Why haven't prey evolved the ability to always outrun their predators?

Why haven't prey evolved the ability to always outrun their predators?

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I can understand why some prey can't outrun a recently evolved species. However, since cheetahs have existed for so long, why haven't its prey evolved to always outrun it, driving cheetahs to extinction. Is it because the cheetah population size is so much smaller than the population size of most of its prey species that its prey species are under very weak natural selection to outrun cheetahs? Is it that those that can run faster have more other biological costly traits?

There are (at least) three important factors to consider here; evolution under selection requires genetic variation upon which to act, selection can act on covarying traits causing trade-offs, and adaptation also occurs in the predator. A lot of this is covered elsewhere on this site (including the effects of the other mechanisms of evolution), but little specific reference is made to predator-prey co-evolution.

Adaptation and genetic variance

One of the mechanisms of evolution is selection, and adaptation occurs when species evolve as a response to selection. For a response to selection to occur there must be genetic variation within the trait, such that the genes an individual carries affect the individuals fitness. For example, there may some genes which give the carrier better muscle structure for fast running, and this increases survival which, in turn, increases reproductive output. The importance of genetic variation is often overlooked, but is highlighted by the breeders equation. Genetic variance can arise by novel mutations, or can exist as standing genetic variance.

Selection on covarying traits

Selection is seldom, if ever, a univariate process. That means fitness is not determined by a single characteristic, so fitness in a prey species may be determined by the speed at which it can run, but also its metabolic rate, stamina, ability to acquire nutrients, the way it provisions nutrients for growth and repair etc… Genetic covariance between traits (induced by linkage or pleiotropy) can impact the response to selection, because any selection acting on a covarying trait can cause affect the adaptation occurring in the focal trait (strengthening, weakening, neutralising, or even reversing the direction of response). Therefore, if running speed covaries with other traits it may be difficult for selection to increase running speed. Think about athletes, I doubt Usain Bolt could run 10,000 metres as fast as Mo Farah and vice versa, because there is a trade-off between speed and stamina.

Adaptation in other species

If selection did cause speed to increase in the prey species then that would strengthen selection for increased speed in the predator. This is called an evolutionary arms race, where the adaptive the evolution of two species interacts causing adaptation and counter-adaptation. For example, genes which allow faster running might spread through a population of gazelles (prey) because carriers are more likely to outrun the lion (or the other gazelles), but this will increase selection on lions (predator) to increase speed (or adopt other strategies) which will spread genes to make the lions faster (use other strategies); the result being adaptation and counter-adaptation between the gazelle and lion populations.

There is a little series here about Butch Brodie and his studies of coevolutionary arms races between garter snakes and toxic newts. It would be good to read on the Red Queen Hypothesis, which describes how species, under interaction with other species, must continue to evolve just to prevent extinction. Also worth noting that prey species often have more than one predator species (and predator species often have more than one prey); interaction and coevolutionary networks are complex. What may be adaptive in some interactions may also be maladaptive for other interactions (see specialist and generalist, and selection on covarying traits).

There are two reasons for this: evolutionary trade-offs and coevolution (the "Red Queen hypothesis", as mentioned in the comment above by Luigi).

Evolutionary trade-off describes situations where one trait cannot increase without a decrease in one or more others. Some hypothetical examples:

  • longer legs may help run faster, but past a certain point, it will increase the risk of injury, decreasing survival;
  • lower body weight may increase top speed, but past a certain point, it will decrease starvation tolerance;
  • more muscle may help acceleration but will increase energy requirements.

All changes have costs and benefits. In situations where the outcome of an event is simply "success/failure", there is therefore an evolutionary incentive to evolve to be just good enough. Once you are good enough getting better only increases the cost (I am oversimplifying this a bit).

Change in an extrinsic driver will change the balance of cost and benefit, shifting evolutionary pressure. For example, when predators are absent (island populations) birds sometimes become flightless, because one benefit of flight (escape from predators) no longer applies.

The interesting bit happens when the "extrinsic driver" is another living thing that is also capable of evolving. In this case you suddenly get an evolutionary arms race where each side is constantly subject to a selective pressure to be slightly better than the other, which is a moving target. You therefore get an arms race situation (or the extinction of one or the other side). The Red Queen hypothesis is named after a quote by the Red Queen in "Alice through the looking glass" (Carroll, 1871):

Now, here, you see, it takes all the running you can do, to keep in the same place.

Using the example of a lion and a gazelle: lions run fast enough to catch and eat the slowest gazelles. The remaining gazelles are on average faster, for whatever reason(s). Some of those reasons will be heritable and the next generation of gazelles will be faster. The slowest lions will starve, and some of the reasons for the remaining lions being faster will be heritable so the next generation will be a bit faster, and then you're back where you started. Rinse and repeat.

Predation obviously operates on a much faster timescale than selection, so this doesn't always occur (putting a fox in a chicken coop won't evolve fast chickens).

Coevolutionary "arms races" can be seen in predator-prey interactions, mimicry and much more (including for example the evolution of sex, but that's off-topic for this answer).

There are both costs and benefits to being able to run faster, both as a predator and as a prey animal. In short, maintaining the large muscles necessary to outrun a cheetah every time is metabolically expensive.

So it isn't a matter of being able to always outrun a predator--it's a matter of how to optimally allocate precious resources either to metabolically expensive running muscles or to other things like grazing, eyesight, brain, etc. Outrunning a cheetah some of the time is generally good enough for most prey animals to live long enough to produce the next generation of offspring.

Predators always have to be much better hunters than the prey - they must eat every few days after all. But they can only get so good.

Predator/prey population balance will tend to look like a competition where if the predators are too efficient they will kill off the prey. If that happens they start to starve to death.

If the prey outrun the predators (or at least escape all the time) then the predators will starve to death. Then they breed until there are so many that they eat all the grass/vegetation and then they die off.

While both of these have certainly happened in natural history what is more stable for predators and prey to evolve in competition with each other such that their populations look like a stable equilibrium. If not, one of the would just disappear. Then later via migration another animal would come in to replace them.

