Giant tortoises taste good. At least they did to pirates and whalers who had spent months at sea eating the same dry rations day in, day out. Soon after the discovery of the Galapagos Islands in 1535 they became a regular stop-off for hungry crews to stock up on these handy living larders.
Over the centuries this pillaging took its toll. Several distinct populations of Galapagos tortoises were wiped out, while others were brought perilously close to extinction. Now, though, researchers have found another consequence of the sailors’ taste for tortoise meat. Unwanted tortoises were often dumped on islands far from home, so populations that were well down the path to becoming different species found themselves thrown back together.
Now conservationists are putting the tortoises back where they belong, so that geographically separate populations can continue on their distinct evolutionary paths. Using genetic techniques borrowed from forensic science they are identifying long-lost relatives and even entirely new subspecies. They are also combing the world’s zoos for individual tortoises that might inject much-needed genetic diversity into existing populations, and are making progress in finding a partner for Lonesome George, the only surviving tortoise from Pinta Island and the world’s rarest living creature.
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When conservationists began to take stock of Galapagos tortoises in the early 1960s they identified 11 different lineages, each isolated on its own island or volcano. Ever since 1835, when the governor of the Galapagos Islands boasted to Charles Darwin that he could tell tortoises from different islands apart just by looking at them, there have been those who believed these lineages deserve full-blown species status. The truth is that the physical differences are not that clear-cut, so taxonomists have lumped them together as related variants of the same species.
Galapagos conservationists, though, have always managed each island population as if they were separate species, doing all they can to avoid any gene flow between islands. This purist approach is largely down to Charles Darwin. His legacy has ensured that Galapagos conservation is not just about preserving the islands’ unique biodiversity but also about maintaining evolutionary processes. “Geographic isolation is one of the leading factors in the speciation process,” says Bryan Milstead, head of vertebrate research for the Charles Darwin Foundation at the Charles Darwin Research Station (CDRS) on Santa Cruz. “Organisms that are naturally separated geographically should always be managed as separate units,” he says.
It was this philosophy that lay behind one of the world’s most successful reintroduction programmes, which started up on Española Island in the 1970s. From just 15 surviving individuals – three males and 12 females – conservationists bred 1500 tortoises over three decades and gradually repatriated them to Española.
It was only when geneticists at Yale University began to collect blood samples from Galapagos tortoises in the early 1990s and study their mitochondrial DNA that it became clear that the effort to keep the Española bloodline pure had been worth it. Their first major analysis, published in 1999, revealed that the tortoises had colonised the southernmost islands of the Galapagos by floating out from mainland South America several million years ago before heading north-west to colonise the rest of the islands. These pioneers evolved into at least 14 genetically distinct forms, three of which are now extinct.
The Yale team found that the genetic differences between island bloodlines are so clear that it’s possible to take a tortoise with an unknown history, run its genes through a DNA database of more than 3000 tortoises and confidently assign it to a particular group. In 2003, the team used this approach to untangle the origins of 23 captive animals confiscated from private collectors or given to the CDRS.
It is also possible to detect new tortoise lineages. On the central island of Santa Cruz, there are two main populations, one on each side of the volcano. Until the geneticists got to them, both groups were assumed to be the same kind. In fact, they are as distant from each other genetically as any of the other major tortoise types, says Jeff Powell, an ecologist and evolutionary biologist at Yale. The goal now is to look for morphological or behavioural differences that would justify classifying them as distinct species, he says.
The approach has also exposed the meddling of past sailors. Genetic analysis has revealed “alien” tortoises on Wolf Volcano – the northernmost volcano on Isabela – with a DNA signature that singles them out from the rest of the population (Evolution, vol 56, p 2052). Their genes suggest they are relatively recent descendants of tortoises on Española and San Cristóbal, islands from which there is no obvious natural means of crossing. “We suspect these are the descendants of animals that were moved by whalers and buccaneers 100 and 200 years ago,” says Powell.
“These are the descendants of animals moved by whalers centuries ago”
Ironically, it isn’t just sailors of centuries past who are responsible for mixing up tortoise genes. Conservationists have also played a part. DNA profiling of captive-bred tortoises returned to Española turned up a cross between a female from Española and a male from Pinzón (BMC Ecology, DOI: 10.1186/1472-6785-7-2). The researchers say the most plausible explanation is that a Pinzón individual was accidentally mixed in with the Española tortoises being shipped out from the CDRS on Santa Cruz.
