After All, Size Does Matter: Size and Trachemys Scripta Reproduction Traits at the Western Limit of Its European Distribution

In invasive species, reproductive success may determine invasion outcomes and influence the choice of appropriate control and eradication. Trachemys scripta (Schoepff, 1792) is one of the world's most widespread invasive turtle species. Although some previous studies have focused on T. scripta reproduction, both in native and in non-native ranges, only one study has been carried out in Portugal, which reported clutch sizes. The aim of this study is to determine which morphological traits affect reproductive traits of T. scripta in Algarve, Portugal. We used shell measurements and body condition to explain reproductive organ morphometrics, fecundity, and clutch size, using both generalized linear models and a Bayesian framework. Overall, in females, reproductive variables were only explained by plastron length, except for the number of stage IV follicles, which was also explained by body condition. In males, only the gonadosomatic index was explained by plastron length and plastron width as well as body condition index. Female T. scripta in Portugal showed a strong size-dependent reproductive relationship, with plastron length positively influencing all major reproductive traits. Our Bayesian analysis indicated that females reach sexual maturity at 15-18 cm. In males, morphology was poorly related to reproductive traits variation, although lower body condition was associated with larger gonads. These results show that environmental conditions in southern Portugal support successful reproduction, reinforcing the species' potential for long-term establishment. These findings highlight the importance of large reproductive females in driving population growth and provide essential demographic parameters for modeling and managing invasive T. scripta populations in Europe.