Generally speaking, predators will always be faster than prey at a certain given level of biological (or technological) evolution. This, indeed, follows from the obvious observations:

  1. Herbivores consume food with low energy density. This means:

    a. Substantial percentage of their time is spent eating and processing food.

    b. Substantial fat and water stores must be present in the herbivore's body for it to be able to feed at all.

    Both these factors contribute to the tendency of herbivore species to get larger bodies (to accommodate for stomachs, intestines, big salivary glands and other processing facilities).

  2. On the other hand, predators rely on foods with high energy density. They don't need a complex digestive system and thus can evolve much better muscle power to weight ratios. They can also allow themselves to be physically much smaller than the herbivores. Essentially, it's exactly "sport car" vs "work truck" situation.
  3. Not surprisingly, "sport car" vs "truck" analogy works in the other direction as well: large adult herbivores (elephants, hippopotami, etc) are essentially safe from any existing predators, due to their sheer size and weight.

The above reasoning applies to evolutionary "steady state", yet species can, of course, change their roles in the ecosystem under the right circumstances. Pandas is one good example of carnivore turned herbivore: one may say, that all of its prey was able to "outrun" it. :-)

ELI5: Why isn't the fastest animal in the world a prey animal, This would let them outrun all predators right?

Why is the cheetah the fastest land animal, and a Falcon is the fastest animal over all? Would it not be more beneficial for prey to be fast in order to survive? Why is the fastest animal always a predator?

A prey species can survive (as a species) by having more offspring. As long as enough reach maturity and procreate, the species continues. Their evolutionary pressure is to be more fertile (up to a point) rather than use resources to become faster, stronger, bigger. As a species, they are better off having more babies than becoming faster as individuals.

Predators on the other hand, would suffer as a species if their fertility were too high - if there are too many cheetahs no cheetah gets enough to eat over the long term. Instead, evolutionary pressures make them better survivors AS INDIVIDUALS, so they do better getting faster, stronger, etc. rather than having larger litters, laying more eggs, etc.

Of course, it's not as simple as this, and prey species do have some evolutionary pressure to get faster so they're not sitting ducks, but not at the expense of decreased fertility.

I think it's because speed is not the only factor in surviving an encounter with a predator. Agility for example is very important, as well as stamina. Prey like the Thompson's Gazelle are more agile than cheetahs and can outrun them over time due to better stamina.

Speed requires sacrifice, the animal must be extremely lean and powerful to be incredibly fast.

This makes it an excellent hunter, but it’s also a problem if there’s a brief downturn in the weather and prey becomes scarce. There’s no fat reserves and there’s a high resting energy burn rate. The animal starves quickly and has poor stamina.

Eventually it becomes more advantageous to stop trying to outrun the speed demon and try some other tactic. Grow too large for them to take down. Gather in a herd so they can’t get behind you. Work on agility to juke the animal that’s all in on speed. Get smaller so it can’t chase you down into the holes. Just outbreed the hell out of them.

Some prey animals definitely are extremely fast, but that’s not the only tool in their evolutionary arsenal.

Because evolution doesn't optimise, it says "good enough". For evolution to occur, a trait must naturally occur through random mutation. That change must then provide a benefit to the ability of the animal to reproduce, and the ability of that animal's offspring to reproduce, and so on. If it didn't, it wouldn't be selected for, and if it was detrimental it would be selected against.

Predators actually aren't that significant of a selection pressure on prey. They're a pressure absolutely, but only to a limit. The strongest selection pressure on prey species is availability of food. The ability to be fast burns a lot of resources, which means it requires more food. It also tends to require smaller size, because it's hard to shift a lot of weight around, and being smaller means you more easily lose heat to the atmosphere, and so need to burn extra resources to maintain your body heat and keep everything working properly. However, plants are a very inefficient source of food. You have to eat loads of them and it takes a long time to digest them. This means the better evolved you are for speed, the smaller your species' population can afford to be and the more time you have to spend eating, which actually makes you more vulnerable to predation.

To evolve speed, first you need to evolve a more efficient energy source than plants. and that's where predators come from. The shift to eating meat means spending much less of your time eating because meat is a much more efficient source of energy than plants. However, now you're a fast predator, not a fast herbivore. Meanwhile, predators naturally select for speed because they need to be able to outsprint their prey - but that's not true in all situations. For example, whales tend to be pretty slow, even though they're carnivorous, because the things they eat are so tiny and so slow that they can just swim along with their mouths open and let food get caught in their baleen.

Herbivores tend to favour other strategies, like being tanky shits that are a pain in the arse to bring down (cows, elephants, giraffes) and where to become effective against them, predators would need to evolve durability and sheer power themselves. Or being sneaky shits that are a pain in the arse to find, like rodents. Sneaky shits also typically go for a "have lots and lots of children" approach - basically, have so many children that the owls couldn't possibly eat them all. There are even more unique and interesting variations on these strategies too. For example, magicicada species lie dormant for periods of around 15 years and then all emerge at once for a huge mating spree. The long dormancy prevents a species that exclusively eats magicicada from evolving, because they're an unreliable food source and the predator species will starve to death in the intermediary periods.

This can go very, very dark, but it's also essentially true in the natural world. Maybe your prey's society wants to rid themselves of certain elements. Maybe they're just fatalistic and feel that only the strongest of their own deserve to live. (Christopher Anvil's Advance Agent may give you some inspiration.)

Human history has plenty of examples of human sacrifice you could use for inspiration. Sacrifices could be willing, or otherwise (the latter are usually prisoners of war). This could work especially well if the predators do something for the prey in return, besides just "honoring them". Maybe there are "good" predators and "bad" predators, and the prey see being eaten by "bad" predators as horrific the "good" predators could protect them from being eaten by the "bad" predators.

The Human Evolution Blog

Signaling Theory: How Prey Animals Communicate with their Predators

(A longer discussion of animal communication can be found in my book.)

It’s no surprise that animals communicate with one another, but we normally think of animal communication as between members of their own species. It turns out that predators and their prey have evolved elaborate systems of communication as well.

I am dangerous!

A few prey animals have evolved ways to let their potential predators know that they are dangerous and it might be better if they moved on and picked another target.

