Baseline peripheral serotonin levels in two sympatric Podarcis lizard species.

Serotonin (5-hydroxytryptamine, 5HT) plays a crucial role in modulating physiological and behavioral traits in vertebrates, nonetheless little is known about its peripheral function in reptiles. Differences in central serotonin levels and behavior under serotonergic control have been observed in two sympatric lacertid lizard - Podarcis siculus and P. melisellensis. We compared baseline peripheral serotonin and tryptophan (Trp) levels, along with intestinal morphology, in these two species known to differ in behavior and invasiveness. Serum 5HT and Trp were measured using High-performance liquid chromatography (HPLC)-DAD, and intestinal enterochromaffin (EC) cell density and tissue morphology were assessed histologically (N = 82). Podarcis melisellensis exhibited significantly higher serum 5HT concentrations, particularly in males, with no corresponding differences in Trp levels or EC cell density. Histological analysis showed preserved intestinal architecture and similar structures in all examined individuals. Histomorphometric analysis revealed sex- and species-specific variation in intestinal structure, independent of sampling location. These findings suggest that interspecies differences in peripheral serotonin homeostasis may arise from physiological mechanisms beyond EC cell abundance, possibly involving altered precursor availability, synthesis, or degradation. Our results highlight peripheral serotonin as a potential physiological marker underlying behavioral and ecological divergence between closely related lizard species.