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The number of species of Galapagos giant tortoises that exist is in dispute and the result could influence how we conserve these endangered animals.

Researchers have defined 16 populations of giant tortoises (Chelonoidis spp.) spread across the Galapagos islands based on differences in their shell shape, behaviour and genetics. Three of these are now extinct because of hunting, the loss of their habitat for agriculture and the introduction of invasive species, such as goats, which compete for their food.

Whether the remaining 13 tortoise populations should be classed as many subspecies within one species or as multiple species has been a matter of debate for decades, but now it seems to be coming to a head.

鈥淚t doesn鈥檛 matter to the tortoises what we call them, right?鈥� says at Newcastle University in the UK. 鈥淏ut how we label the tortoises can have implications for conservation.鈥�

From 2012, the International Union for Conservation of Nature (IUCN) recognised and one extra 鈥渋ndeterminate鈥� grouping, based on genetic studies. But in 2021, the IUCN鈥檚 Turtle Taxonomy Working Group decided all the populations were part of just one species of the Galapagos giant tortoise, following a study .

The 2021 study looked at 15 tortoises in total from the 13 populations and found that differences in the DNA within mitochondria 鈥� which produce energy inside cells 鈥� between some members of different Galapagos giant tortoise populations are smaller than those seen between members of other tortoise species, suggesting the Galapagos giant tortoises are one species, says , part of the Turtle Taxonomy Working Group.

However, the study relied on data from very few tortoise samples, says , director of the Gal谩pagos Conservancy, which looks after tortoise populations locally.

The 2021 study 鈥渞eally boils down to them making the point that Galapagos populations are younger than other species鈥�, says Jensen. 鈥淏ut there is no rule about how old a lineage needs to be to be called a species.鈥�

Jensen and her colleagues want the 2021 IUCN update to be reversed. 鈥淥ur way to rebut this update was to do a study,鈥� she says.

The researchers have now analysed the genomes of 38 tortoises from the 13 living groups. They also used a computational model to compare 50 regions 鈥� each 1000 DNA letters, or base pairs, long 鈥� that were randomly located across the genomes.

This approach has been shown to accurately distinguish between different species of mammals, birds and frogs in previous studies, whose classification is widely agreed on, but has never been applied to these tortoises until now, says team member at Princeton University.

Jensen and her colleagues conclude that there are at least five species of Galapagos giant tortoises and potentially as many as 13.

The single species decision hasn鈥檛 yet been reflected in the which defines the extinction risk of species worldwide and influences conservation efforts, says Jensen, and she hopes it won鈥檛 be.

Funding is often allocated based on the species, rather than subspecies, level, so lumping species together could stop endangered populations receiving financial support, says Jensen.

Tapia thinks similarly. 鈥淚f, for example, the Galapagos National Park asks the Ecuadorian government to create a breeding centre for a subspecies, the possibility to get money is very low, but if you ask for money to conserve a species, normally the government takes it more seriously,鈥� he says.

But , a member of the Turtle Taxonomy Working Group and director of the Turtle Conservation Program at Re:wild, thinks it is unlikely that grouping tortoise lineages together will reduce funding. 鈥淭he Galapagos islands are such a unique location that a species or subspecies status is really not particularly relevant for people who donate to conservation efforts,鈥� he says.

In light of the recent preprint鈥檚 findings, Rhodin now says the answer may be that the Galapagos has a number of separate species and also several subspecies of some of those species. 聽鈥淢y personal feeling is that each genetically identifiable population should be considered a species,鈥� he says.

鈥淯ltimately, it comes down to philosophical differences in how you classify these things,鈥� says Gaughran. 鈥淭here鈥檚 no definitive way to define a species, I just hope that people continue to reassess new data and make decisions from there.鈥