The rattlesnake is probably the most striking example of this. Rattlesnakes are themselves predators, but their distinctive rattle call is certainly not part of their hunt. Why would they want to advertise their presence to their prey?!

Particularly as juveniles, rattlesnakes are the frequent targets of hawks, eagles, crows, raccoons, coyotes, skunks, and even other snakes. With the exception of whipsnakes (who have evolved immunity to rattlesnake venom), all of these predators run a very high risk when hunting the very poisonous rattlers.

The only hope of nabbing a rattlesnake without dying in the process is to sneak up on them. When a threat is spotted, rattlers shake their tail and make a very distinct and conspicuous sound. Clearly, the predators have learned this signal because they almost always retreat, rather than risk being dealt a fatal injury.

I am poisonous!

While I would like to focus on intentional communication in this post, I would be remiss if I didn’t at least mention something called aposematic coloring. This is a phenomenon in which a prey animal evolves an anti-predator defense mechanism AND a way to communicate that defense to their predators.

The most famous example are the poisonous tree frogs of the Amazon rain forest. These little guys secrete extremely potent toxins into their skin. So toxic are these secretions that Amazonian tribes wipe their arrows on the backs of these frogs to make a truly lethal weapon.

The value of the poison is obvious: if a predator eats the frog, she will then die. However, even when the poison works as it should, the frog is still dead. An even better way to use a poisonous deterrent is to train predators not to eat you in the first place.

This is where their bright coloring comes in. The poisonous frogs have evolved obnoxious coloring so that they are as recognizable as possible, the exact opposite of camouflage. This coloring then “trains” the predators to recognize and avoid the frogs.

This training is not about true learning: the predators wouldn’t survive the “learning” process. Instead, it is evolutionary learning. Through mutation and selection, the predators evolve the avoidance behaviors. Predators with no aversion to the frogs eat them and die, while predators who are genetically disposed to avoid them survive and leave offspring.

Once this training is in place, the poisonous tree frogs are largely left alone by their predators. The “communication” is complete.

One last thing about coloring. Nature has a common tendency to reward cheaters and poison advertising is no exception. After one species has gone to the trouble of evolving poison and then training predators to avoid eating them, others can piggyback. There are also species of tree frogs that are brightly colored but with no poison. These are nature’s freeloaders and this term is called Batesian mimicry.

(Can you tell which of these frogs are poisonous? Neither can their predators. Answer: the top row are all poisonous species, the bottom row are all nonpoisonous mimics.)

I am fast and healthy!

For some prey species, it would be silly to feign aggression and pretend that you could fight off your predator. Imagine a mouse trying to convince a cat that he’s big and tough and will fight back.

However, some prey animals have evolved means to tell their would-be killers, “I see you. I’m faster than you. Don’t bother trying to chase me because I’ve got a head start and you’ll just be wasting your energy.” This phenomenon is called signaling theory and these displays usually involve a prey animal engaging in feats of strength to show predators that they are strong, fast, and/or alerted to their presence.

One of the most famous examples of signaling theory is a behavior in gazelles called stotting or pronking.

The main predator of gazelles is the cheetah. The cheetah can sprint faster than the gazelle, but has less endurance and cannot maintain speed as well while turning. This means that the cheetah must sneak up on the gazelle stealthily and make a sudden surprise attack if she is going to catch him. If the gazelle has a head start, the cheetah has no chance.

When a gazelle spots a stalking cheetah, he will start jumping very high, straight up in the air. It’s a rather remarkable sight, actually. At first blush, this seems kind of stupid. Here is this gazelle being stalked by a cheetah, and when he notices, rather than running away, he makes himself incredibly obvious to the cheetah.

However, what happens next demonstrates the purpose of the stotting: the cheetah gives up the hunt and walks away. The stotting is the gazelle’s way of telling the cheetah that he sees her, has a head start, and that a chase would be futile.

I am honest!

Stotting is often referred to as an “honest” signal because, since the gazelle has to be in good physical shape to perform the signal, it is a true display of fitness. This is a fascinating example of co-evolution because the gazelle has evolved to perform the signal and the cheetah has evolved the ability to interpret it. Both species benefit from this communication because they’ve been spared the bother of a fruitless chase.

The gazelle is happy to evolve a way to avoid having to outrun the cheetah every time, and the cheetah is happy to evolve a way to reduce its record of unsuccessful hunts. Chases are energy-expensive after all and they are also are very loud and obvious. After a chase, every potential prey animal in the area is suddenly aware of the cheetah. They get one shot. If it doesn’t work out, there probably won’t be any successful hunts the rest of that day.

Communicating About Predators

Stotting also communicates other things to nearby animals. Of course, the slotting warns fellow gazelles of a cheetah in the area. That may have been the reason that the behavior first evolved and then Cheetahs learned it later, but we’ll probably never know for sure. Further, stotting may be part of the courtship behavior of gazelles. Given its utility in avoiding both predation and in saving energy, stotting seems like as good a display of fitness as any other.

The many species of gazelle are not the only animals that stot. Their cloven relatives impalas, antelopes, and wild sheep are all thought to stot. Although domestication seems to have diminished stotting in adult sheep livestock, young lambs are prone to periodically engage in bizarre spastic jumping behavior that seems playful and may be the remnant of the stotting instinct. Other forms of pursuit-deterrent signal have been discovered in motmot birds, Eurasian jays, rabbits, mice, curly-tailed lizards, and even guppies.

As I’ve written about before, Diana monkeys have been shown to give specific calls to warn their fellow monkeys of specific kinds of predators. Their vocabulary includes distinct calls for each of their main predators.

Interestingly, two of the Diana Monkey predators – leopards and eagles – hunt by surprise attack. Accordingly, they have “learned” what the calls mean and when they hear the monkeys making those calls, they give up the hunt. Thus, these calls also function as pursuit-deterrents for those predators, not unlike slotting in gazelles.

However, chimpanzees also hunt Diana monkeys, but they do so through sustained stalking and chase, not surprise attacks. Consequently, they are not at all dissuaded by the warning calls. For the chimps, they don’t rely on the element of surprise anyway so they could care less if the Dianas are aware that they are being hunted.

