Slow and steady wins the race (of life, that is). Despite its leisurely wander and slow pace of life – or, perhaps, because of it – the giant tortoise outlives the vast majority of animals on the planet, including humans.
Now, an international team of scientists has deciphered the genomes of two giant tortoises and compared the readings to those of related species in order to uncover the secrets behind their large body size and longevity. The results have been published in the journal Nature Ecology & Evolution.
The two tortoises involved in the study are actually two different species. The first is Lonesome George, who was famously (and tragically) the last of his species, the now extinct Pinta Island tortoises (Chelonoidis abingdonii). George died in in 2012 when he was 100 or so, though his exact age remains something of a mystery. Unfortunately for George and his ancestors, their tasty meat and slow meander made them an appetizing (and convenient) snack for sailors visiting the Galapagos Islands, causing their rapid spiral to extinction from the 16th century onwards.
The second is an Aldabra giant tortoise (Geochelone gigantea), the only living species to exist in the Indian Ocean and currently listed as vulnerable on the IUCN Red List. It is not at all unusual for these tortoises to reach their centenary, with many living to 150 years or more. In fact, Jonathan the tortoise is an Aldabra and at a very impressive 186 years old, he is the oldest known living animal on the planet. Although it is his sex life (not his age) that seems to draw the most headlines.
In the genomes of both tortoises, the scientists detected signatures of positive selection of certain gene families related to the regulation of metabolism, the repairing of DNA, and the immune system's response. The team suspects that these gene families are the ones responsible for the size and long lifespan of the two species.
Specifically, the scientists highlight the expansion of tumor suppressors in both tortoises. This is an important thing to note because longer-lived organisms – like humans – are typically at a higher risk of cancer than their shorter-lived neighbors. Some species (including elephants) have evolved to have specific genes that greatly reduce the risk of deadly cancer cells. So too have giant tortoises, as it turns out. But the scientists have yet to confirm whether this is an adaptation unique to giant tortoises or whether it is a mechanism found across all tortoises and turtles.
As well as offering up a model to study traits like longevity and disease related to age, the researchers hope the results can help conservation efforts geared towards saving the remaining giant tortoise species.
"A better understanding of the processes that we have studied may help to further elucidate the biology of these species and therefore aid the ongoing efforts to conserve these dwindling lineages," wrote the study authors, who hail from Yale University, University of Oviedo, Galapagos National Park Service, Flinders University and nine further institutions.
"Lonesome George – the last representative of C. abingdonii, and a renowned emblem of the plight of endangered species – left a legacy including a story written in his genome whose unveiling has just started."