Captive-introduced tortoises in wild populations: can we identify them by shell morphology?

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Direct exploitation and wildlife trade pose significant threats to global biodiversity, particularly impacting reptiles, such as tortoises, which are highly vulnerable. Moreover, the releases of captive-held animals into the wild can lead to potential negative consequences, including genetic introgression, disease transmission, and parasite spread. Therefore, diagnostic assessments are essential to evaluate the impact of release practices. In this study, we analyzed the shell morphology of Testudo graeca tortoises to assess the effectiveness of morphological analysis in identifying captive-introduced tortoises and quantifying tortoise releases in SE Spain. Despite being illegal, the cultural tradition of keeping and breeding tortoises at home persists in this region. Principal component analyses identified significant morphological differences between captive and wild tortoises. Captive tortoises generally exhibited larger shell sizes and greater morphological variability compared to wild tortoises. Accordingly, linear discriminant analyses accurately identified approximately 99% of wild tortoises, but the identification of captive tortoises was only 50% accurate because some captive tortoises show shell morphologies identical to wild ones. These results likely reflect the diverse origins and growth conditions of captive tortoises. Although this is likely an underestimation, our approach classified 7% of 125 wildlife tortoises across 13 natural sites such as captive-introduced tortoises. These findings highlight the potential impact of tortoise releases in wild populations and underscore the value of morphological analyses as an initial tool for diagnosing such impacts. Furthermore, integrating genetic and isotopic methods could further enhance the precision of these assessments.