While this Diana monkey alarm system almost certainly evolved as a warning to conspecifics, the predators have “learned” what they mean. For eagles and leopards, the game is up when they have been spotted and so they give up and move on.

Signaling theory depends on the signals being truthful. What if Diana monkeys went around making eagle calls randomly, just to protect themselves in the off-chance that an eagle was nearby? After a while, the eagles would lose their training and no longer be dissuaded from attacking based on the calls alone. The dishonest Diana monkeys would find themselves to be victims of eagle attacks, possibly even more often than chance alone because the calls might actually attract the eagles. If the cheating behavior were genetic, dishonesty would quickly be bred out of the population and balance would be restored. Thus, honesty is self-perpetuating in signaling theory.


Interestingly, some animals appear to have evolved to “eavesdrop” on predator-warning calls made by other species. There is a species of iguana on the island of Madagascar that has evolved very respectable hearing, despite the fact that they don’t hunt using sound, nor do they communicate among themselves using any auditory communication.

They do, however, respond to the predator-warning calls of a species of flycatcher bird that also lives on the island. Both the iguanas and the flycatchers are sometimes preyed upon by large raptors—birds of prey. The iguanas never evolved a warning call for raptors because they didn’t have to. They could just listen out for the calls made by flycatchers.

Just like the nonpoisonous frogs that mimic poisonous ones, cheaters can prosper.

Why Hasn’t Evolution Made Another Platypus?

Originally published at Nautilus on September 14, 2017.

S nuffling through the underbrush, the shaggy little creature wanders through the sylvan night, sticking its nose in one place, then another, seeking the aroma of…


Special thanks to John Calambokidis, Ari Friedlaender, and David Johnston and the crew of the Research Vessel Truth for spearheading field operations to Jo Welsh for anchovy specimens to Madison Bashford, Ben Burford, and Diana Li for experimental assistance to Jake Linsky for analytical assistance to Jessica Bender for sea lion illustrations and to Alex Boersma for the remainder of the illustrations. Thanks should be extended to the three anonymous reviewers whose careful considerations strengthened the manuscript. This work was funded with NSF Integrative Organismal Systems Grant 1656691, Office of Naval Research Young Investigator Program Grant N000141612477, and Stanford University’s Terman and Bass Fellowships. All procedures were conducted under institutional Institutional Animal Care and Use Committee guidelines and National Marine Fisheries Service permit 16111.

Why We Age

As you read in the last chapter, we age because every time our cells reproduce, our telomeres shorten. Each shortening of the telomere leads to increasingly poor cellular functioning. This, in a nutshell, is the telomere theory of aging, and it tells us an enormous amount about how we age, as you will see in the rest of this book.

In this chapter, however, I want to take a brief diversion to ask another question: Why do we age? Earlier theories of aging tend to avoid this question. If we are aging because of wear and tear or free radicals, the answer is clear: We age because aging is inevitable. Despite our bodies’ best efforts, the accumulated damage eventually overcomes our ability to repair that damage.

But the telomere theory of aging makes the question of why we age far more interesting. Our cells all have the gene for telomerase. They could express telomerase, just as our germ and stem cells do. But they don’t. Apparently, our bodies, rather than aging as the result of an inevitable physical process, are, in fact, designed to age. Our bodies age on purpose.

Evolutionary Thinking

“Evolution is cleverer than you are.” Leslie Orgel, evolutionary biologist

Any time we ask why something happens in biology, we are asking an evolutionary question. The functioning of every living thing on this planet is the result of billions of years of evolution, and virtually every aspect of your body is the result of the relentless workings of evolution. If aging didn’t make good evolutionary sense for a species, then organisms wouldn’t age. Somehow, aging makes it more likely that our genes will replicate and our species will survive.

Asking why can feel like an endless children’s game, but in biology the whys generally end with an evolutionary explanation.

A: Because we haven’t eaten in a while.

Q: Why does not eating make you hungry?

A: Because when you haven’t eaten your body produces less leptin, and that causes you to feel hungry.

Q: Why does your body do that?

A: Because animals that didn’t get hungry didn’t work as hard to find food, and didn’t survive to reproduce, while animals that got hungry survived. People inherited this trait from their animal ancestors.

Note that it’s very tempting to say that your body “wants” you to eat or that evolution “wants” you to eat. I’ll use this shorthand occasionally, but it’s important to remember that saying “want” is only convenient shorthand. Evolution doesn’t “want” anything, but if you don’t eat, then your genes don’t survive. In this chapter we are asking, “Why does our body want to age?” but this is shorthand. The real question is this: Why did animals that age out-survive and out-reproduce animals that didn’t age? Why does aging make a species (and its genes) more likely to survive?

Because multicellular animals began aging billions of years ago, any answer to the question of why we age is inherently speculative. But asking the question can teach us a lot about evolutionary reasoning and the nature of the evolutionary process.

Evolutionary Cost-Benefit

Why aren’t lions faster? Lions can run at an impressive fifty miles per hour for short bursts, but their prey are equally fast. Wildebeest can run at fifty miles per hour as well. Zebras and Cape buffalo can run at around forty miles per hour. Because lions survive by chasing down prey, why haven’t they evolved to run as quickly as a cheetah, which can run seventy miles per hour?

Because, as with everything in evolution, there are tradeoffs. Cheetahs can reach incredible speeds because their slender bodies, small heads, and long, thin legs are very aerodynamic. They have oversized hearts and large lungs and nostrils to allow their muscles to stay oxygenated at these speeds. But these benefits come at a price. The cheetah’s small head means smaller teeth and weaker jaws than most predators.

A faster running lion might find it easier to catch prey, but may need to be less muscular and more aerodynamic. This faster lion might fall victim to competitions with other lions and never live to reproduce.

The point is that to understand why evolution takes a certain direction, it’s necessary to look at tradeoffs between the costs and the benefits.

This brings us to the question of aging. Wouldn’t it be a big evolutionary advantage to live and reproduce indefinitely? Animals that didn’t age could theoretically produce many times more offspring than their aging competitors.

