Since 2009, researchers from the National Museum of Natural Sciences (MNCN-CSIC) have been visiting two ponds in the Sierra de Guadarrama (central Spain).
They capture, mark and release all the amphibians (toads, frogs or newts) they can catch.
The monitoring of thousands of specimens for more than a decade has made it possible to estimate how long these animals live and how they age in the wild.
Their life expectancy is greater than previously believed and the senescence process in some species is extraordinarily slow.
These observations seem to be repeated in most of the so-called cold-blooded animals, according to two studies published now in the journal
Science
with everything that is known about their longevity.
Evolutionary theories of aging predict that all living organisms weaken and deteriorate with age in a process called senescence that ends in death.
In mammals, the most studied animal class, development and fullness is reached with the entry into reproductive age.
Once the essential mission of preserving the species has been fulfilled, these animals, including humans, begin a weakening process that translates into an increased rate of aging.
Until now, this pattern was thought to be repeated in other classes of the animal kingdom, such as reptiles and amphibians.
But not.
In the ponds of Guadarrama there is an appreciable population of gallipatos.
These are newts (
Pleurodeles waltl
), the largest amphibians in Europe, endemic to the Iberian Peninsula and North Africa.
MNCN researcher Íñigo Martínez-Solano, co-author of one of the Science studies, says that several Gallipatos were already tagged in 2009 as adults "and this spring we have detected them again, which could mean that they live for more than 15 years" .
Specimens that reached 20 years had been observed in captivity, but another thing is the natural environment, where life expectancy is always much lower.
Tuatara and not turtles are the longest living reptiles.
They live on average 137 years.
Despite their appearance, they are not iguanas.Sarah Lamar
Knowing the real life expectancy is one of the main contributions of this first work, in which more than 100 researchers have participated who, like Martínez-Solano, have spent years and even decades studying reptile and amphibian species.
They are ectothermic animals (also fish), that is, they do not generate internal heat, hence they are called cold-blooded.
Many biologists maintained that ectothermy forced a low metabolic rate (the direction of this connection may be reversed).
On the other hand, the thermoregulatory hypothesis maintained that for equal size, these animals live longer than mammals or birds and have much lower aging rates.
But the reality is much more complex.
Indeed, 26 of the 30 longest-living species are ectotherms.
And yes, 80% of the reptiles and amphibians studied have lower aging rates than humans.
However, from the study of 107 populations in the wild out of 77, it can be deduced that both these ratios and the life expectancy of ectotherms are highly variable.
There are species well above and also below known rates for endotherms (mammals, birds, and the occasional fish) of similar size.
This suggests that the way an animal regulates its temperature (cold-blooded versus warm-blooded) is not necessarily determinant of its rate of aging or its longevity.
It is specified by David Miller, an ecologist at Pennsylvania State University (United States) and senior author of the study:
“We found no support for the idea that a lower metabolic rate means that ectotherms age more slowly.
That relationship is only true for turtles, which suggests that they are unique among ectotherms."
The popular belief that turtles live a long time is confirmed by this study.
In fact, several species, such as those of the Galapagos, live well beyond 100 years.
However, according to a second investigation focused on these reptiles, the average age of the 52 species of turtles studied is 39 years, far from the 137 years lived by tuatara, endemic New Zealand reptiles similar to iguanas.
But what is really special about turtles, whether they are terrestrial or marine, is their very low aging rate, even with negative rates if conditions allow it.
In mammals, once sexual maturity has been reached, the risk of mortality increases with age, but this does not happen with many reptiles and amphibians and with almost all turtles.
By studying the data housed in Species360, a database with information from zoos and aquariums around the world, the authors of the second study found that turtles are the reptiles with the lowest aging rate, almost close to zero.
That is, the probability of dying remains stable.
Like all animals, turtles grow, devoting a lot of energy to development.
However, when they reach reproductive age, they continue to grow while others stop.
Fernando Colchero, researcher at the University of Southern Denmark
Fernando Colchero, a researcher at the University of Southern Denmark, is co-author of this second work focused on turtles.
“The latest theories suggest that aging is the result of a balance of where to put energy when reaching sexual maturity, in survival or in reproduction.
There are animals that deposit almost all of it during reproduction, with many progeny, but they die very quickly.
Others are extremely long-lived, putting less energy into reproduction at any given time, but spreading it out over time, like humans or elephants.
But the turtles don't fit in,” Colchero points out.
“Like all animals, they grow, devoting a lot of energy to development.
However, when they reach reproductive age, they continue to grow while the others stop.
Turtles are also even capable of reversing senescence.
When comparing the longevity and aging rate of tortoises in zoos with those that live in the wild, they found that the former live longer and have a much lower ratio.
"This means that they have a lot of plasticity to regulate their aging," says Colchero.
And he compares it to the situation with other living beings.
Last year they published a study on aging in primates, including humans.
“When you change the environmental context, what you achieve in primates is to lower infant or juvenile mortality, but it does not clearly reduce the aging rate.
In turtles, yes.
For Colchero, the turtles force us to reformulate the evolutionary theories of aging.
"If we now find that our theories only manage to explain a part, but not all, by reformulating our ideas it is very likely that we can find mechanisms that were previously unknown."
One possible explanation, not exclusive of others, is that their efficiency in repairing damage has to do with the energy they dedicate to their progeny.
Unlike what happens with mammals and birds, cases of parental care between reptiles and amphibians are very rare.
They make up for it with many clutches and for many years, so they achieve the essential goal: that their genes are passed on to the next generation.
But Colchero acknowledges that new studies at the physiological or molecular level are necessary to unravel the mystery.
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