According to the biographies of long-lived people, the secret of immortality is hidden in an active and vibrant life. For example, the biography of Jeanne Kalman, whose 122 years of life is still a record among documented achievements, suggests: eat chocolate, spin the pedals and spare no wine - and your hundred years! But if we take a look at the rest of the animal kingdom, then we come across much less encouraging advice. Champions among fish and mammals, birds and invertebrates do not mature for a long time, they hide in cold dark corners and are in no hurry to breed. "Attic" will try to draw a collective image of a long-lived animal and evaluate to what extent a person corresponds to him.
Whose path is longer
When it comes to long-livers, numbers are on the table as the main argument. Let's start with them and we. Who comes to your mind when you hear about long-lived organisms? Elephant, turtle, parrot? Whale? All of them did not even get close to the podium. In the first place, with a huge margin from the rest, sponges were fixed. The record holder among them - as far as it was possible to determine from the mineral skeleton - is about 11 thousand years old. The second place is firmly held by the corals Leiopathes sp. and Gerardia sp. (4265 and 2742 years respectively). The third place, having lost hope of catching up with the first two, is occupied by the bivalve mollusk Arctica islandica, 507 years old. It is followed by the Greenland shark (new to this list, about 400 years old), shellfish, sea urchin and some fish (including, for example, the Aleutian perch). But they all did not cross the border of 200-250 years of life.
Most of the winners belong to invertebrates - vertebrates practically did not get places on this podium. And our closest relatives - mammals - are not among them at all. Of those whom we habitually consider long-livers, only the bowhead whale could compete with this team: according to some reports, it managed to hold out for 211 years. There is no naked mole rat among the champions - the icon of modern gerontology. It can often be found in articles as an example of an ageless organism, because it is 10 times ahead of its mouse relatives in average age and hardly changes with age, but it lives only about 30 years.
However, the places on this pedestal are changeable and will constantly change hands as new dates appear: sooner or later they will find another mollusk that lives a little longer than the previous one, or a new perch that has overtaken the sea urchin, and so on ad infinitum. There are no definitive values for maximum life expectancy. Every time we say that someone "lives up to 500 years", we have to constantly add "latest data", because data is constantly arriving. But can we measure aging with constantly fluctuating numbers?
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Moreover, in its pure form, this list will hardly be useful to us. If we really want to take advantage of other people's secrets of long life, it would be good if this organism was a little like ours, at least in terms of a set of organ systems.
On either side of the straight
In humans, the main statistical indicator of aging is the Gompertz-Makeham curve, which reflects the dependence of the risk of dying from natural causes on a person's age, or, simply put, the inevitable approach of death. The curve on the graph is constantly growing, that is, it signals that the body is becoming more and more fragile and every year it runs more and more risks. This implies the simplest - and one of the most used now in science - definition: aging is a growing risk of dying.
For any population of people, the mortality curve will look the same, except that it can move left-right depending on living conditions or slightly flatten towards the end. But animals have different options. The graphs below are based on long observations of different creatures. The thin blue line represents the survival rate (as a percentage of the total population). The red curve is the relative risk of dying (1 corresponds to the average risk for an adult). Finally, the thick blue line is the relative ability to reproduce (unit is the average number of offspring that an adult of a given species produces). The charts start at puberty (that is, childhood is not counted) and end at the age when only 5% of the original population is alive.
In many animals, the shape of the curves is generally similar to that of humans. The only fundamental difference between our charts and those of a lion or chimpanzee is that mortality does not grow smoothly, but sharply and from a certain age. Probably, the fact is that the average risk of dying in our population is low, and we tend to take care of the elderly until the moment when we can no longer help them - at this moment the curve soars up. Nevertheless, the trends are the same for us and for the lion: the ability to reproduce decreases over time, and the curve of survival, curved outward (that is, up), literally falls down after a certain age.
But sometimes everything is exactly the opposite. For example, in the red-footed frog (the dashed line indicates the lack of data for analysis) or the desert turtle, the survival line is concave at some stage. In fact, this means that at a certain age, individuals of this species are at risk of dying less and less. This phenomenon has been called negative aging. And if we look for an example of victory over the inevitable in nature, then this should be exactly it - not a movement towards death, but an escape from it.
