Death comes for all, but aging doesn’t — at least for some animal species
Two new studies published Thursday (June 23) in the journal Science found that turtles and tortoises have remarkably slow rates of aging. In captivity, without the stress of finding food and avoiding predators, some may not age at all.
"That is super fascinating," said one study's lead author Rita de Silva, who conducted the research while at the University of Southern Denmark and who is now a biologist at the Universidade do Porto in Portugal. What makes it even more interesting is that modern humans have yet to unlock this benefit, even though we inhabit a world with fewer challenges to our day-to-day survival than the habitats of our early ancestors.
"As modern humans, we tend to live in really good conditions, so the environment for us would be close to ideal [as well]," de Silva told Live Science. "And still, we cannot lower our aging rate."
How the turtles avoid aging is a bit of a mystery, but the secret may lie in their shells.
Aging and death
Understanding this turtles' anti-aging superpower first requires a quick tutorial on the mind-bending notion that while death is inevitable, aging might not be. There are a couple of ways to think about age. One is longevity, or the maximum lifespan of a species. Scientists often define longevity as the age at which a large proportion, say 95%, of adults in a population are dead. For humans, that pegs longevity at around 100 years.
The other way to think about aging is senescence. Senescence is the weakening of an organism as it ages. It’s easy to see this in humans; with age, immune systems falter, bones become brittle, energy flags. What’s more, death becomes statistically more likely with each year of age. For example, according to Social Security Administration actuarial tables, a 50-year-old man in the United States has a 0.48% chance of dying within the next year. An 80-year-old man has a 5.6% chance of death within a year. For a centenarian man, there’s a nearly 35% chance that he won’t ring in the next year.
Related: Why do turtles live so long?
The new research reveals that this accumulation of risk is much, much slower in turtles, and in some cases may not exist at all. In other words, in some species, age doesn’t increase the risk of death. Eventually, all turtles will die, because if there’s even a 1% chance of death each year, it’s inevitable that the age-defying reptile’s number will eventually come up. But that chance of death may be the same whether the turtle is 5 or 25 years old — or in the cases of some long-lived species, perhaps even 125 years old.
Scientists have long noted that turtles and tortoises can live extremely long lives. This year, for instance, a Seychelles giant tortoise (Aldabrachelys gigantea hololissa) named Jonathan turned 190, making him the oldest tortoise ever and the oldest recorded land animal. Research on turtle biology suggests that turtles and tortoises are able to quickly kill off damaged cells and that they are resistant to DNA damage that accumulates over time as cells divide; this protects individuals even as they enter extreme old age, Live Science previously reported.
The evolution of aging
The big question, evolutionarily speaking, is how turtles accumulated these incredible anti-aging powers — and why the rest of us are stuck growing old. In the two new studies, researchers investigated the question in both wild and captive turtle and tortoise populations.
The study led by de Silva looked at 52 species in captivity whose records were available in the Species360 Zoological Information Management System, software used by zoos to track data about animal husbandry. They found that about 75% of species showed zero or negligible aging rates. In some cases, there was a wide range of uncertainty around the rate of a species’ aging, but in others, the numbers were consistent around zero, meaning that these species probably age very slowly or not at all. Some of these consistent negligible agers included the Greek tortoise (Testudo graeca) and the black marsh turtle (Siebenrockiella crassicollis). The Aldabra giant tortoise (Aldabrachelys gigantea) showed a negligible aging rate and a particularly long average lifespan of 60 years or more in captivity. The Galápagos tortoise (Chelonoidis niger), one of the species studied by scientist Charles Darwin on his voyage to the islands of the same name in 1835, also lived 60 years or more on average.
The second study, conducted by a different research group, looked at aging in wild populations. One hypothesis for why turtles age slowly has long been that because they are cold-blooded, they don’t have to spend energy on maintaining their body temperature, perhaps allowing them to direct energy toward cellular repair. Northeastern Illinois University biologist and lead author Beth Reinke, co-author and Pennsylvania State wildlife population ecologist David Miller, and colleagues wanted to test this idea by comparing cold-blooded animals’ aging rates to warm-blooded animals’ aging rates, controlling for factors such as body size.
To do this, they had to pull together data from multiple scientists across the world who mark or tag animals within a wild population and then go back year after year to see if they can recapture those animals. These long-term field studies are one of the few ways to learn about animal longevity and demographics in the wild.
"I'm just so in awe that we were able to get so many researchers willing to contribute their data that they've sweated hours for in the field," Reinke told Live Science.
To their surprise, the researchers found that cold-blooded animals didn’t age slower than warm-blooded animals did; instead, the cold-blooded creatures displayed a much broader range of aging's effects, with some aging more quickly than similarly sized warm-blooded animals, and some aging more slowly. At least one species in each of four groups (frogs and toads, crocodilians, squamate lizards, turtles) showed negligible rates of aging. However, as in de Silva and colleague’s study, the turtles stood out.
"What we found is there are some really consistent patterns in turtles, which is that they live a long time, and they age really slowly," Miller told Live Science.
Because cold-bloodedness couldn’t explain this slow aging, the researchers tested some other possible factors that might explain why some species age fast and others age slowly. They looked at the average local temperatures in each species’ range, but found varying patterns: Hotter climates increased the rate of aging in reptiles, but decreased it in amphibians. They also found that longevity was linked to later sexual maturity, indicating a slower life pace for long-lived cold-blooded creatures.
How to die without aging
But one of the most interesting findings was that the slowest-aging cold-blooded creatures were also those that had the most robust defenses to protect them from predators. In particular, physical protection like shells was associated with low aging rates.
Shells keep turtles from being eaten, meaning that their mortality rates from outside sources are lower than animals without such protection. (Imagine the likelihood of a young box turtle surviving an attack from a fox, compared with the chances of a young bunny.) This low mortality rate across all ages means turtles are likely to survive long enough to take advantage of their cellular protections against aging, Miller said.
"If a lot of animals get eaten or die by disease. not many survive long enough for there to be any benefit from the sort of cellular processes that slow aging down," Miller said. Protective features may allow animals to live long enough for evolution to act on anti-aging protections, in other words.
There are potential evolutionary parallels with humans, Miller said, many of whom today live in cushy conditions with easy access to food and shelter – not unlike captive turtles. Turtles and tortoises may seem enviable in their slow aging, but humans are actually not slouches in the longevity department, Miller said. Humans age quicker than the average turtle, but a lot slower than many other species.
The biology of turtles and tortoises could help unlock anti-senescence secrets for humans, but a lot more research is required to get there, Reinke said. More work is needed to understand the evolution of aging in other animals, too. For example, there isn’t much data on extremely long-lived species, especially looking at whether or not aging rates accelerate at some point in animals that live very long lives. Jonathan the tortoise, for example, is blind, can’t smell, and must be hand-fed, University of Alabama, Birmingham biologist Steven Austad and University of Southern California aging researcher Caleb Finch wrote in an editorial accompanying the two studies.
"Even if many of these fascinating species lack significantly increasing mortality with age," Austad and Finch wrote, "some clearly incur infirmities of aging."
Originally published on Live Science