Monday, January 9, 2012

EXTINCTION MAY NOT ALWAYS BE FOREVER: Bringing an extinct tortoise back from the grave

I suppose that a tortoise that that went extinct nearly 200 years ago is not actually a fossil species, but being declared extinct should at least make you an honorary member of The Land Of The Dead blog.

Charles Darwin studied and collected specimens during his stop at the Galapagos Islands between Sept 15 – Oct 30 1835. Darwin served as the naturalist aboard the HMS Beagle on its five year circumglobal travels. One of the striking things that Darwin noticed were the large tortoises on the islands. In fact, islands got their name “Insulae de las Galopegos” (Islands of the Tortoises) in the 18th century because of their abundance of tortoises (1). The island-to-island differences in Galapagos tortoise species helped spark Darwin’s thoughts about evolution through natural selection.

Fig 1  The Galapagos Islands.  Floreana Island in green. Isabela Island  in red.


The Galapagos were used extensively as a stopover by pirates and whaling ships. In the first half of the 19th century some 200,000 tortoises were killed for food by the whalers. The tortoises were large, heavy (up to 880 lbs.), and slow moving, making them an easy target. It also made them a target for extinction. Habitat destruction and the introduction of rats, pigs, etc. also contributed to their demise (2). In spite of these depredations each island still has its own living species of tortoise (with greatly reduced populations), but the species on Floreana Island, the giant Galapagos tortoise (Chelonoidis elephantopus), went extinct. It was rare when Darwin visited the Island and extinct by 1850.

So it was surprising when in 2008 Nikos Polakakis and colleagues (3) reported that genetic studies of Galapagos tortoises revealed that 11 hybrid tortoises (Chelononidis becki) on Isabela Island in the Galapagos had genes from C. elephantopus. For the technically inclined, they were looking at single nucleotide polymorphisms that vary among the tortoises on different islands.

This discovery provided a great opportunity. By selectively breeding individuals of C. becki having C. elephantopus genes, in about four generations (approximately 100 years) there would be individuals with about 96% of the C. elephantopus genetic markers. This would allow them to reintroduce the species to with some of its original genetic diversity back onto Floreana Island (1, 2).


Fig 2  "This hurts me more than it does you."  Collecting blood samples for genetic analysis.

However, in a more stunning paper just published (4) Ryan Garrick and his colleagues (including Nikos Polakakis) the story becomes more remarkable. They have found that for 84 of the tested C. becki on Isabele Island, genetic analysis shows that they are the result of a hybridized breeding event that involved a purebred C. elephanoptus as one of the parents Given that 30 of these 84 these individuals are less than 15 years old and that Galapagos tortoises have lifespans of over 100 years, there is the very strong possibility that pure genetic strains of G. elephantopus are still alive and crawling around Isabela Island. The search is now underway to find those purebred individuals.


Figure 3.  The  Galapagos tortoises, by virtue of their large size and great weight, have a real impact on vegetation and erosion.  Imagine thousands of such beasts per island!  Their removal  radically and negatively changes the island ecosystem they evolved in.

Tortoises are important members of the island ecosystems and their presence, or absence, greatly influences the plants and animals on the island. Purebred C. elephantopus could be used to establish a breeding program to produce young that could be used reintroduced to Floreana Island, bringing the "extinct" tortoise back to the island it once inhabited. This is the first time that genetic tracking in hybrids has led to the rediscovery of a (thought to be) extinct species.

One final question. If each Galapagos island has its own species, how did C. elephantopus get to Isabela Island? It is well known that pirates and whalers moved them. Maybe some of that was intentional, but when whalers left the Galapagos they would toss overboard any tortoises still on the ship to make room for a far more valuable cargo than meat --- whale blubber. (2) Although large, the tortoises don’t sink like stones and some of them probably made it to the shores of Isabela Island to breed amongst themselves and survive cryptically until today.

Photos 

Fig 1(modified): http://en.wikipedia.org/wiki/Gal%C3%A1pagos_tortoise
Figs 2, 3:  http://www.yalescientific.org/2010/09/bringing-back-the-dead-reviving-an-extinct-tortoise-species/


References 
(1) Galapagos tortoise (Wikipedia):en.wikipedia.org/wiki/Galapagos_tortoise>
(2) Muller, B. 2010. Bringing back the dead: reviving an extinct tortoise species. Yale Scientific Magazine (posted in 83.2 News)  www.yalescientific.org/2010/09/bringing-back-the-dead-reviving-an-extinct-tortoise-species/
(3) Pulakakis, N. et al. 2008. Historical DNA analysis reveals living descendants of an extinct species of Galapagos tortoise. Proceedings of the National Academy of Sciences USA 105: 15464-15469.

(4) Garrick, R.C. et al. 2012. Genetic rediscovery of an “extinct” Galapagos giant tortoise species. Current Biology 22(1): R10-R11 (copy available for free download at:

2 comments:

  1. Hi Dan:

    You noted that the Floreana tortoises may have made their way to Isabela via human intervention. It's also been reported that giant tortoises in the Aldabras will swim between islands. This may also be the method by which the original tortoise species colonized the Galapagos Islands.

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  2. The original colonization is probably via large vegetation mats swept into the Pacific Ocean. Such mats have been observed floating far out to sea and are large enough to carry lizards, turtles, etc. Swimming the distance to the Galapagos is unlikely as the distance is great.

    I suppose they could have swam between islands but given that each island had its own turtle species it is unlikely that happened very often, otherwise speciation between islands would not have occurred. Several herpetologists who work on Galapagos tortoises favor the pirate/whaler intervention since there is a documented historical record for such activity. They made no mention of swimming in the the Aldabra tortoises in their papers.

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