Smaller, slower, deeper
Useful tips about longevity from record-breaking animals
Polina Loseva, "The Attic"
If you believe 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 still remain a record among documented achievements, suggests: eat chocolate, pedal and do not spare wine – and a hundred years are yours! But if we take a look at the rest of the animal world, we will meet with much less inspiring advice. Champions among fish and mammals, birds and invertebrates do not grow up for a long time, hide in cold dark corners and do not hurry to reproduce. "Attic" will try to draw a collective image of a long-lived animal and assess to what extent a person corresponds to it.
When it comes to centenarians, numbers are the main argument on the table. We will start with them. Who comes to your mind when you hear about long-living organisms? Elephant, turtle, parrot? A whale? All of them didn't even get close to the podium. In the first place, with a huge gap 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 old, 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 (a newcomer to this list, about 400 years old), more shellfish, sea urchin and some fish (including, for example, Aleutian perch). But all of them did not cross the border of 200-250 years of life.
Most of the prize–winners belong to invertebrates - vertebrates practically did not get places on this pedestal. And our closest relatives – mammals – are not among them at all. Of those whom we habitually consider long-livers, only a bowhead whale could compete with this team: according to some reports, he managed to last 211 years. There is no naked digger among the champions – an icon of modern gerontology. He can often be found in articles as an example of an ageless organism, because he is 10 times ahead of his mouse relatives in average age and almost does not change with age, but he 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 individual of perch that has overtaken the sea urchin, and so on indefinitely. The maximum life expectancy does not have final values. Every time we say that someone "lives up to 500 years", we have to constantly add "according to the latest data", because data is constantly arriving. But can we measure aging with constantly fluctuating numbers?
In addition, in its pure form, this list is unlikely to be useful to us. If we really want to take advantage of other people's secrets of a long life, it would be good if this organism was a bit like ours, at least in terms of a set of organ systems.
On both sides of the straight line
In humans, the main statistical indicator of aging is the Gompertz-Makeham curve, reflecting the dependence of the risk of dying from natural causes on the age of a person, or, simply put, the inevitable approach of death. The curve on the graph is continuously growing, that is, it signals that the body is becoming more fragile and is risking more and more every year. This implies the simplest – and one of the most commonly used in science now – definition: aging is a growing risk of dying.
In any population of people, the mortality curve will look the same, except that it can move left or right depending on living conditions or smooth out a little towards the end. But animals have different options. The graphs below are based on long observations of various creatures. A thin blue line indicates survival (as a percentage of the total population). The red curve is the relative risk of dying (one corresponds to the average risk for an adult). Finally, the thick blue line is the relative ability to reproduce (the average number of offspring produced by an adult of a given species is taken as a unit).
The graphs begin at puberty (i.e. childhood is not taken into account) and end at the age when only 5% of the original population remains alive.
In many animals, the shape of the curves is generally similar to that of humans. The only fundamental difference between our graphs and those of a lion or a chimpanzee is that mortality does not increase 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 as for Leo: the ability to reproduce decreases with time, and the survival curve, curved outward (that is, upward), literally collapses down after a certain age.
But sometimes it's exactly the opposite. For example, in a red-legged frog (the dotted line indicates a lack of data for analysis) or a 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 in nature for an example of victory over the inevitable, then this should be exactly it – not a movement towards death, but an escape from it.
However, do not be surprised ahead of time. There is also such a period in a person's life, he just did not get on the charts of these authors, because he is earlier than the period of time they are considering. Even in the most civilized society of people, infant mortality is higher than that of children, and up to some age it is even higher than that of adults. 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, are moving from death, which means we are getting younger before our eyes. However, this does not mean that people are ready to live forever – just like desert turtles. Although their risk of dying does not increase with age, as in humans, but at every moment of time some individual will die, of course. Therefore, eternal life for some of them is possible only in a hypothetical population of infinite size.
Neglect, getting old
Since negative aging is actually synonymous with childhood, then where to look for truly non-aging animals? The graph of their mortality should be perfectly straight, like a string, not deviating either inwards (into childhood) or outwards (into old age). This is how, for example, the graphs for some species of hydra and the sea ear mollusk look like. They are called negligibly aging. This term represents a 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" appeared long before the construction of these curves. It was proposed by gerontologist Caleb Finch in 1990. He also put forward his own criteria for assigning this honorary title to an animal: 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. According to data to date, only six animals meet these stringent requirements:
- tailed amphibian Proteus European (Proteus anguinus, maximum lifespan 102 years),
- American swamp turtle (Emydoidea blandingii, 77 years old),
- box turtle (Terrapene carolina, 138 years old),
- Aleutian perch (Sebastes aleutianus, 205 years old),
- sea urchin (Strongylocentrotus franciscanus, 200 years old),
- and a bivalve mollusk (Arctica islandica, 507 years old).
