Ecological Study on the Populations of Two Sympatric Species of Tortoises – Testudo hermanni and T. graeca (Testudines: Testudinidae) in South-Western Bulgaria

Although studies on Testudo hermanni boettgeri and T. graeca ibera have been conducted for more than a century in Bulgaria, little or no data have been published on some aspects of their ecology. Moreover, most of the local populations have still not been examined. The purpose of the study was to examine two local populations of the Hermann's and the Spur-thighed Tortoise occurring together. The study was carried out during two different periods from 2012 to 2023. Here we present data on the numbers, age structure, and sex ratio of the two populations. We also provide information on their home range and real and potential threats. The results indicated that T. hermanni boettgeri is much more numerous than T. graeca ibera. The age structure of both species was very similar – the adults predominated over subadults and juveniles. The sex ratio was slightly in favor of males in the Hermann's Tortoise, whereas it was female-biased in the Spur-thighed Tortoise. The mean home range of the Hermann's Tortoise was smaller compared to other populations of the species. Females had a larger home range than males. The mean home range of the Spur-thighed Tortoise was larger than that of the Hermann's Tortoise and similar to the home range of other populations on a species level. The biggest threat identified was related to partial deforestation within the habitat of the tortoises. Overall the two species seemed to successfully coexist even in the presence of threats as their number did not decline during the study but rather was stable.