Giant tortoises, which can live for over 100 years in captivity, are among the world's longest-living animals.

Take Lonesome George, a famous Pinta Island tortoise. He was estimated to be between 80-100 years old when he died. Harriet, a Giant Galapagos Land Tortoise, was 176 years old when she passed away in 2006. Adwaita, an Aldabra giant tortoise who died at Alipore Zoological Gardens in Kolkata, was believed to have lived up to 250 years. 

So how com giant tortoises can live for such a long time? 

The new study conducted by researchers at Yale University, the University of Oviedo in Spain, the Galapagos Conservancy, and the Galapagos National Park Service provides hints.

They sequenced the genomes of two giant tortoises of the Galapagos and found that they possessed a number of gene variants linked to DNA repair, immune response, and cancer suppression not possessed by shorter-lived vertebrates.

According to the study published in the journal Nature Ecology & Evolution on December 3, they detected "lineage-specific variants affecting DNA repair genes, inflammatory mediators and genes related to cancer development."

Apart from this, giant tortoises' armored shells and their island habitat are also considered contributing to their longevity. As an island species, it has enjoyed natural protection from predators, and their shells can be a strong defense against predation.

The current International Union for Conservation of Nature (IUCN) conservation status for giant tortoises is "vulnerable" to extinction. 

It is threatened by species introduced to the islands, such as dogs and cats, which prey on young tortoises, and cattle, which compete for grazing vegetation, according to the World Wildlife Fund (WWF).

"Uncovering the secrets of longevity will help with efforts to restore giant tortoise populations," said the Galapagos National Park director Jorge Carrion.

(With input from AFP)