Please note that not all the record holders for life expectancy are included in this list. There is no hydra in it, and there is no sea ear clam. Perhaps the fact is that not enough data has been accumulated about all animals in order to check all the criteria. The classic hydra observation experiment, for example, lasted only four years. During this time, it was possible to show that hydra does not age, but what happens to it next is unknown. There are no mammals on this list either. Even a naked digger– an animal that is often called negligibly aging – turned out to be unworthy of this title. Finch himself, reviewing his criteria decades later, admitted that the digger did not meet them. The reason for this was the individual observations of gerontologists, according to which the cubs of "elderly" diggers are less viable than those of "young" – and Finch considered this a sign of a decrease in the reproductive abilities of the animal.
There is a crisis of example to follow: the most long-lived species are too unlike us. The record holders closer to us do not pass the criterion of negligible aging. Then who should we focus on and whose path should we follow? Statistics come to the rescue here. In the world of people, it is useless to listen to the advice of each individual, 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 managed to cope with aging. So: elephant, whale, proteus, turtle, shark, parrot, digger, perch – what unites them?
Harsh patterns
The first thing that matters for a long life is size. Most centenarians are larger than their relatives. This helps them to slip out from under the pressure of natural selection: the elephant is less threatened by predators 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 the others, their long age is only a natural consequence of their impressive size. It is more interesting in this sense to look at those who did not come out either in length or height, but still managed to survive the others. Among mammals, this is, for example, the notorious naked digger, as well as tree squirrels and bats. Each of them has found his own way to escape from predators: burrow underground, climb a tree or even take to the air and live in the dark.
The second important advantage that size gives is protection from cancer (not so much from the risks of its occurrence, as reducing the threat from each individual tumor). Imagine that you rule a huge state with millions of citizens. If there is an uprising 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 the animal's body. If a small tumor has appeared 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 entire body.
Therefore, strategies to fight cancer, as well as with predators, in animals depend on size. Very small animals, like mice, having no way to escape from an external enemy, capitulate to the internal one. Small, but long-lived animals, like a naked digger, acquire early protection mechanisms. Their cells do not get a chance to even begin to multiply 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 fighting mechanisms, for example, the enhanced launch of programmed cell death, do not work immediately, and are designed for those tumors that did not die by themselves at the early stages of their development.
At the same time, if you forbid your cells to multiply, then how to cope with damage in the body? This dilemma probably explains why there are so few vertebrates among the long-lived champions: they have acquired too many organs that are extremely difficult to repair without giving the cells additional powers. Bones are much worse updated than skin, muscles regenerate worse than fat, and brain tissue is almost impossible to restore at all. One of the most popular theories of aging is based on this contradiction – the theory of "disposable soma" (disposable soma), which is easier to translate as the theory of "body on release". From the point of view of reproduction of the organism, only the germ cells are important. The rest of the soma body is just a superstructure over them. And the more attention it requires, the more effort it takes to update it, the fewer resources the germ cells get. Therefore, vertebrates with their structures that cannot be restored live less than invertebrates: their body eventually ceases to have enough energy for repair, and it is sent "to be thrown out". And advanced regeneration abilities can only boast of a shark and tailed amphibians (to which proteus belongs).
Finally, looking at the list of centenarians, you can also find a climatic pattern: most of them live in the cold. This is true primarily for cold-blooded animals (clam 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 trained to constantly warm themselves up, still tend to find a colder place. As an example, we can recall the bowhead whale. Or the same naked digger who almost became cold-blooded back by burrowing 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 a warm climate brings with it many hardships. The higher the temperature, the faster the chemical reactions in the animal's body go, the more metabolic byproducts 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. Interestingly, cold-blooded centenarians, who can only warm up in the rays of the sun, also tend to hide away from it. They have another reason to prefer cold to heat – and this is a long childhood.
As we remember, childhood corresponds to a period of negative aging. Therefore, the longer the body drags on entering adulthood, the more time passes before its mortality begins to grow. Living in cold conditions is a great way to slow down development for a cold–blooded animal. Warm-blooded ones can, again, take advantage of their size: an elephant needs much more time to grow up than a rabbit. There is a third way to stretch childhood – slowing down development. Its most radical type is neoteny, reproduction in the larval state. So, for example, does proteus, like other tailed amphibians. Apparently, a similar fate befell the naked digger: although he does not spend his life as a larva, but his development is slowed down – throughout his life he resembles the embryo of a mouse or rat and does not grow to the appearance of a "real adult" rodent. These clever moves allow the animals to bypass the dilemma of "body to discard". Germ cells begin to absorb energy only with the onset of puberty, and the "eternal child" can afford to direct all his efforts 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 tricky. Predators are not interested in them, it rarely gets cancer and has its own defense mechanisms against it – it hits the enemy from afar or waits for him "in ambush". It regenerates well and tends to live in the cold, regardless of the base temperature of its body. Finally, he prolongs his 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-breaking animals. The naked digger is not capable of regeneration, sharks have no special mechanisms of protection against cancer, and bats live with a surprisingly high body temperature. This only means that in each case, a long life arose by itself, and there is no general recipe. Each winner went his own way, compensating for innate shortcomings with new acquisitions.
But a person fits well into the image of a long-lived animal. We are rather small compared to champion mammals, rarely suffer from predators, live better in the cold than in the heat, and develop more slowly than our primate ancestors. As for cancer protection and regeneration, we have long discovered these shortcomings and are working to improve them. And when we finalize it, it is still unknown who will have to learn longevity from whom.
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