For mammals like humans, aging is inevitable. No matter how many vitamins we take, skin sags, bones soften and joints stiffen over time. However, turtles and tortoises age more gracefully. Despite their wrinkled skin and toothless gums, species like Galápagos giant tortoises seem unscathed by the ravages of aging. Some show few signs of slowing down as they plod into their 100s.
To determine what drives these ageless wonders, two groups of researchers examined turtles, tortoises and their ectothermic, or coldblooded, brethren in a pair of studies published Thursday in the journal Science. Prior aging research has largely revolved around warm-blooded animals like mammals and birds. But ectotherms like fish, reptiles and amphibians dominate the longevity record books. For example, salamanders called olms slither through subterranean 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 compiled data sets on 77 species of wild reptiles and amphibians including Komodo dragons, garter snakes and tree frogs. The team utilized decades of monitoring data to analyze traits like metabolism to determine their impact on aging and longevity.
“We had these awesome data sets to get at questions of aging in a way that hasn’t been done before,” said Beth Reinke, an evolutionary biologist at Northeastern Illinois University and an author of the new study. “Getting at the heart of the issue of how aging evolves can only be done with this broad taxonomic approach.”
Living so long requires a gentle aging curve. After most animals reach sexual maturity, much of their energy is devoted to reproduction at the expense of mending aging tissue. This physical deterioration, or senescence, often causes an uptick in mortality risk as older animals become susceptible to predators or disease. But several coldblooded animals experience little senescence as they age.
One theory is that coldblooded animals are better equipped to manage the wear of aging because they rely on the environment to calibrate their body temperatures instead of the energy-draining metabolisms of endothermic, or warm-blooded animals. But what Dr. Reinke and her colleagues found was more complex. They discovered that some ectotherms aged much much faster than similar-sized endotherms, while others aged much slower. 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 at all were turtles and tortoises.
The other new study drilled deeper into the aging of these timeless turtles. The researchers examined age-related decline in 52 species of captive turtles and tortoises in zoos and aquariums. They found that 75 percent of the species, including Aldabra giant tortoises and pancake tortoises, exhibited low or negligible senescence. A few, like Greek tortoises and black marsh turtles, even displayed negative rates of senescence, meaning their mortality risk decreased as they aged. Around 80 percent had aging rates slower than those of modern humans.
Turtles being the anti-aging standard makes sense, considering their sluggish metabolisms. Researchers have also linked their sturdy shells to longer lives. As herbivorous turtles and tortoises spend their lives munching on veggies (well, mostly), snug suits of armor provide protection to even grizzled geezers.
These lethargic aging rates are unsurprising considering the pampered lives of captive turtles. But unlike humans, who age regardless of the fantasy of cryogenic preservation, captive turtles provide evidence that ideal environments in zoos can slow aging because the reptiles lounge in ideal temperatures and enjoy a balanced diet of fruits and greens.
“We compared the populations in zoos to wild populations and found that the ones under protected conditions were able to switch off senescence,” said Rita da Silva, a population biologist at the University of Southern Denmark and an author of the tortoise study. “For humans, our environment continues to get better and better, but we are still not able to switch off senescence.”
While the mortality risk in long-living turtles and tortoises remained stagnant over the decades, they have not obtained eternal youth according to Caleb Finch, a gerontologist at the University of Southern California who studies aging in humans. Like elderly humans, eventually eyesight and hearts weaken in turtles and tortoises.
“Some of them get cataracts and are feeble to the point where they need to be fed by hand,” said Dr. Finch, who was not involved with the new studies. “They would not survive in the real world, so there is no question that they do age.”
While these lumbering reptiles cannot outpace death, they may hold insights for prolonging longevity and decreasing age-related decline.
“If we continue to study the evolution of aging in turtles, at some point we’ll find a clear connection between turtles and human health and aging.” da Silva said.