- A giant Galapagos tortoise, made famous by Darwin and thought to be extinct, still exists on a remote tropical island.
- Genetic analysis found that hybrids resulted from matings between the "extinct" tortoise and another species.
- This is the first time that a species has been rediscovered using genetic footprint tracking.
A species of giant tortoise believed extinct for 150 years was actually just moved from its original home and now lives on the volcanic slopes of the northern shore of Isabela Island in the Galapagos archipelago.
A genetic analysis, published in the latest Current Biology, found that DNA footprints of the long lost tortoise species, Chelonoidis elephantopus, exist in the genomes of its hybrid offspring. These tortoises turn out to be a mix of C. elephantopus and another giant tortoise from the area, C. becki.
While researchers have yet to isolate a purebred C. elephantopus individual, such tortoises must exist, based on the DNA data. The study marks the first time that a species has been rediscovered by way of tracking the genetic footprints left in the genomes of its hybrid offspring.
"This work also underscores the importance of museum collections in facilitating new discoveries," co-author Ryan Garrick told Discovery News. "Here, we were able to extract DNA from tortoise bones that were collected many decades ago, and use this DNA to characterize the gene pool of purebred C. elephantopus."
Garrick is a former Yale postdoctoral researcher who is now an assistant professor at the University of Mississippi.
He and his colleagues visited Volcano Wolf on the northern tip of Isabela Island and took blood samples from more than 1600 tortoises. The scientists then compared them to a genetic database of living and extinct tortoise species.
The matching process detected the genetic signatures of "extinct" C. elephantopus in 84 of the Volcano Wolf tortoises, meaning one of their parents was a purebred member of the missing species.
In 30 cases, breeding took place within the last 15 years. Since the lifespan of the tortoises can exceed 100 years, there's a high probability that many purebreds are still alive.
The Galapagos tortoises are famous for their influence on Charles Darwin's ideas about evolution by natural selection. It's no wonder they captured his attention: individuals can weigh nearly 900 pounds and grow to almost 6 feet in length.
The tortoise's size and slow moves also led to its downfall. Garrick explained that "populations were heavily decimated by buccaneers in the late 1600s and 1700s, and then by whalers, fur sealers, merchantmen and the crews of naval vessels."
"Largely owing to the exploitation of tortoises for oil and as a source of food aboard ships, C. elephantopus from Floreana Island (its original home) was reported to be extinct soon after Charles Darwin's historic voyage to the Galapagos in 1835," he added. "Indeed, it has been estimated that up to 200,000 tortoises were eliminated from the archipelago within only two centuries of intensive harvesting."
Co-author Gisella Caccone, a senior research scientist in Yale's Department of Ecology and Evolutionary Biology, told Discovery News she hopes to now find actual C. elephantopus individuals and restore them to their island of origin.
"We plan to go back to Volcano Wolf in December 2012 and attempt to find all the individuals of mixed ancestry (we have them all tagged with PIT transmitters), and if we are lucky also the pure ones," she said.
The work is important, she continued, "as these animals are keystone species playing a crucial role in maintaining the ecological integrity of the island communities."
It remains unclear how the tortoises wound up on Isabela Island, but Caccone and her colleagues suspect that humans transported them as food. Whalers may have also thrown them overboard or simply left them on the shore of this other island.
Garrick said he's hopeful that other "extinct" species might be rediscovered using the same genetic methods used to detect the missing giant tortoise.
There's already hope that the giant tortoise C. abingdoni from Pinta Island, now represented by the single known purebred individual "Lonesome George," is alive and well elsewhere.