We used to think there was just one species of giant tortoise living on Santa Cruz Island in the Galapagos archipelago. But after a thorough genetic analysis, researchers discovered that the few hundred tortoises living on the east side of the island belong to a previously unknown species completely separate from Chelonoidis porteri living on the west side.
The newly named eastern Santa Cruz tortoise (Chelonoidis donfaustoi), described in PLoS One this week, brings the total giant Galapagos tortoise tally up to 12.
A population of 2,000 to 4,000 giant tortoises live on the island’s southwestern slopes in a 156-square-kilometer (60-square-mile) sector called La Reserva. About 20 kilometers (12 miles) to the east, a second population of just 250 tortoises occupy a dry 40-square-kilometer (15-square-mile) swath of land called Cerro Fatal. The two groups don’t mix much and their domed upper shells (called carapaces) are of different sizes and shapes. Despite that, they were considered members of the same species. However, recent studies have suggested that the two are spatially and evolutionarily distinct lineages, and they likely derived from separate colonization events from different source islands.
While there’s a large amount of variation, multiple morphological measures on several individuals are needed to distinguish them, Yale’s Adalgisa Caccone explained to IFLScience. Her team turned to genetic characters. They examined mitochondrial and nuclear DNA extracted from three museum specimens: a skull (pictured below, right) and part of a carapace collected from the Cerro Fatal area, as well as the C. porteri holotype collected in 1902, upon which the species description was made. Additionally, to estimate levels of their genetic diversity, the team looked at 70 mitochondrial DNA (mtDNA) sequences from Reserva tortoises and 51 from Cerro Fatal tortoises using previous studies. This combined dataset was compared with sequences from all Galapagos tortoise species – extinct and living – that have been identified by mtDNA studies.
The eastern and western populations on Santa Cruz, the researchers revealed, are as genetically distinct as different species living on different islands. In fact, they’re not even that closely related.
Cerro Fatal tortoises are a sister species to those from San Cristóbal Island, and those two are grouped with tortoises from Pinta, Española, and Santa Fe islands. Meanwhile, the lineage from Reserva belongs in the same grouping as tortoises from Isabela, Floreana, Fernandina, and Pinzón Islands. Reserva tortoises are part of the archipelago’s oldest lineage, having diverged around 1.74 million years ago; the much younger Cerro Fatal tortoises diverged around 0.43 million years ago.
The researchers named the new species Chelonoidis donfaustoi after Fausto Llerena Sánchez (also known as Don Fausto), who had been a park ranger in the Galapagos National Park Directorate for 43 years. The team also propose that the common name for Chelonoidis porteri should be changed from simply Santa Cruz tortoise to the western Santa Cruz tortoise.
Recognizing the eastern Santa Cruz tortoise as a new species could help promote efforts to protect them – especially with their smaller range, lower numbers, and lower genetic diversity. The team did find that 3% of the tortoises are a mix of the two. They’re likely the result of being transported by humans, since the two species’ ranges are now linked by an agricultural zone (pictured in gray on the map) that was created in the last century.
Images in the text: N. Poulakakis et al., PLOS ONE 2015
UPDATED 22 OCTOBER 2015 AT 3:00 PM ET