Advances in Biology & Earth Sciences

Understanding how environmental gradients shape phenotypic and ecological variation is essential for interpreting population-level responses of reptiles to heterogeneous landscapes. We investigated geographic variation in body size, body mass, body condition index (BCI) and digestive tract helminth abundance in Tenuidactylus caspius across six contrasting regions of Azerbaijan encompassing lowland semi-deserts, xeric foothills and humid montane forest-edge habitats. A total of 395 individuals were examined using standardized morphometric and parasitological methods. Mean snout-vent length and body mass were highest in lowland and foothill semi-arid environments and decreased toward humid and montane regions, while intermediate values were observed in forest-edge habitats of the Greater Caucasus. Montane populations exhibited greater within-site variability in both body size and body mass. In contrast, variation in BCI was comparatively limited across sites, indicating relatively consistent proportional body condition despite geographic differences in absolute size and mass. Digestive tract helminth abundance varied markedly among populations, with higher parasite loads recorded in humid forest-associated habitats and lower values in arid and semi-arid environments, accompanied by substantial individual-level variability. Overall, the results demonstrate coherent spatial structuring of morphological traits, body condition and parasite abundance in T.caspius that aligns with major climatic and habitat gradients. These patterns are consistent with broad ecological frameworks describing phenotypic variation in ectothermic vertebrates and highlight the ecological flexibility of this widespread gecko in both natural and human-modified landscapes.