Specimen identification from time-series photographs using plastron morphometry in Testudo graeca ibera

The spur-thighed tortoise Testudo graeca ibera Pallas, 1814 is a good example of a vulnerable long-lived CITES species, which is widely bred as a pet and can be easily reproduced in captivity. However, the high number of bred tortoises makes it difficult to monitor the international pet trade. Despite microchipping being widely recommended as an identification method, it is unsuitable for hatchlings and young tortoises because implanting the chip in chelonians can sometimes cause health problems. We believe that these disadvantages can be resolved by plastron photo documentation but this method is still not considered a valid alternative. The objective of this study was to find a method for identifying a specimen from a photograph database using morphometrics, an approach potentially more reliable than other fingerprinting methods currently used for tortoises. We propose a combined dissimilarity measure calculated from the relative differences in seam distances between two plastron photographs. The reliability of identification is studied on a dataset of 90 spur-thighed tortoises, 20 of which were photographed at ages two, eight, 14 and 20 months. We achieved a reliability of 99.3-99.9% in identifying time-lagged photographs of the same individual against photographs of unrelated individuals, and 97.5-99.8% in identifying time-lagged photographs against photographs of siblings. We conclude that a properly taken plastron photograph of Testudo graeca enables precise identification during the first years of ontogenetic development.