Thankfully, more than half of the repatriated tortoises have been sampled and only one hybrid has turned up, so it doesn’t look like the Pinzón stowaway has spread his genes too widely. In fact, it’s amazing there has been so little contamination, says Gisella Caccone, who heads the Yale project along with Powell. “It’s a tribute that only one mistake happened in more than 20 years of this programme,” she says.
In an ideal world, it would be nice to mount an expedition to locate and remove this tortoise and any other individuals on Española “contaminated” with Pinzón genes. But the Yale geneticists estimate the purge could cost around $50,000 – perhaps not the best way to spend limited funds, says Linda Cayot, former head of protection at the Charles Darwin Foundation and now science advisor for the US-based Galapagos Conservancy. “There are so many pressing needs in the Galapagos,” she says. “The costs of finding the Pinzón tortoise and any offspring outweigh the benefits.” What’s more, it would be near-impossible to be absolutely certain the job was complete. “What would be your end point?” asks Cayot.
Others agree. “A small contamination shouldn’t matter if it’s a healthy system,” says Justin Gerlach, scientific coordinator of the Nature Protection Trust of Seychelles, the only other place on Earth where giant tortoises still survive. Since these reptiles occasionally drift between islands anyway, the odd bit of genetic mixing is normal and can even be beneficial, he says. “Our obsession with genetic purity may not always be helpful.”
In fact, impurity may turn out to be the saving grace for Lonesome George – the only Pinta tortoise to survive the sailors. For more than 35 years, George has been in captivity at the CDRS on Santa Cruz patiently awaiting the discovery of a suitable female.
In research published last month the Yale team revealed that a tortoise tested on Wolf Volcano, Isabela, back in 2000 has clear signs of Pinta ancestry (Current Biology, vol 17, p 317). This news has electrified the Galapagos because until now, everyone believed the Pinta lineage would die out with George.
There’s a catch, however. The tortoise in question is a male – which somewhat limits the breeding options for George. Worse, although his parents could still be alive and breeding it’s clear from his mitochondrial DNA that his Pinta genes came not from his mother, but his father. That means that George’s best reproductive hope is that the new male has some sisters. Finding them won’t be easy. “We are unsure of exactly how many tortoises there are on Wolf Volcano,” says Bryan Milstead of the Charles Darwin Foundation. The best estimate is that there could be more than 1500 on the western slopes where this tortoise was tested, he says.
It’ll certainly take a lot of perseverance, not least because it means an expedition to the inhospitable slopes of Wolf, a steep volcano where the tortoises are crawling with ticks. “It’s probably one of the least pleasant places to mount a collecting trip,” Powell says. “But we’re going to have to do it.”
Even the tiniest chance has got to be worth the risk, says Powell. “This would boost the morale of not only Galapagos conservation biologists, but everyone worried about the future of life on Earth.”
Reconstructing George
To find a genetic match for Lonesome George, researchers at Yale University needed samples from others of his kind – otherwise they ran the risk of rejecting bona fide Pinta tortoises on the basis of individual genetic differences. But how do you find out what it means, genetically speaking, to be a Pinta tortoise, when you only have one sample?
Fortunately, they were able to extract DNA from six of George’s long-dead ancestors – two at the California Academy of Sciences in San Francisco and four at the Charles Darwin Research Station. Without these historical samples, the Pinta ancestry of the Wolf tortoise discovered last month would have been missed, says lead researcher Michael Russello, formerly at Yale and now an assistant professor at the University of British Columbia, Okanagan.
This reconstruction of Pinta’s population genetics has the potential to open up another reproductive avenue to Lonesome George: it should be possible to detect Pinta ancestry in captive tortoises around the world. Russello has got his hands on DNA from 98 tortoises on show in hotels, in university and private collections, and in zoos on three continents. One of them – a tortoise called Tony at Prague Zoo in the Czech Republic – has a shell shaped like Lonesome George’s.
Unfortunately, none of the animals sampled so far has shown any signs of Pinta ancestry, report Russello and his colleagues (Animal Conservation, in press). Nevertheless, the search has not been entirely in vain. The Prague tortoise, for example, could help boost the genetic diversity of tortoises on Pinzón, where it originally came from. “That individual did have some unique alleles that may be useful for reinjecting into the population in the future,” says Russello.
This kind of matchmaking was never part of the plan, says Jeff Powell, professor of ecology and evolutionary biology at Yale. But as the DNA database grows, so do the conservation possibilities. “I suspect there’ll be plenty more surprises yet,” he says.
