designed for killing gazelles. But the very same designer has equally evidently strained every nerve to design a gazelle that is superbly equipped to escape from those very same cheetahs. For heaven’s sake, whose side is the designer on? When you look at the cheetah’s taut muscles and flexing backbone, you must conclude that the designer wants the cheetah to win the race. But when you look at the sprinting, jinking, dodging gazelle, you reach exactly the opposite conclusion. Does the designer’s left hand not know what his right hand is doing? Is he a sadist, who enjoys the spectator sport and is forever upping the ante on both sides to increase the thrill of the chase? Did He who made the lamb make thee?
Is it really part of the divine plan that the leopard shall lie down with the kid, and the lion eat straw like the ox? In that case, what price the formidable carnassial teeth, the murderous claws of the lion and the leopard? Whence the breathtaking speed and agile escapology of the antelope and the zebra? Needless to say, no such problems arise on the evolutionary interpretation of what is going on. Each side is struggling to outwit the other because, on both sides, those individuals who succeed will automatically pass on the genes that contributed to their success. Ideas of ‘futility’ and ‘waste’ spring to our minds because we are human, and capable of looking at the welfare of the whole ecosystem. Natural selection cares only for the survival and reproduction of individual genes.
It’s like the trees in the forest. Just as each tree has an economy, in which goods that are put into trunks are not available for fruits or leaves, so cheetahs and gazelles each have their own internal economy. Running fast is costly, not just in energy ultimately wrung from the sun but in the materials that go into the making of muscles, bones and sinews – the machinery of speed and acceleration. The food that a gazelle ingests in the form of plant material is finite. Whatever is spent on muscles and long legs for running has to be taken away from some other department of life, such as making babies, on which the animal might ideally ‘prefer’ to spend its resources. There is an extremely complicated balance of compromises to be micro-managed. We can’t know all the details but we do know (it is an unbreakable law of economics) that it is possible to spend too much on one department of life, thereby taking resources away from some other department of life. An individual that puts more than the ideal amount into running may save its own skin. But in the Darwinian stakes it will be out-competed by a rival individual of the same species, who skimps a little on running speed and hence incurs a greater risk of being eaten, but who gets the balance right and ends up with more descendants to pass on the genes for getting the balance right.
It isn’t just energy and costly materials that have to be correctly balanced. There’s also risk: and risk, too, is no stranger to the calculations of economists. Legs that are long and thin are good at running fast. Inevitably, they are also good at breaking. All too regularly a racehorse will break a leg in the heat of a race, and usually is promptly executed. As we saw in Chapter 3, the reason they are so vulnerable is that they have been overbred to be fast, at the expense of everything else. Gazelles and cheetahs have also been selectively bred for speed – naturally, not artificially selected – and they too would be vulnerable to fractures if nature were to overbreed them for speed. But nature never overbreeds for anything. Nature gets the balance right. The world is full of genes for getting the balance right: that is why they are there! What it means in practice is that individuals with a genetic tendency to develop exceptionally long and spindly legs, which are admittedly superior for running, are less likely to pass on their genes, on average, than slightly slower individuals whose less spindly legs are less likely to break. This is just one hypothetical example of the many hundreds of trade-offs and compromises that all animals and plants juggle. They juggle with risks and they juggle with economic trade-offs. It is, of course, not the individual animals and plants that do the juggling and balancing. It is the relative numbers of alternative genes in gene pools that are juggled and balanced, by natural selection.
As you would expect, the optimum compromise in a tradeoff is not fixed. In gazelles, the trade-off between running speed and other demands within the economy of the body will shift its optimum depending upon the prevalence of carnivores in the area. It’s the same story as for the guppies of Chapter 5. If there are few predators around, the gazelle’s optimum leg length will shorten: the most successful individuals will be the ones whose genes predispose them to shunt some energy and material away from legs and into, say, making babies, or laying down fat for the winter. These are also the individuals who are less likely to break their legs. Conversely, if the number of predators increases, the optimum balance will shift towards longer legs, greater danger of fractures, and less energy and material to spend on those aspects of the body’s economy that are not concerned with running fast.
And just the same kinds of implicit calculation will balance up the optimum compromises in the predators. A cheetah who breaks her leg will undoubtedly die of starvation, and so will her cubs. But, depending on how difficult it is to find a meal, the risk of failing to catch enough food if she runs too slowly may outweigh the risk of breaking a leg through being equipped with the wherewithal to run too fast.
Predators and prey are locked in an arms race in which each side is unwittingly pressing the other to shift its optimum – in the economic and risk compromises of life – further and further in the same direction: either literally in the same direction, for example towards increased running speed; or in the same direction in the looser sense of being aimed at the predator/prey arms race rather than some other department of life such as milk production. Given that both sides have to balance the risks of, say, running too fast (breaking legs or skimping on the other parts of the bodily economy) against the risks of running too slowly (failing to catch prey, or failing to escape, respectively), each side is pushing the other in the same direction, in a sort of grim folie à deux.