It turns out that aging has a much lower cost than it might seem. While there is great variation in lifespan from species to species, most animals do not live a full lifespan and die of old age. Starvation, interspecies competition, predation, disease, and cancer kill most animals long before they reach old age. Aging isn’t a factor in their deaths. The “cost of aging” only applies to those animals who reach a point where aging is a factor in their death.

There’s another factor that is a little more subtle. Organisms function in their own ecological niches, which can only support so many of each species. The limiting factor in the number of deer, for example, isn’t the rate at which deer can reproduce. It’s based on the availability of food and prevalence of predators. If a given area can support a population of 1,000 deer, what would happen if another 1,000 deer were suddenly added? Starvation and predation would soon reduce the population back to 1,000.

With this in mind, imagine a small group of ageless deer that reproduce indefinitely. These deer are a small subset of the total population of deer, which age normally. The aging deer, like all species, continually evolve in response to changes in the environment.

But the ageless deer are producing offspring that represent an earlier evolutionary phase. With each generation the offspring of the ageless deer are less fit than the offspring of aging deer. These ageless deer would be quickly be crowded out.

This last point is the key issue in the evolution of aging. If a species has a long lifespan, then it can’t adapt as quickly as a species with a shorter lifespan. It’s a bit like the turning radius of a car—if the turning radius is shorter, the car can turn sharper corners. If a species lives a long life and has offspring late in life, then the “turning radius” of the species may not keep up with rapid changes in its physical or biological environment. If the temperature, oxygen, pH, or some other physical aspect of the environment changes, a species needs to change with it. If the biological competition or a prey species changes quickly, then once again, a species with a shorter lifespan can adapt more quickly and is more likely to survive. On the other hand, if the environment—physical or biological—is stable, then longer lifespans are advantageous to survival. Lifespan and the rate of aging have to be finely tuned not just to the environment, but also to the rate at which the environment changes.

So the benefits of “agelessness” are much lower than it might seem, for two reasons: Most deaths occur before aging decreases fitness, and agelessness slows the rate of evolution. Aging has benefits to a species, but the costs to an individual—aging and disease—are severe.

Historically, we’ve sometimes assumed that aging was simply part of being multicellular. As it turns out, some multicellular organisms (like hydra) don’t age, while some unicellular organisms (like yeast) do age.

The Multicellular Dilemma

Multicellular life first evolved around one billion years ago, after an estimated 2.6 billion years during which only single-celled organisms lived. Early multicellular life was in the form of cooperative colonies, in which single-celled organisms could thrive better than they could on their own over time.

Multicellular life eventually learned to differentiate cells to allow for more sophisticated organisms with specialized germ cells for reproduction. Consider how radical this change was for the cells of multicellular creatures. For billions of years cells had evolved to survive and reproduce. The single-celled organisms that reproduced most rapidly and most successfully crowded out those single-celled organisms that were not as aggressive.

Now, as part of a multicellular life form, cells had to learn a very different way of behaving. Cells had to operate responsibly to perform their roles in support of the whole organism. They had to divide only when needed for the benefit of the organism. A cell that divides too rapidly—when the organism doesn’t need it to divide—is a cancer cell, which kills the organism. Organisms with cells that reproduced willy-nilly were selected against organisms that carefully controlled their cells survived and prospered.

Multicellular creatures evolved to control their cells’ reproduction. What was the mechanism of that control? Part of that mechanism was cell aging.

The Hayflick Limit provides a harsh but powerful tool for controlling cell reproduction. After a certain number of divisions, cells simply couldn’t reproduce any further. With each cell division, the telomeres shorten, and after forty or so divisions, most cells simply can’t divide any further. This mechanism of control came at a price—aging and death from aging—but, as we saw earlier, this price wasn’t all that high, evolutionarily speaking, and may have had some benefit in fine-tuning the rate at which a species can adapt to environmental changes.

Why Do We Age?

While the complete answer to why we age may never be known, it seems quite likely that aging was a product of evolution—a tool to enhance a species’ ability to adapt quickly to environmental change. So if evolution “chose” aging, can scientists develop tools to “unchoose” it in whole or in part? This brings us to the next chapter, in which we leave theory behind and examine the progress we’ve made applying the telomere theory of aging to improving health and lengthening lives.

About the Author:

The world’s foremost expert on the clinical use of telomerase for age-related diseases, Dr. Michael Fossel has lectured at the National Institute for Health and the Smithsonian Institute, and continues to lecture at universities, institutes, and conferences throughout the world. He has appeared on Good Morning America, ABC 20/20, NBC Extra, Fox Network, CNN, BBC, Discovery Channel, and regularly on NPR. He is currently working to bring telomerase to human trials for Alzheimer’s disease.

Snake v. newt

The three species of newts from the genus Taricha defend themselves with a lethal poison called tetrodotoxin. It kills by plugging up molecular pores on the surface of nerve and muscle cells that act as channels for sodium ions. If these ions are denied passage, nerve cells can’t fire and muscles can’t contract. The heart stops, breathing becomes impossible and death soon follows. There is no antidote.

The skin of a single newt is laced with enough tetrodotoxin to kill 10-20 humans, or thousands of mice. But not the common garter snake (Thamnophis sirtalis) some individuals have become immune to tetrodotoxin, by changing the structure of their sodium channels so that the poison no longer blocks them.

To study the arms race between snake and newt, Hanifin surveyed different populations across their entire shared range, a 2,000 km stretch of land between British Columbia and the southern tip of California. While many arms-race studies look at a single pair of populations, that’s a bit like spotlighting on two actors on a crowded stage instead, Hanifin wanted to look at a large geographical stage to watch populations at different stages of escalation.

Together with two Edmund Brodies (Jr and III), he measured the levels of tetrodotoxin in newts from 28 locations across the west coast. They also measured how resistant local snakes were by injecting them with the poison and measuring its effect on their slithering speed.

As expected, they found massive differences in both toxicity and resistance. Some populations haven’t entered the arms race at all in British Columbia, for example, non-resistant snakes live alongside poisonless newts. As the newts become more toxic, the snakes become more resistant and the conflict escalates until both poison and resistance are magnified by a thousand times.

