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A special turtle species has found a brilliant way to reduce aging

Does this mean they are immortal?

 A special turtle species has found a brilliant way to reduce aging
A Sternotherus odoratus (Common musk turtle) with a healthy cap of algae in Spring Lake, Texas. Francis L. Rose

Jonathan looks 70 but is actually closer to 190.

The Seychelles giant tortoise is estimated to have been born in 1832, which means that he turned, or will turn, 190 years old sometime in 2022, according to the Guinness World Records. For a clearer picture - Jonathan was born before Queen Victoria ascended the British throne in 1837. He lives on the volcanic South Atlantic island of St. Helena, a British Overseas Territory. 

Jonathan is apparently blind and has lost his sense of smell. But he continues to engage in his favorite pastimes: sun-bathing, sleeping, eating, and mating, as though he were still in his prime. He's definitely a proponent of the words of Crush the sea turtle in Disney's Finding Nemo - "Hundred and fifty, dude, and still young. Rock on!"

Unbelievable. This contrasts with humans who begin to show signs of aging in their 30s.

Thanks a ton, senescence.

Senescence is the gradual process of deterioration with age and the loss of the cells' powers to divide and grow. For a long time, there were hypotheses about the existence of slow or even negligible senescence among turtles and tortoises. Now, in research published Thursday in Science, researchers across two studies have investigated aging in cold-blooded tetrapods. They reveal surprisingly little evidence of senescence, or physical aging, in several turtle species.

Change is not inevitable for all of us

This means that senescence may not be inevitable for all organisms, according to biologist Rita da Silva, the lead researcher behind one of the analysis, who was at the University of Southern Denmark when the work was done.

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Even though evidence has suggested that evolution could be kind enough to permit some species to reduce or at least partially avoid the effects of senescence.

Species that continue growing after reaching reproductive maturities, such as turtles and tortoises, are the primary candidates for escaping senescence.

"I think we, as humans, have a deep obsession with aging, [and] the fact that aging among other species does not compare to ours is extremely interesting," Fernando Colchero, associate professor at the University of Southern Denmark, one of the researchers on da Silva's study, tells IE in an interview.

"[The coolest part was] confirming long-standing hypothesis on the existence of slow or even negligible senescence among turtles and tortoises while opening the door to reconsidering the evolutionary theories of senescence to explain patterns in species that are not closely related to humans," he said. 

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Chart
Average rate of aging for two species of turtles, one tortoise and humans, for comparison. Turtles and tortoises icons in red represent females and blue ones represent males. Source: Based on Da Silva et al. 2022; Icons credits: T. kleinmanni- Andrew A. Farke, shell lines added by Yan Wong, A. gigantea - Roberto Díaz Sibaja

Turns out a relatively slow pace of life can help

Together, the two studies investigated the impacts and patterns of aging in these closely related species that differ significantly in their aging rates, despite some fundamental similarities. "By investigating the nature of [this] variation, something new may be learned about aging in humans,” write Steven Austad, the University of Alabama at Birmingham, and Caleb Finch, University of Southern California, in the related Perspective section of the journal.

In one analysis, Beth Rinke from the Northeastern Illinois University/Pennsylvania State University and colleagues provided a comparative study of aging rates and lifespan across wild, cold-blooded tetrapods.

They utilized data from long-term field studies of 77 species from 107 wild populations, including turtles, amphibians, snakes, crocodilians, and tortoises. They evaluated how factors such as thermoregulatory mode, environmental temperature, protective adaptations, and pace of life contribute to physical aging.

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The researchers found greater diversity in aging rates in the groups studied compared to birds and mammals. Ectotherm longevity (estimated as the number of years between first reproduction and 95 percent of adults having died) ranged from 1 to 137 years. For comparison, primate longevity ranges from 4 to 84 years.

Little evidence of aging was found in multiple chelonian species, some salamanders, and the tuatara. In the case of turtles, characteristics like bony shells and a relatively slow pace of life helped explain the negligible aging.

Tuatara
A side view of a tuatara (Sphenodon punctatus). Source: Sarah Lamar

The one where they never grow old

In another analysis, da Silva and colleagues examined the change in mortality rate with age in captive animals, focusing on 52 turtle, terrapin, and tortoise species in zoo and aquarium populations. 

She found that among 52 species, 75 percent of the species showed extremely slow senescence, while 80 percent of them have slower senescence than modern humans.

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Their study found that the aging pattern did not resemble that in humans or other animals. Most of the species age slowly, and in some cases, their senescence appears negligible.

This is particularly interesting because the researchers found that some turtle and tortoise species may reduce physical aging in response to more ideal environmental conditions. As the 'facilities' around them improve, they can allocate more energy to cellular repair rather than protecting themselves or searching for food, thereby extending their lifespans. 

No such luck with human beings

According to an article in the April 2019 Journals of Gerontology entitled, Inconvenient Truths About Human Longevitythe rise in human longevity is one of humanity's "crowning achievements". Improved health care and hygiene, better medical care, and reduced child mortality have all contributed to this, and we can now expect to live much longer than our ancestors could just a few generations ago.

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Yet, research on humans and non-human primates have revealed that improved living conditions do not modify the rate of aging to a significant degree. In other words, more people reach an older age, but the maximum lifespan possible has not improved.

Among primate species, infants and juveniles are most susceptible to environmental changes and age-independent causes of death such as predation or extreme conditions.

"The difference is that turtles have much lower aging rates than humans, while they can significantly reduce them when environmental conditions improve. We haven’t yet found any evidence that humans, or any other primate for that matter, can do that," said Colchero.

Now, according to some evolutionary theories, senescence appears after sexual maturity, more like a tradeoff between the energy an individual invests in repairing damages in its cells and tissues and the energy invested in reproduction. Among other things, this particular tradeoff implies that individuals stop growing and start experiencing aging after reaching sexual maturity, as per the paper.

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This prediction has been confirmed for species such as mammals and birds.

But, organisms that continue to grow even after sexual maturities, such as turtles and tortoises, are believed to have the potential to keep investing in repairing cellular damages. Therefore, they are considered ideal candidates for reducing and avoiding old age.

But that doesn't automatically mean that their risk of death doesn't increase with age. All of them will eventually die due to unavoidable causes of mortality such as illness, said Colchero.

"Our results are sufficiently reliable"

In their paper, the researchers stress that current evolutionary theories of senescence have primarily been tested on birds and animals. "To fully understand how senescence molds vital rates and how environmental conditions affect senescence, further studies comparing populations under human care and natural environments are needed, particularly for underresearched tetrapods," it states.

Next, the researchers will continue to explore aging in other reptile species, "while we would want to explore the evolutionary mechanisms of our findings," said Colchero.

Though their data goes back to the early to mid-1900s, Colchero said that it would have been "great to have data that goes back even further in time for the most long-lived species. Still, despite these limitations, our results are sufficiently reliable to open further research avenues," added Colchero.

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