A study published in the journal Organisms Diversity & Evolution has shown that it is variation between populations and not, as previously thought, between sexes that determines whether Hermann’s tortoises have four or five digits on their front feet.

It might be hard to imagine humans having different numbers of fingers depending on the population to which they belong, but it is true of Hermann’s tortoises (Testudo hermanni), which have either five or, less frequently, four digits on each of their front feet. A study published recently in Organisms Diversity & Evolution has concluded that some of them have lost their first digit due to genetic differences between populations in different regions.

“We based our work on 1669 tortoises from six different populations – one from the Ebro Delta and five from Menorca – and found that the presence of five digits doesn’t depend on the sex of the tortoise as some authors had previously suggested”, says Mariona Ferrandiz, a CREAF researcher and Autonomous University of Barcelona lecturer who participated in the study.

“Digit loss in animals is a common phenomenon in evolution and is mainly due to non-use”, explains Àngel H. Luján, a researcher from the Miquel Crusafont Catalan Institute of Palaeontology and the study’s lead author. “According to our results, age, sex and the type of substrate a tortoise inhabits aren’t determining factors”, he continues. “The key lies in belonging to one population or another.”

“Our X-rays of different tortoises confirm that the specimens with only four digits have lost the phalanges, metacarpal and carpal bones of their first digit”, states Albert Bertolero, a member of the Tortoise and Freshwater Turtle Specialist Group of the International Union for Conservation of Nature. “That means the loss of the digit is anatomically real and not just an external appearance”, he explains.

Two genetic lineages

While the researchers did not perform a genetic analysis to identify the genetic lineages of the tortoises they studied, previous scientific work has shown there to be two such lineages and described their geographical distribution. The researchers have thus been able to establish how the presence or absence of the first digit is related to each lineage. Almost 70% of the tortoises belonging to the first lineage have five digits, falling to 60% in the case of the second. In a third group, in which the two lineages are mixed, there is a fifty-fifty split between tortoises with four and tortoises with five digits.

There are two lineages: a continental lineage, corresponding to mainland Europe, and a Menorcan lineage.

As regards the geography of the two lineages, one is of continental origin (shared by populations from mainland Spain, France and Italy) and the other has so far only been found in Menorca (although it cannot be considered to have originated on the island as its fossils have never been found there). The two lineages are mixed in some parts of Menorca as a result of tortoises having been released into the wild some years ago. “We were very surprised to find that half of the individuals in the interbreed population have lost a digit”, remarks Ferrandiz. “One of our hypotheses is that the gene responsible for digit loss may be overexpressed in that population”, she concludes.

Referenced article:

Luján, À.H., Ferrandiz-Rovira, M., Torres, C. et al. Intraspecific variation in digit reduction in Testudo: the case of the Hermann’s tortoise. Organisms Diversity & Evolution (2019). https://doi.org/10.1007/s13127-019-00413-3