In general, the most resistant snakes lived alongside the most toxic newts. But Hanifin also found that the animals’ abilities were often mismatched and in every single case, it was the snakes that came out ahead. In a third of the locations they sampled, even the least resistant snakes were more than capable of eating the most toxic newts. Taking mouthfuls of one of the most lethal of animal poisons barely slowed them down.

In these locations, the snakes have escaped from the cyclic nature of the evolutionary arms race. Their advantage is so great that there isn’t a newt toxin they can’t handle, and as such, they are under no impetus to become even more resistant.

The Basics of Equine Behavior

A horse’s vision is its primary detector of danger. Even though they have poor color vision, they can differentiate blue and red from gray hues. However, they have more trouble differentiating yellow and green from gray. Horses also have poor depth perception when only using one eye. They can’t tell a trailer from an endless tunnel, or a mud puddle from a bottomless lagoon. Their perception is improved by about 5 times when using both eyes (binocular vision). They can instantly change their focus from near to far objects. This is why horses cock their head in different ways to see close vs. distant objects. Horses have an acute ability to detect movement. This is why a horse is much flightier on windy days things that are normally stationary are now moving and perceived as a potential threat. Horses are able to see fairly well at night however, the contrast sensitivity is less than that of a cat.

The mechanics of a horse’s vision are different from our own. They can see almost panoramically, with a small spot directly in front and directly behind as their blind area (see Figure 1). Never approach a horse without talking to them in these areas if frightened they will use one of their defense mechanisms, e.g., kick or run. A horse can see two things at once, one from each eye. That allows each side of its brain to work separately. Like humans, horses have a dominant side (right-handed or left-handed) however, unlike humans, horses need to be taught things twice: on the right side and on the left side. The expression in a horse’s eye is often thought to be a good indicator of their behavior, e.g., wide open with white showing (and not an Appaloosa), scared half closed, sleepy, etc.

A horse’s hearing is much keener than ours. They use their hearing for three primary functions: to detect sounds, to determine the location of the sound, and to provide sensory information that allows the horse to recognize the identity of these sources. Horses can hear low to very high frequency sound, in the range of 14 Hz to 25 kHz (human range = 20 Hz to 20 kHz). Horses’ ears can move 180 degrees using 10 different muscles (vs. 3 for the human ear) and are able to single out a specific area to listen to. This allows the horse to orient itself toward the sounds to be able to determine what is making the noise.

Horses’ tactile sensation or touch is extremely sensitive. Their entire body is as sensitive as our fingertips. They can feel a fly on one single hair and any movement of the rider.

Body Signals
Horses are good at letting us know exactly how they are feeling the only problem is most people don’t know how to speak “horse”. So here are some tips on reading a horse’s body language.

If a horse’s tail is:

  • High: they are alert or excited
  • Low: it is a sign of exhaustion, fear, pain or submission
  • Held high over its back: (as seen in most foals) they are playful or are very alarmed
  • Swishing: they are irritated.
  • Pawing: they are frustrated
  • One front-leg lifted: can be a mild threat (or a normal stance sometimes when eating
  • A back-leg lifted: is often a more defensive threat
  • Stamping: indicates a mild threat or protest (or they may be getting rid of insects or flies biting their legs).

Some horses’ facial expressions include:

  • Snapping: This is seen in foals showing submission to an older horse. They will open their mouths and draw back the corners, then open and shut their jaws.
  • Jaws open with teeth exposed: this shows aggression or possible attack.
  • The Flehmen response: This is caused by an intense or unusual smell, usually in stallions when they sense a mare in heat. They stick their nose in the air and curl the upper lip over their nose.
  • Flared nostrils: usually means they are excited or alert.
  • Showing white around the eyes: usually means they are angry or scared. (White around the eyes is also a normal characteristic of the Appaloosa breed.)

The horses’ ears are a unique feature:

  • Neutral: is when the ears are held loosely upward, openings facing forward or outward.
  • Pricked: ears held stiff with openings pointed directly forward means the horse is alert.
  • Airplane ears: the ears flop out laterally with openings facing down, usually meaning the horse is tired or depressed.
  • Drooped ears: hang down loosely to the side, usually meaning tiredness or pain.
  • Ears angled backward (with openings directed back towards a rider): usually mean attentiveness to the rider or listening to commands.
  • Ears pinned flat against the neck: (see picture below) the means watch out! The horse is angry and aggressive.


Horses have a variety of methods of vocal and non-vocal communication. Vocal noises include a squeal or scream which usually denotes a threat by a stallion or mare. Nickers are low-pitched and quiet. A stallion will nicker when courting a mare a mare and foal nicker to each other and domestic horses nicker for food. Neighs or whinnies are the most familiar: high pitched, drawn out sounds that can carry over distances. Horses whinny to let others know where they are and to try to locate a herd mate. They also respond to each other’s whinnies even when out of sight.

Blowing is a strong, rapid expulsion of air resulting in a high pitched “whooshing” sound, which usually is a sign of alarm used to warn others. Snorting is a more passive, shorter lower pitched version of blowing and is usually just a result of objects entering the nasal passage.

I n contrast to signals of aggression within a herd, there are also signs of friendship. Mares and foals nudge and nuzzle each other during nursing or for comfort, and mutual grooming, when two horses nibble at each other, is often seen.

A herd of wild horses consists of one or two stallions, a group of mares, and their foals. The leader of the herd is usually an older mare (the “alpha mare”), even though one stallion owns the herd. She maintains her dominant role even though she may be physically weaker than the others. The older mare has had more experiences, more close encounters, and survived more threats then any other horse in the herd. The requirement of the lead horse is not strength or size if this were so, then humans could never dominate a horse. Dominance is established not only through aggression but also through attitudes that let the other horses know she expects to be obeyed.

