When it comes to longevity, numbers are the main argument on the table. Let’s start with them. Who comes to your mind when you hear about long-lived organisms? An elephant, a turtle, a parrot? A whale? None of them are even close to the podium. Sponges took the top spot by a huge margin. The record-breaker 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 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 Arctica islandica, 507 years old. It is followed by the Greenland shark (a newcomer to the list, about 400 years), more mollusks, sea urchin and some fish (including, for example, the Aleutian perch). But all of them haven’t crossed the 200-250 year life span threshold. Most of the winners belong to invertebrate animals – vertebrates are almost no places on this pedestal. And our closest relatives – mammals – are not among them at all. Of those we habitually consider long-lived, only the bowhead whale could compete with this team: according to some data, he managed to last 211 years. There is no naked digger – an icon of modern gerontology – among the champions. It can often be found in articles as an example of an ageless organism, because it is 10 times ahead of the average age of its relatives mice and almost does not change with age, but it lives only about 30 years. However, the places on this pedestal are fickle 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 species of perch that has overtaken the sea urchin, and so on to infinity. Maximum lifespans don’t have definitive values. Every time we talk about someone “living to be 500 years old,” we have to keep adding “according to the latest data,” because the data keeps arriving. But can we measure aging with constantly fluctuating numbers? Besides, in its pure form, this list is hardly useful to us. If we really want to tap into someone else’s secrets of long life, it would be good if that organism were a bit like ours, at least in terms of organ systems.

In humans, the main statistical indicator of aging is the Gompertz-Makeham curve, which reflects the dependence of the risk of dying of natural causes on the age of a person, or, simply put, the inevitable approach of death. The curve on the graph is continuously increasing, that is, it signals that the body is becoming more and more fragile and riskier with each passing year. The simplest – and one of the most used definitions in science today – follows from this: aging is an increasing risk of dying. In any population of humans, the mortality curve will look the same, except that it may move left to right depending on living conditions or flatten out a bit toward the end. But in animals, different variations are possible. The graphs below are based on long observations of different creatures. The thin blue line represents survival rates (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 start at sexual maturity (i.e., childhood is not considered) and end at the age when only 5% of the original population remains alive. In many animals, the shape of the curves is broadly similar to that of humans. The only fundamental difference between our graphs and those of a lion or a chimpanzee is that mortality increases not smoothly, but sharply and from a certain age. Probably, the point is that the average risk of dying in our population is low, and we tend to take care of the elderly until we can no longer help them, at which point the curve rises. Nevertheless, the trends are the same for us as for the lion: the ability to reproduce declines over time, and the survival curve, curved outward (i.e. upward), literally collapses downward after a certain age. But sometimes the opposite is true. For example, in the red-legged frog (the dashed line indicates a lack of data to analyze) or the desert tortoise, the survival line is concave at some point. What this actually means is that at a certain age, individuals of that species are at less and less risk of dying. This phenomenon has been called negative aging. And if we look for an example of victory over the inevitable in nature, it should be just that – not a movement towards death, but an escape from it. However, we should not be surprised before the time. There is also such a period in human life, it is just that it did not get on the charts of these authors, because it is earlier than the time segment they consider. Even in the most civilized society of people, infant mortality is higher than child mortality, and up to some age – even higher than adult mortality. So up to a certain age (up to about age 9), our survival curve is also concave, and we too – according to the statistical definition of aging – are moving away from death, and thus getting younger before our eyes. Nevertheless, this does not mean that humans are prepared to live forever – just like desert tortoises. Although their risk of dying does not increase with age as it does in humans, but at any given time some individual will die, of course. So eternal life for some of them is only possible in a hypothetical population of infinite size.

If negative aging is in fact synonymous with childhood, then where to look for truly ageless animals? Their mortality graph should be perfectly straight, like a string, deviating neither inward (into childhood) nor outward (into old age). This is what the graphs for some species of hydra and the abalone clam look like, for example. They are called negligibly senescent. This term represents some 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 that allow to award 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. To date, only six animals have met these stringent requirements: Amphibian tail amphibian Proteus europeanus (Proteus anguinus, maximum lifespan 102 years), American bog turtle (Emydoidea blandingii, 77 years), box turtle (Terrapene carolina, 138 years), Aleutian perch (Sebastes aleutianus, 205 years), sea urchin (Strongylocentrotus franciscanus, 200 years), and bivalve (Arctica islandica, 507 years). European Proteus (Proteus anguinus) Tatiana Dyuvbanova / Shutterstock Note that this list does not include all the record holders for longevity. There is no hydra or abalone mollusk in it. Perhaps the reason is that not all animals were not able to accumulate enough data to check all the criteria. The classic hydra experiment, for example, lasted only four years. During this time, it was possible to show that the hydra does not age, but what happens to it next – it is unknown. Mammals are not on the list. Even the naked digger, an animal often described as negligibly aging, was unworthy of the title. Finch himself, reviewing his criteria decades later, recognized that the mole did not meet them. The reason for this was the separate observations of gerontologists, according to which the cubs of “old” diggers are less viable than in the “young” – and this Finch considered a sign of decline in reproductive capacity of the animal. There is a role model crisis: the longest-lived species are too unlike us. Closer record-breakers do not meet the criterion of negligible aging. Who should we look up to and whose path should we follow? This is where statistics comes to the rescue. In the world of people it is useless to listen to the advice of each individual – you need to study long-livers in general. In the world of animals, too, it is impossible to find an ideal, so you need to look at all your successful relatives from afar and try to make some kind of a collective image of an animal that managed to cope with aging. So, elephant, whale, protheus, turtle, shark, parrot, digger, perch – what do they have in common?