However, do not be surprised ahead of time. In a person's life there is also such a period, it just did not get on the charts of these authors, because it is earlier than the period of time they are considering. Even in the most civilized society of people, infant mortality is higher than infant mortality, and up to a certain age - even higher than adult mortality. Therefore, up to a certain age (up to about 9 years), our survival curve is also concave, and we, too, according to the statistical definition of aging, move away from death, which means we are getting younger before our eyes. However, this does not mean that humans are ready to live forever - just like the desert turtles. Although the risk of dying in them does not increase with age, as in humans, at any moment in time some individual will die, of course. Therefore, eternal life for some of them is possible only in a hypothetical population of infinite size.
Ignore aging
Since negative aging is actually synonymous with childhood, then where to look for truly ageless animals? The graph of their mortality should be perfectly straight, like a string, not deviating either inward (in childhood) or outward (in old age). This is how, for example, graphs for some species of hydras and the sea ear mollusk look like. They are called negligible aging. This term represents a kind of compromise between scientists, who (for the most part) believe that aging is inevitable, and the results of experiments in which it is not always possible to detect its immediate signs. But in fact, it follows from this graph that there is no aging in their lives.
However, the term "negligible aging" itself appeared long before the construction of these curves. It was suggested by gerontologist Caleb Finch in 1990. He also put forward his own criteria that allow the animal to be awarded this honorary title: 1) mortality does not increase with age, 2) fertility does not decrease with age, 3) there are no age-related diseases that worsen health over time. To date, only six animals meet these stringent requirements: the European tailed amphibian Proteus (Proteus anguinus, maximum life span of 102 years), the American marsh turtle (Emydoidea blandingii, 77 years old), the box turtle (Terrapene carolina, 138 years old), Aleutian perch (Sebastes aleutianus, 205 years old), sea urchin (Strongylocentrotus franciscanus, 200 years old) and bivalve mollusk (Arctica islandica, 507 years old).
Please note that not all lifespan champions are included in this list. There is no hydra in it, no sea ear clam. Perhaps the fact is that not all animals have been able to accumulate enough data in order to verify all the criteria. The now classic hydra observation experiment, for example, only lasted four years. During this time, it was possible to show that the hydra does not age, but what happens to it next is unknown. There are no mammals on this list either. Even the naked mole rat - an animal often called negligible aging - was not worthy of this title. Finch himself, revising his criteria decades later, admitted that the excavator did not meet them. The reason for this was the individual observations of gerontologists, according to which the cubs of the "elderly" mole rats are less viable than those of the "young" ones.- and Finch saw this as a sign of a decline in the animal's reproductive abilities.
There is a crisis of role model: the most long-lived species are too different from us. The champions who are closer to us do not pass the criterion of negligible aging. Who, then, should be guided by and which path to follow? This is where statistics come to the rescue. In the human world it is useless to listen to the advice of each individual person - you need to study centenarians as a whole. It is also impossible to find an ideal in the animal world, so you need to look at all your successful relatives from afar and try to make up some kind of collective image of an animal that has managed to cope with aging. So, elephant, whale, proteus, turtle, shark, parrot, mole rat, perch - what unites them?
Harsh patterns
The first thing that matters for long life is size. Most centenarians are larger than their relatives. This helps them to slip out of the press of natural selection: predators threaten the elephant less than the shrew, which means that long-lived elephants have every chance of leaving more offspring than their short-lived relatives. In this sense, the elephant, whale and shark are no different from others, their long life is only a natural consequence of their impressive size. In this sense, it is more interesting to look at those who did not come out either in length or in height, but still managed to outlive others. Among mammals, this is, for example, the notorious naked mole rat, as well as tree squirrels and bats. Each of them found his own way to escape from predators: burrow underground, climb a tree, or even rise into the air and live in darkness.
The second important advantage that size gives is protection against cancer (not so much from the risks from its occurrence, but rather a decrease in the threat from each individual tumor). Imagine that you are ruling a huge state with millions of citizens. If an uprising occurs in one of a thousand cities, then it will hardly affect the life of the country, unless this city is the capital. But if you are the prince of tiny Liechtenstein and there is a revolution in one of half a dozen of your towns, then you are in serious trouble. Unfortunately, the same simple arithmetic works in an animal's body. If a small tumor has developed in it, say, weighing 3 grams, then some capybara (55 kg) may not notice it at all, while for a mouse (30 g) it is a tenth of the whole body.
