For mammals like humans, aging is inevitable. No matter how many vitamins we take, the skin sags, bones become softer and joints stiffen over time. Turtles and tortoises, however, age more gracefully. Despite their wrinkled skin and toothless gums, species such as Galápagos giant tortoises appear unscathed by the ravages of age. Some show little sign of slowing down as they plod into their hundred.
To determine what drives these timeless wonders, two groups of researchers examined turtles, tortoises and their ectothermic, or cold-blooded brethren in a pair of studies published Thursday in the journal Science. Prior research on aging has largely revolved around warm-blooded animals such as mammals and birds. But ectotherms such as fish, reptiles and amphibians dominate the longevity record books. For example, salamanders called olms glide through underground caves for nearly a century. Giant tortoises can live twice as long — earlier this year, a Seychelles tortoise named Jonathan celebrated his 190th birthday.
In one of the new studies, researchers collected data sets on 77 species of wild reptiles and amphibians, including Komodo dragons, garter snakes and tree frogs. The team used decades of monitoring data to analyze traits such as metabolism to determine their impact on aging and longevity.
“We had these amazing data sets to answer questions about aging in a way that hasn’t been done before,” said Beth Reinke, an evolutionary biologist at Northeastern Illinois University and author of the new study. “The crux of the issue of how aging evolves can only be reached with this broad taxonomic approach.”
Living that long requires a gentle aging curve. After most animals reach sexual maturity, much of their energy is spent on reproduction at the expense of repairing aging tissue. This physical decline, or aging, often causes an increase in mortality risk as older animals become more susceptible to predators or disease. But several cold-blooded animals experience little aging as they age.
One theory is that cold-blooded animals are better equipped to manage the wear and tear of aging because they rely on the environment to calibrate their body temperature rather than the energy-guzzling metabolism of endothermic or warm-blooded animals. But what Dr. Reinke and her colleagues found was more complex. They found that some ectotherms age much faster than endotherms of similar size, while others age much more slowly. The aging rates for lizards and snakes were scattered, but were remarkably low in certain crocodiles, salamanders and the enigmatic tuatara. However, the only group that barely aged were tortoises and tortoises.
The other new study drilled deeper into the aging of these ageless turtles. The researchers examined age-related decline in 52 species of tortoises and tortoises in zoos and aquariums. They found that 75 percent of the species, including Aldabra giant tortoises and pancake tortoises, showed low or negligible aging. A few, such as Greek tortoises and black terrapins, even showed negative aging rates, meaning their mortality risk decreased as they got older. About 80 percent had a slower aging rate than modern humans.
Turtles as an anti-aging standard make sense, given their slow metabolism. Researchers have also linked their sturdy shells to longer life. While herbivorous tortoises and tortoises spend their lives munching on vegetables (well, most of the time), cozy harnesses offer protection to even gray dudes.
This lethargic aging is not surprising given the spoiled life of turtles in captivity. But unlike humans, who age regardless of cryogenic conservation fantasy, captive turtles provide evidence that ideal zoo environments can delay aging because the reptiles lounge at ideal temperatures and enjoy a balanced diet of fruits and vegetables.
“We compared the populations in zoos with wild populations and found that those under protected conditions could disable aging,” said Rita da Silva, a population biologist at the University of Southern Denmark and author of the turtle study. “For humans, our environment is getting better, but we’re still not able to turn off aging.”
Although the mortality risk in long-lived tortoises and tortoises has stagnated in recent decades, according to Caleb Finch, a gerontologist at the University of Southern California who studies aging in humans, they have not achieved eternal youth. Like older people, eyesight and hearts eventually weaken in turtles and tortoises.
“Some of them get cataracts and are so weak that they have to be hand fed,” said Dr. Finch, who was not involved in the new investigations. “They wouldn’t survive in the real world, so there’s no question they’re getting older.”
While these unwieldy reptiles can’t outrun death, they may hold insights for extending lifespan and reducing age-related decline.
“If we continue to study the evolution of aging in turtles, at some point we will find a clear link between turtles and human health and aging,” said Dr. da Silva.