The first thing that matters for long life is size. Most long-livers are larger than their relatives. This helps them slip out from under the pressure of natural selection: an elephant is less threatened by predators than a shrew, which means that long-lived elephants have a good 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 life span is just a natural consequence of their impressive size. It is more interesting in this sense to look at those who did not come out neither length nor height, but still managed to outlive others. Among mammals, this includes the notorious naked mole, as well as tree squirrels and bats. Each of them has found its own way to escape predators: burrow underground, climb a tree, or rise into the air and live in the dark. The second major benefit that size offers is protection against cancer (not so much the risks from cancer, but reducing the threat from each individual tumor). Imagine you rule a huge state with millions of citizens. If there is an uprising in one of a thousand cities, the life of the country will hardly be affected, unless that city is the capital. But if you are a prince of tiny Liechtenstein and there is a revolution in one of your half a dozen towns, you are in serious trouble. In the animal body, unfortunately, the same simple arithmetic works. If there is a small tumor, say 3 grams, some capybara (55 kg) may not notice it at all, while for a mouse (30 g) – a tenth of the body. Therefore, the strategies for fighting cancer, as well as predators, in animals depend on their size. Very small animals like mice, having no way to escape an external enemy, capitulate to an internal one. Small but long-lived animals, like the naked digger, 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, long-lived animals like elephants and turtles, on the other hand, rely on late defense against cancer. Their coping mechanisms, such as enhanced triggering of programmed cell death, are not triggered immediately and are designed to target tumors that did not kill themselves early in their development. Naked mole (Heterocephalus glaber) Photo: Neil Bromhall / Shutterstock At the same time, if you forbid your cells from multiplying, how do you deal with the damage in your body? This dilemma probably explains why there are so few vertebrates among the long-lived champions: they have built themselves too many organs that are extremely difficult to repair without giving cells additional powers. Bones are much worse at renewing themselves than skin, muscle regenerates worse than fat, and brain tissue is almost impossible to repair at all. This contradiction is the basis of one of the popular theories of aging – the theory of “disposable soma” (disposable soma), which can be more easily translated as the theory of “body disposal”. From the point of view of reproduction of the organism, only sex cells are important. The rest of the body – soma – is just a superstructure over them. And the more it requires attention, the more energy is spent on its renewal, the less resources go to the sex cells. That is why vertebrate animals with their non-repairable structures live less than invertebrates: their body eventually ceases to have enough energy for repair and it is sent to “throw away”. And advanced abilities to regenerate can boast only a shark and tail amphibians (which includes Proteus). Finally, looking at the list of long-lived, you can find a climatic pattern: most of them live in the cold. This is true primarily for cold-blooded animals (the mollusk Arctica islandica, the protheus, the Aleutian perch and the Greenland shark), which are unable to regulate their body temperature from within. But even warm-blooded vertebrates, seemingly specially trained to constantly warm themselves, still tend to find a colder place. The bowhead whale is a case in point. Or the same naked shrew, which almost became cold-blooded back by burrowing deep underground. Now its constant body temperature is about 33 degrees, which is much lower than its rodent relatives. Greenland polar shark, or small-headed polar shark (Somniosus microcephalus) Photo: Dotted Yeti / Shutterstock The fact is that a warm climate brings with it many adversities. The higher the temperature, the faster the chemical reactions in the animal’s body, the more metabolic byproducts are formed and the faster the body wears out. Therefore, from the point of view of long life, being warm-blooded is not so advantageous. It is interesting that cold-blooded long-livers, 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 to warm, and that is a long childhood. As we remember, childhood corresponds to a period of negative aging. Therefore, the longer an organism delays entering maturity, the more time passes before its mortality rate begins to rise. Living in cold conditions is a great way to slow development for a cold-blooded animal. Warm-blooded ones can again take advantage of their size: an elephant takes a lot longer to grow up than a rabbit. There is a third way to stretch childhood – slowing down development. Its most radical form is neoteny, reproduction in the larval state. This is what Proteus does, for example, like other tailed amphibians. Apparently, a similar fate befell the naked mole: although it does not spend its life in the form of a larva, but its development is slowed down – throughout its life it resembles the embryo of a mouse or rat and does not grow to the form of a “real adult” rodent. These clever moves allow the animal to get around the “body to discard” dilemma. Sexual cells begin to absorb energy only at puberty, and the “eternal child” can afford to devote all its energy only to maintaining its own health. *** So, let’s now compose in our minds a typical long-lived animal. It’s either quite large or quite small, but it’s very cunning. It is not interested in predators, 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 its basal body temperature. Finally, it prolongs its childhood by remaining a perpetual larva or simply slowing its development, and takes its time to reproduce, conserving resources. Our collected portrait does not describe any of the actual animals of record. The naked shrew is incapable of regeneration, sharks have no special defense mechanisms against cancer, and bats live with surprisingly high body temperatures. This only shows that in each case long life arose on its own, and there is no general recipe. Each winner went its own way, compensating for innate shortcomings with new acquisitions. But the image of a long-lived animal fits well with humans. 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 defense and regeneration, we discovered these weaknesses long ago and are working to improve them. And when we do, it remains to be seen who will have to learn longevity from whom. Source: Chrdk. Photo: yandex.ru