Bimodal Copulatory Locomotion and Ritualized Male–Male Combat in the Amazonian Lancehead (Bothrops atrox): Linking Sexual Size Dimorphism to Reproductive Mechanics and Sexual Conflict

Reproductive behaviors in secretive Neotropical pitvipers are rarely documented under natural conditions, limiting empirical assessments of sexual selection, locomotor mechanics, and mating systems. Here, we present focal videography and photographic documentation of free-ranging Amazonian Lanceheads (Bothrops atrox) from coastal French Guiana during the rainy season. A prolonged copulation (~3 h) revealed a remarkable bimodal locomotor sequence: the pair moved from terrestrial litter across a 20 m-wide forest stream and back to land without cloacal separation. Video analysis demonstrates female-driven locomotion, wherein the substantially larger female pulls the coupled male through the water column against his counter-directional swimming movements. This behavior illustrates female locomotor dominance under sexual size dimorphism and the widely mechanical resilience of the spine-bearing crotaline hemipenis during forced aquatic displacement. Sequential photographic documentation captures ritualized male–male combat with synchronized vertical displays, anterior-trunk swaying, neck hooking, and topping attempts, confirming that stereotypic motor patterns described in western Amazonia are conserved in eastern Guiana Shield populations. These postural dynamics represent transient physical contests over spatial priority rather than permanent dominance hierarchies. Together, this evidence indicates that empirical validation for female behavioral control during copulation and expands the geographic framework supporting the prolonged mate-searching polygyny hypothesis in B. atrox.