Therefore, cancer control strategies, as well as predators, are size dependent in animals. Very small animals, like mice, having no way to escape from the external enemy, surrender to the internal one. Small, but long-lived animals, like the naked mole rat, acquire early defense mechanisms. Their cells do not even get a chance to start multiplying if there is no need for it, for example, if they are surrounded by dense connective tissue without damage. Large centenarians like elephants and turtles are betting on late protection from cancer. Their mechanisms of struggle, for example, enhanced triggering of programmed cell death, do not work immediately and are designed for those tumors that did not die on their own in the early stages of their development.
At the same time, if you forbid your cells to multiply, then how to deal with damage in the body? This dilemma probably explains why there are so few vertebrates among long-living champions: they have acquired too many organs that are extremely difficult to repair without giving the cells additional powers. Bones are much worse renewed than skin, muscles regenerate worse than fat, and brain tissue is almost impossible to restore. This contradiction is the basis of one of the popular theories of aging - the theory of "disposable soma" (disposable soma), which is easier to translate as the theory of "body to discharge." From the point of view of the reproduction of the organism, only the sex cells are of importance. The rest of the body - soma - is just a superstructure above them. And the more it requires attention to itself, the more energy is spent on its renewal,the less resources go to the sex cells. Therefore, vertebrates with their structures that cannot be restored live less than invertebrates: their body eventually ceases to have enough energy for repairs and it is sent “to be discarded”. And only the shark and tailed amphibians (to which the Proteus belongs) can boast of advanced regeneration abilities.
Finally, looking at the list of centenarians, you can find a climatic pattern: most of them live in the cold. This is especially true for cold-blooded animals (mollusk Arctica islandica, Proteus, Aleutian perch and Greenland shark), which do not know how to regulate body temperature from the inside. But even warm-blooded vertebrates, seemingly specially taught to constantly warm themselves up, still strive to find a colder place. An example is the bowhead whale. Or the same naked mole rat who almost became cold-blooded back, buried deep underground. Now his constant body temperature is about 33 degrees, which is significantly lower than that of his rodent relatives.
The fact is that the warm climate brings with it many hardships. The higher the temperature, the faster chemical reactions occur in the animal's body, the more metabolic by-products are formed and the faster the body wears out. Therefore, from the point of view of a long life, being warm-blooded is not so profitable. It is interesting that cold-blooded centenarians, who can already warm themselves only in the rays of the sun, also tend to hide away from it. They have another reason to prefer cold over warmth, and this is a long childhood.
As we remember, childhood corresponds to a period of negative aging. Therefore, the longer the organism pulls from entering maturity, the more time passes before its mortality begins to rise. Living in cold conditions is a great way to slow down development for a cold-blooded animal. Warm-blooded animals can again take advantage of their size: an elephant takes much longer to grow than a rabbit. There is also a third way to stretch childhood - slowing down development. Its most radical type is neoteny, reproduction in the larval state. So, for example, proteus does, like other tailed amphibians. Apparently, a similar fate befell the naked mole rat: although he does not spend his life as a larva,but its development is slowed down - throughout its life it resembles an embryo of a mouse or a rat and does not grow to the appearance of a "real adult" rodent. These clever moves allow animals to bypass the body-to-throw dilemma. Sex cells begin to absorb energy only with the onset of puberty, and the "eternal child" can afford to direct all his forces only to maintain his own health.
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So let's now put together in our mind a typical long-lived animal. It is either quite large or very small, but very cunning. It is not interested in predators, it rarely gets cancer and has its own defense mechanisms against it - it hits the enemy from afar or waits for it "in ambush". It regenerates well and tends to live in the cold, regardless of the base temperature of its body. Finally, it prolongs its childhood, remaining an eternal larva or simply slowing down development, and is in no hurry to reproduce, saving resources.
Our collective portrait does not describe any of the real record-holding animals. The naked mole rat is incapable of regeneration, sharks have no special defense mechanisms against cancer, and bats live with surprisingly high body temperatures. It only says that in each case, long life arose by itself, and there is no general recipe. Each winner went his own way, compensating for inborn shortcomings with new acquisitions.
But a person fits well into the image of a long-lived animal. We are rather small compared to mammalian champions, rarely suffer from predators, live better in cold than warm, and develop more slowly than our primate ancestors. As for the protection against cancer and regeneration, we have long ago discovered these shortcomings and are working to improve them. And when we finish it, it remains to be seen who will have to learn longevity from whom.
Author: Polina Loseva