Post-ejaculatory thermal sensitivity of sperm performance in the high-altitude lizard Phymaturus extrilidus

Rising environmental temperatures may differentially affect physiological processes in ectotherms, with sperm function representing a potentially critical but understudied vulnerability. We experimentally evaluated post-ejaculatory thermal sensitivity of sperm kinematics in the high-altitude viviparous lizard Phymaturus extrilidus. Ejaculated sperm from adult males were exposed to three ecologically relevant temperatures (28°C, 35°C, and 38°C), and motility and swimming parameters were quantified over a 2-hour incubation period using computer-assisted sperm analysis. Sperm performance was strongly affected by temperature, incubation time, and their interaction, indicating progressive thermal damage. Contrary to our prediction, peak sperm motility and swimming performance occurred at 28 °C rather than at the preferred body temperature (35 °C). Incubation at 35 °C and especially 38 °C caused marked, declines in motility, curvilinear velocity (VCL), and average path velocity (VAP), while straight-line velocity (VSL) remained comparatively stable. Increased linearity and straightness at elevated temperatures suggest reduced trajectory complexity and flagellar oscillation amplitude. These results reveal a mismatch between the thermal optimum for sperm function and locomotor performance, highlighting a physiological trade-off between somatic and reproductive traits. Under climate-warming scenarios, reduced access to cooler microhabitats may accelerate sperm senescence and compromise reproductive success in high-mountain ectotherms.