Population-Level Laterality of Ambush Posture in the Chinese Green Tree Pit Viper (Trimeresurus stejnegeri) in Northern Taiwan

Behavioral lateralization, a consistent directional asymmetry in behavior, has been documented across many vertebrates, yet population-level evidence in snakes remains limited and debated. We examined ambush-posture laterality in the Chinese green tree pit viper (Trimeresurus stejnegeri), a nocturnal sit-and-wait predator, using a large dataset compiled from nocturnal field surveys (2014–2025) and verified citizen-science records from iNaturalist (2013–2025). Ambush-posture direction was classified from photographs using a standardized, anatomically referenced procedure based on head-aligned reference lines, and ambiguous cases were excluded unless observer classifications were concordant. We tested for population-level bias among sites using chi-squared tests and binomial logistic regression, and evaluated effects of age class and perching height. Across 283 observations from three localities, snakes at Yangmingshan exhibited a significant rightward bias in ambush posture (36 of 55 individuals = 65.5%; χ² = 5.25, df = 1, p = 0.022), whereas no population-level bias was detected at the other two localities. Neither age class nor perching height significantly affected laterality. These results provide the first evidence for site-specific population-level ambush-posture laterality in a snake and suggest that local ecological or demographic contexts, potentially linked to prey community composition and availability, may modulate the expression of laterality. Future work across additional populations and taxa, together with performance-based tests comparing capture success between left- versus right-curved strikes, will be essential for evaluating the adaptive significance of this pattern.