The stallion’s job is to be the herd’s guardian and protector, while maintaining reproductive viability. The stallion’s harem usually consists of 2 to 21 horses, with up to 8 of those being mares and the rest their offspring. When the colts are old enough to be on their own they will form a bachelor herd. The fillies will either remain in their natural herd or more commonly disperse into other herds or form a new herd with a bachelor stallion. As soon as a stallion becomes too old to maintain his status as herd owner he is replaced by a younger stallion from a bachelor herd. The average time for a stallion to remain leader is about 2 years, but some can last more than 10 years.

Horses are most vulnerable when they are eating or drinking. So, when a horse is being submissive, it will simulate eating by lowering its head, chewing, and licking its lips (similar to snapping mentioned above). Dominance occurs when a horse forces the other to move against its will. One horse will move its body in the direction of or in contact with the other forcing it to move. Fighting usually occurs when the dominant horse is challenged by the other horse not moving, or responding aggressively.

Vices are negative activities that occur due to various causes, including stress, boredom, fear, excess energy, and nervousness. Horses naturally graze for 12 to 16 hours a day. When kept in stalls we prevent them from engaging in many natural activities such as grazing, walking, or playing with other horses. Not enough natural stimuli will cause a horse to invent its own stimuli. Once these habits start they are difficult to eliminate.

Cribbing occurs when the horse bites onto a fixed surface (e.g., stall door edge, grain bin, fence rail), arches his neck and sucks in air, making a grunting noise. This causes a release of endorphins which relieves the unpleasant situation. Cribbing becomes addictive even when removed from the unpleasant situation the horse may still crib. Some horses even prefer cribbing to eating! Cribbing can lead to weight loss, poor performance, gastric colic, and excessive tooth wear.

occurs when the horse stands by the stall door and rhythmically shifts its weight back and forth on its front legs while swinging its head. This is also caused by boredom or excess energy, and can lead to weight loss, poor performance and weakened tendons.

Stall kicking
, stall walking, pawing, or digging, and biting over the stall door are also vices that are caused by boredom from being kept in a stall. To decrease the frequency of this behavior, you might try adding another mealtime, placing toys in the stall, or providing more roughage or turn out time.

Wood chewing, eating bedding, or dirt, and self-mutilation are caused by lack of exercise or boredom. However, nutritional deficiencies could also cause these vices. To eliminate this as a cause, provide more roughage to the diet, and free choice salt or minerals. This may decrease the frequency of the vice.

Super-predatory humans

Predators have roamed the planet for 500 million years. The earliest is thought to be some type of simple marine organism, a flatworm maybe or type of crustacean, perhaps a giant shrimp that feasted on ancient trilobites. Much later came the famous predatory dinosaurs such as T. rex, and later still large toothed mammals such as sabre toothed cats or modern wolves.

But one or two hundred thousand years ago, the world’s most powerful predator arrived.

We lacked big teeth or sharp claws, huge tentacles or venomous bites. But we had intelligence, and the guile to produce tools and artificial weapons. And as we became ever better hunters we started harvesting animals on a great scale.

We wiped out the passenger pigeon, the dodo, the great herds of North American bison. Last century we decimated great whale populations. Today the world’s fishing fleets routinely take more fish than scientists say is sustainable, leading to crashes in cod numbers for example, while people kill more large mammals in North America than all other causes put together.

But out of our mass consumption of the world’s fauna appears a curious conundrum.

Predators and prey are normally locked in an evolutionary arms race. As predators evolve to run faster, their prey too is selected to become fleeter of foot. As predators evolve sharp teeth, herbivores evolve horns for protection. Some carnivores hunt in packs, so their prey form defensive herds.

But animals don’t appear to have evolved defences against us. Which raises the question why?

Is it that these animals simply haven’t had time to evolve defences, or lack the variation in their genes to produce them? Or is it to do with the way we hunt them?

These questions are raised by Professor Geerat Vermeij of the University of California at Davis, US, in a scientific paper just published in the journal Evolution. He has been studying the effects of predators on evolution for more than thirty years.

“Usually, when new, more powerful predators evolve or come in from elsewhere, the local species can often adapt by themselves becoming better defended through a variety of means but this option seems to be closed when it comes to the evolution of humans as super-predators,” he tells me.

Even huge blue whales have become potential prey

In his paper he investigates why this is so.

First he examines how animals adapt to other non-human predators. He shows how prey animals consistently, and successfully, evolve certain types of defence.

The first is growing big. If you can grow big enough, it becomes very difficult, even for predators hunting in packs, to tackle you without injury and bring you down.

Scientific studies have shown that large terrestrial herbivores are by weight up to ten times bigger than their largest predators, which can’t grow mouths large enough to cope with their outsized prey. It explains why lions, wolves and orca tend to avoid fit adult buffalo, moose and whales respectively, targeting more often the weak and young (which are smaller).

If species can’t grow big, then they evolve other defences, such as the passive armour afforded by shells. As predators evolved to drill through shells, many prey species evolved to become toxic. The evolutionary arms race once more. A good example here, says Prof Vermeij, is the cephalopods, animals including squid and octopi. Early versions of these animals had armour, but as they were eaten by fish and toothed whales, they were replaced by lineages that were faster, more aggressive, venomous or toxic.

But then humans came along.

“The spread of modern humans represents one of the great ecological and evolutionary transformations in the history of life,” Prof Vermeij writes in Evolution.

We hunted and gathered on land, but soon began exploiting intertidal zones, taking shellfish and fish. Such intertidal zones were important food sources for prehistoric human populations living in places as far and wide as South America, South Africa, California and Oceania.

Boar hunting depicted in the 14th Century

Then we started taking big animals. When we did the very adaptations that offered protection against natural predators attracted rather than deterred human hunters. The huge size of mammals such as bison or whales made them juicy targets for meat-hungry humans for example.

Other defensive ornaments became disadvantageous as humans evolved into super-predators. Elephants were killed for ivory, crabs and lobsters fished for their large meaty claws. These once advantageous traits became liabilities in the modern, human-dominated world.

We didn’t just take large species, we also preferentially harvested out the largest individuals of smaller species, a problem that persists today.

Prof Vermeij has examined the degree to which this happens.

He looked at one group of animals, marine molluscs and echinoderms such as starfish, and surveyed all the scientific research into how they have been exploited by humans. We select the largest individuals among 35 of 40 species studied, he discovered.

