Aquatic foraging in two independently evolved species of snake: Nerodia rhombifera (Colubridae) and Agkistrodon piscivorus (Viperidae)

This study compares piscivory in two species of sympatric snakes, the highly aquatic natricine, the diamond back water snake, Nerodia rhombifera (Colubridae), and the generalist crotaline, the cottonmouth, Agkistrodon piscivorus (Viperidae) in terms of the relationship among foraging behavior, ontogeny and the habitat. Current issues of interest in snake foraging are discussed, focusing on the role of ectothermy and problems peculiar to piscivory.

Piscivory was studied in the laboratory in a large naturalistic pool, and in the field at Reelfoot Lake, Tennessee, Ontogenetic changes in the foraging of both species were studied in the laboratory. Each study examined water depth, the use of cover and open water, the amount of time spent above and below the water, and specific types of emergent and submergent behaviors. Prey variables included live versus dead fish and relative size. Predatory behaviors were divided into attacking, capturing and handling prey, and the ingestion of prey.

The two species exhibited very different foraging tactics. The negatively buoyant Nerodia rhombifera attacked mainly live fish from the water. Fish were handled and ingested where captured, including underwater. Size and sex differences were found. The positively buoyant Agldstrodon piscivorus foraged from the water's edge, selecting mainly dead fish. Fish were handled and ingested largely out of water, and always above water. There was very little overlap in habitat use between the two species,

Ontogenetic changes were found in both species, Nerodia rhombifera neonates initially used sit-and-wait attacks at the surface, and later attacked more underwater and used more types of attacks. The open-mouthed foraging considered typical of the species developed over time. Field work on N. rhombifera corroborated laboratory work, including size and sex differences. Neonate Agkistrodon piscivorus initially attacked live prey from the edge and later attacked from below the surface as well. Although juveniles employed more methods of attack, they still attacked only from the edge. Dead fish were taken more frequently by larger snakes, although the juveniles tested still preferred live fish. Predation on fish was similar to predation on rodents in terms of prey handling techniques, including the use of envenomation.

This study clarified the relationship between foraging behavior and habitat in Nerodia rhombifera and Agkistrodon piscivorus. Additionally, ontogenetic changes in foraging in these precocial species were documented, with open-mouthed searching and other underwater behaviors developing after birth. The generalist-specialist continuum of snake piscivory was more clearly defined as a result of this study. Open-mouthed searching was found to be an efficient tactic for both generalist and specialist piscivores.