Well, perhaps folie (madness) doesn’t quite do justice to the seriousness of the matter, for the penalty of failure on either side is death – murder on the side of the prey, starvation on the side of the predator. But à deux captures handily the feeling that, if only hunter and hunted could sit down together and hammer out a sensible agreement, everybody would be better off. Just as with the trees in the Forest of Friendship, it is easy to see how such a compact would benefit them, if only it could be made to stick. The same sense of futility as we encountered in the forest pervades the predator/prey arms race. Over evolutionary time, predators get better at catching prey, which prompts prey animals to get better at evading capture. Both sides in parallel improve their equipment to survive, but neither necessarily survives any better – because the other side is improving its equipment too.

On the other hand, it is easy to see how a central planner, with the welfare of the whole community at heart, might umpire an agreement in the following terms, along the lines of the Forest of Friendship. Let both sides ‘agree’ to scale down their armoury: both sides shift resources to other departments of life, and all will do better as a result. Just the same, of course, can happen in a human arms race. We wouldn’t need our fighters if you didn’t have your bombers. You wouldn’t need your missiles if we didn’t have ours. We could both save billions if we halved our armaments spending and put the money into ploughshares. And now, having halved our arms budget and reached a stable stand-off, let’s halve it again. The trick is to do it in synchrony with each other, so that each side remains exactly as well equipped to counter the other’s steadily de-escalating arms budget. Such planned de-escalation has to be just that – planned. And, once again, planned is precisely what evolution is not. Just as with the trees in the forest, escalation is inevitable, right up until the moment when it no longer pays a typical individual to escalate any further. Evolution, unlike a designer, never stops to consider whether there might be a better way – a mutualistic way – for all concerned, rather than bilateral escalation for a selfish advantage: an advantage that is neutralized precisely because the escalation is mutual.
The temptation to think like a planner has long been rife among ‘pop ecologists’, and even academic ecologists sometimes come perilously close to it. The tempting notion of ‘prudent predators’, for example, was dreamed up not by some tree-hugging airhead but by a distinguished American ecologist.
The idea of prudent predators is this. Everybody knows that, from the point of view of humanity as a whole, we’d be better off if we all refrained from overfishing an important food species, such as the cod, to extinction. That is why governments and NGOs in stately conclave meet to draw up quotas and restrictions. That is why the precise mesh size of fishing nets is minutely specified by government decree, and that is why gunboats patrol the seas in pursuit of dissenting trawlermen. We humans, on our good days and when properly policed, are ‘prudent predators’. Therefore – or so it seems to certain ecologists – shouldn’t we expect wild predators, like wolves or lions, to be prudent predators too? The answer is no. No. No. No. And it is worthwhile understanding why, because it’s an interesting point, one that the forest trees and this whole chapter should have prepared us for.
A planner – an ecosystem designer with the welfare of the whole community of wild animals at heart – could indeed calculate an optimum culling policy, which lions, for example, should ideally adopt. Don’t take more than a certain quota from any one species of antelope. Spare pregnant females, and don’t take young adults full of reproductive potential. Avoid eating members of rare species, which might be in danger of extinction and might come in useful in future, if conditions change. If only all the lions in the country would abide by the agreed norms and quotas, carefully calculated to be ‘sustainable’, wouldn’t that be nice? And so sensible. If only!
Well, it would be sensible, and it is what a designer would prescribe, at least if he had the welfare of the ecosystem as a whole at heart. But it isn’t what natural selection would prescribe (mainly because natural selection, lacking foresight, cannot prescribe at all) and it isn’t what happens! Here’s why, and it is again the same story as for the trees in the forest. Imagine that, by some quirk of leonine diplomacy, a majority of lions in an area somehow managed to agree to limit their hunting to sustainable levels. But now, suppose that in this otherwise restrained and public-spirited population, a mutant gene arose that caused an individual lion to break away from the agreement and exploit the prey population to the uttermost, even at the risk of driving the prey species extinct. Would natural selection penalize the rebellious selfish gene? Alas, it would not. Offspring of the rebel lion, possessors of the rebel gene, would out-compete and out-reproduce their rivals in the lion population. Within a few generations, the rebel gene would spread through the population and nothing would be left of the original amicable compact. He* who gets the lion’s share passes on the genes for doing so.
But, the planning enthusiast will protest, when all the lions are behaving selfishly and over-hunting the prey species to the point of extinction, everybody is worse off, even the individual lions that are the most successful hunters. Ultimately, if all the prey go extinct, the entire lion population will too. Surely, the planner insists, natural selection will step in to stop that happening? Once again alas, and once again no. The problem is that natural selection doesn’t ‘step in’, natural selection doesn’t look into the future,† and natural selection doesn’t choose between rival groups. If it did, there would be some chance that prudent predation could be favoured. Natural selection, as Darwin realized much more clearly than many of his successors, chooses between rival individuals within a population. Even if the entire population is diving to extinction, driven down by individual competition, natural selection will still favour the most competitive individuals, right up to the moment when the last one dies. Natural selection can drive a population to extinction, while constantly favouring, to the bitter end, those competitive genes that are destined to be the last to go extinct. The hypothetical planner that I have imagined is a certain kind of economist, a welfare economist calculating an optimum strategy for a whole population, or an entire ecosystem. If we must make economic analogies, we should think instead of Adam Smith’s ‘invisible hand’.