That means that size is no longer a refuge. Whereas growing big may have been one defence against natural predators, it offers no defence against human super-predators.

Sticking to rocks, as limpets do, is no good either as humans have invented picks and knives to prise them off.

Prey animals may do better to become toxic instead, and there is evidence that some marine species have become poisonous to people, either producing their own toxins, or by harnessing toxins produced by microbes. Reef fish and crabs are often toxic to people because they contain unpalatable, and sometimes lethal, dinoflagellates, for example.

Elephant tusks attracted rather than deterred human hunters (Ron O'Connor / NPL)

But humans have found ways to get around this too. Many toxins need to be concentrated into organs such as the liver. And humans have learnt to remove these, to avoid their ill effects.

In short the way humans hunt appears to be the main factor preventing animals evolving adaptations to defend themselves from us.

Animals do respond to selective pressures, even over short time scales, and many species have responded to humans being super-predators, says Prof Vermeij.

By eliminating large apex predators, secondary predators have boomed. As cod numbers crashed in the 20th Century, their place was taken by an abundance of shrimp, lobster and crabs, which in turn feed on marine snails. As a result, these snails may have evolved thicker shells to protect themselves against these marauding shell-crunching crustaceans.

But we hunt on too grand a scale, with too much ingenuity, targeting the biggest animals.

“Our arrival and technological history has engendered an enormous change in the evolution of most species on Earth,” says Prof Vermeij.

In evolutionary terms, we leave our prey with nowhere to go. They have no way to defend themselves and simply cannot respond.

And that, says Prof Vermeij, represents a cataclysmic shift for species on this planet, the implications of which, he adds, we have barely begun to understand.

Comments Post your comment

Comment number 1.

Enjoyed the article. Here's a thought. Given the unusual nature of our dominance, it could be considered that defending against us has required unusual "work-arounds" by natural selection.
Animals that have become successful since the rise of Homo sapiens are the domestic dog, the domestic cat, cattle, sheep, goats, etc. In other words, success has come not by competing with us but by becoming commensal or by having a synergistic relationship with us.
You might think it isn't much of a life being a cow or a sheep but, in biological terms, they are currently extremely successful.
For that matter, rhinoviruses are doing quite well, the influenza virus, the Lactobacillae, Escherischa coli is booming, Staph., and so on.
So, our "servant" species and our parasites and micro-predators are doing ok. Perhaps the closest to an "old-fashioned" competitor that is doing well would be the fox, crows and the like. Small, to medium-sized intelligent generalists.
Oh. And of course, cockroaches. :-)

Comment number 2.

Very interesting article. I suppose the same goes for plant evolution, given the scale of human agriculture, urbanisation, impact on climate etc.

As the writer says, we have barely begun to understand the implications of the ecosystem transformations we have caused. At the same time, humans themselves have barely evolved over the last 10,000 years, during which our impact on the natutral world has been greatest. So we are altering our environment dramatically without adapting to this genetically.

Comment number 3.

As humans have wiped out many of the major sea predators such as tuna, sharks etc a new kid on the block is becoming dominant and that is the Humboldt squid and it reproduces in large numbers and even likes human.

Comment number 4.

I thought that one explanation of the extinction of large mammals in America after the arrival of humans was that unlike Africa they had not evolved behavioral defenses.

Comment number 5.

Some interesting observations on the flip side of this matter from @sarahtim.

Far more species have learned and evolved anti-human predator and adaptations than we realise. It is merely that most people are so out of touch with animal behaviour that they never see it. Modern humans are in general extraordinarily poor at reading the behaviour of wildlife. Whilst evolved predator defences can be very effective, they are rarely easily observable as such. This is because if the defence is very effective, the species tends to get ignored by predators, so you won't actually see it in action. The most effective predator protection is probably the most subtle, because of its success. Often protection from predation just involves the numbers game and so whilst greatly reducing the success of predator attacks, it is not a 100% defence against them i.e. it just makes it more difficult for the predator.

All species have limitations of what type of predation they can adapt to. Ground nesting birds have trouble adapting to introduced ground predators, as it would require a major change in their behaviour. Therefore how quickly a species can adapt to predation depends on what it's experience of analogous predators is, and what genetic or phenotypic tool kit it has at its disposal. Self-evidently there is a continuum of protection from near complete protection from predation, to just subtle statistical drops in the efficacy of predator attacks.

Through a lifetime of watching and photographing wildlife I've had to learn to get closer to it. To understand how different species see the world, what scares them and what they are comfortable with. This has left me being much better at reading animals than most. What has become very apparent to me is that wildlife is a lot better at reading and understanding us, than we are at reading them.

For instance in the UK countryside wildlife is far more terrified of humans than even their regular predators. I've seen rabbits ignoring a fox strolling past them, because they've seen it, and it knows that. But the same rabbits will scatter at over 100m if they see a passing rambler. This is because rabbits understand how foxes hunt and when they are safe. People are much harder to read as some have rifles that can kill a rabbit well over 100m away. Nevertheless, whilst not giving themselves complete protection from firearms, in daylight it makes it difficult to do more than pick off the odd rabbit.

There is plenty of anecdotal evidence for how quickly animals learn. The oral history of one Native American tribe has it that wolves did infrequently take their people in the past, but that quickly stopped after European colonisers and firearms arrived in the area. This is another key point. Most of the more modern dangers from human predation, firearms, commercial fishing boats etc, are relatively new, especially in their modern efficient forms.

What I'm trying to say is that I believe that plenty of species already have highly adapted responses to human predation. The reason it isn't documented is for a number of reasons. Firstly it is probably because this behaviour is now taken for granted i.e. it's how that species behaves, because we never knew of its previous behaviour. Most of this adaptation is behavioural, like simply keeping out of sight, and therefore very difficult to document. Finally evolution works the numbers game on many dimensions. Species come and go from an ecosystem perspective. The most likely adaptation of ecosystems with species heavily exploited by humans, is that in the future these niches would get filled by more variable species, less interesting and useful to people, and less commercially exploitable.

Comment number 6.

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