Oct 7 : The world's oldest-known terrestrial animal, an Aldabra giant tortoise named Jonathan and estimated to be about 194 years old, may owe his extraordinary longevity to a combination of genetic traits and an ability to keep key cellular systems functioning in a more youthful way, according to scientists.

Members of this species are known to live more than 100 years, but Jonathan has far exceeded that. Brought to the remote South Atlantic island of St Helena as a fully grown animal in 1882, likely from the Aldabra Atoll or the Granitic Seychelles, the tortoise now resides in a paddock at Plantation House, the governor's residence.

"Other Aldabra tortoises have been claimed to have lived as long as Jonathan, but he has the most documentary evidence supporting his age," said Benjamin Vaisvil, a researcher affiliated with the organization Kallel and co-lead author of the study published on Wednesday in the journal Science Advances.

Scientists have studied Jonathan to explore the secrets of the animal's amazing longevity, sequencing his genome and analysing his epigenome — the chemical markers that control which genes switch on and off.

"We found changes in Jonathan's genome in pathways associated with longevity, like DNA repair, mitochondrial function, insulin regulation and telomere function," Vaisvil said.

Mitochondria are structures considered the powerhouse of a cell. Insulin is a hormone that helps regulate levels of blood sugar by letting it enter the body's cells for energy. Telomeres are protective caps at the ends of chromosomes, structures that carry genetic information inside cell nuclei.

Researchers compared Jonathan's epigenome with four other Aldabra tortoises, ranging from juveniles to older adults. While pretending to feed Jonathan, they waited until he began biting the air, then wedged his mouth open with a gloved hand and scraped under his tongue with a specialized kit. St Helena officials restrict blood draws to prevent infection risks.

To explain the finding, Vaisvil compared the genome to a cookbook and the epigenome to handwritten notes in the margins.

"As the cookbook ages, those notes become more difficult to read clearly. We can think of that as methylation entropy," Vaisvil said, referring to a metric for disorder in a certain biochemical process.

Most of the notes in Jonathan's "cookbook" look very old. But in certain regions, stretches of DNA that act as switches controlling whether genes are turned on or off, Jonathan's methylation entropy was lower than that of every tortoise studied except a 5-year-old juvenile. These regions controlled genes that influence longevity.

The researchers identified 287 genes with variants unique to Jonathan, including some linked to tumour suppression and DNA repair. The tortoise also carries extra copies of a gene that encodes a protein that hunts down ageing and cancerous cells. 'GENTLE GIANT' When younger, Jonathan was known for mischievous antics including overturning chairs and lying on croquet balls, according to the tortoise's longtime veterinarian and study co-author Joe Hollins. Cataracts have since left him mostly blind.

Hollins describes Jonathan today as "a gentle giant." Determining how long giant tortoises can live is challenging because reliable historical records are scarce and many outlive researchers. Record-keeping before the 20th century also was problematic. While a young tortoise's age can be estimated from growth rings on the plates that form the animal's shell, ascertaining the age of adults is more difficult.

The new study involved only one exceptionally old tortoise, making it hard to determine whether the traits identified are unique to Jonathan or common among long-lived tortoises.

"We cannot say for certain that what we found is the reason for his longevity," Vaisvil said.

Environmental factors also may have contributed. Jonathan lives on a tropical island, with an optimal diet carefully managed by trained veterinarians.

The research findings broadly match patterns documented in studies of exceptionally long-lived people, suggesting that some mechanisms of aging may be shared across species.

Vaisvil noted that a recent study of a 117-year-old woman found unusually efficient mitochondrial function, echoing findings in some of Jonathan's regions.

VORACIOUS APPETITE Despite his age, Jonathan retains a voracious appetite, Hollins said, lifting his head and stretching out his long neck when the veterinarian calls out his name.

"I know he knows me, knows my voice and my handling, and I also know it isn't love on his part but a recognition of the arrival of a feast," Hollins said. "And yet I, of course, don't mind. I have a huge affection for him." "He is a remarkable symbol. Icon, I know, is an overused word, but Guinness World Records has just given him 'icon' status this year," Hollins said. Beloved by island residents, Jonathan has lived through world wars and the rise and fall of empires.

"He is symbolic of resilience, longevity, endurance, a persistent presence throughout huge historic events," Hollins said.