Predator-prey interaction between snail-eating snake Pareas carinatus and terrestrial snails in Thailand

Abstract

Ecological interactions between predator and prey can drive their dynamic coevolution in morphology and behavior. Previous studies on East Asian snail-eating specialists, Pareas iwasakii suggested directional asymmetries in striking behavior and dentition function for specialized predation on dextral snails. However their predatory behaviors mostly remain unknown. This study provides predatory behaviors and performances of P. carinatus, which most widely occurs in the genus and coexisting with dimorphic snail prey. The result suggested that predatory behaviors of P. carinatus can be divided into the pre-capture, feeding and post-feeding phases, in which the snake pursues 9 behaviors which probably differ in function. These behaviors can be identified by 15 different displays. The result of morphological study and some behavioral response demonstrated that the direction of either head-tilt or dentition asymmetry is not functionally crucial for predation success to dimorphic prey by P. carinatus. This mean that the specialized handling of asymmetric prey does not require so strong asymmetry in dentition or striking behavior as expected from the previous study. During pre-seizure phase, snakes showed the trend of prey-size selection before predation by relying on visual recognition. The snake frequently avoided approaching or striking at relatively large sinistral prey (23.64 ± 0.87 mm), whereas the size of dextral snail did not affect whether the snake struck. This was also the case when the snake did not flick the tongue. From 29 sinistrals of Dyakia salangana, the snake only approached smaller 17 in the mean shell size (14.6 ± 1.6 mm) than the rest (23.7 ± 0.87). After approach, the snake struck at smaller 10 (11.3 ± 1.6) but not at other larger sinistrals (22.1 ± 1.1). These size-dependent decisions for predation on conspecific preys are not ascribable to prey odor differences. Therefore the snake recognizes prey handedness without relying on vomeronasal chemoreception by tongue-flick. The benefit of preying on the dextral instead of the sinistral snails increased with prey size over 12 mm in shell diameter, in term of soft-body mass gained per retraction (F1,47 = 66, p = 0.024) and the gain per time (F1,47 = 57, p = 0.001). This size-dependent increase of cost for preying on a sinistral instead of a dextral explains that the snake preyed on all of the sinistrals smaller than 12.4, and avoided sinistrals that are larger than this size. The presence of this threshold size supports a hypothesis that the size-dependent increase of cost for preying on a sinistral has driven the evolution of prey-handedness recognition and size-dependent avoidance of sinistral-predation. This arboreal snake is frequently active on trees where dimorphic tree snails abundantly co-occur. These tree snails are almost invariably sinistral in 17.3 % of 900 prey species. While, P. iwasakii rarely encounter sinistral prey with only one sinistral out of 23 potential prey species. Thus, it would be advantageous to evolve an ability to distinguish between prey enantiomorphs and explains the failure of P. iwasakii to capture a given sinistral. In contrast, P. carinatus should be advantageous to avoid predation on costly sinistrals. In this case, predator does not evolve to exploit sinistrals by arms race. Instead the snake has shifted to avoid inefficient sinistral because the easier dextral prey still remains abundant. Predator’s recognition of prey handedness, which benefits both the snake predator and sinistral prey, could further accelerate ecological prey speciation by a reversal gene.