Behavioral Responses of a Dietary Specialist, the Queen Snake (Regina septemvittata), to Potential Chemoattractants Released by Its Prey

Using natricine snakes to test how prey type and size affect predatory behaviors and performance

Introduction

Predation is a complex process for which behavior, morphology, and size of both predator and prey can affect the success and effectiveness of the predator. For predators such as snakes that swallow prey whole, gape ultimately limits prey size, but the behaviors used to select, capture, and consume prey and attributes of the prey can also affect maximal prey size. For example, swallowing live, struggling prey is difficult, but using coiling or envenomation to restrain or kill prey has evolved repeatedly in snakes.

Methods

To test the potential benefits of these behaviors, we manipulated the type and size of prey, and determined how stereotyped predatory behavior was in a snake species (Liodytes rigida) that uses both coiling and envenomation to restrain and immobilize its formidable prey of crayfish. We also studied a close relative (Liodytes pygaea) that eats fish and salamanders to gain insights into the evolution of these traits.

Results

For L. rigida, envenomation of hard-shell crayfish via their soft underside was very stereotyped (100% of feedings). Envenomation of soft-shell crayfish was less frequent (59% of feedings) but became more likely both with increased relative prey size and increased time after molt (hardness). L. rigida coiled more for hard-shell than soft-shell crayfish (77% vs. 30%). The probability of coiling was unaffected by prey size, but it increased with increased time after molt for the soft-shell crayfish. Liodytes rigida waited to swallow crayfish until they were completely immobile in 75% and 37% of the feedings with hard- and soft-shelled crayfish, respectively. Even with large prey L. pygaea never used coiling or envenomation, whereas previous studies of L. alleni, the sister species of L. rigida, observed non-lethal coiling without envenomation when eating hard-shell crayfish.

Discussion

Our findings for the Liodytes clade of three species suggest that coiling evolved ancestral to the crayfish specialists (L. alleni; L. rigida), and envenomation by L. rigida subsequently evolved as an additional means of subduing formidable prey. The proximate benefits observed for coiling and envenomation in L. rigida support the evolutionary scenario that both traits evolved to enhance the feeding performance for more formidable prey.

Crayfish Eating in Snakes: Testing How Anatomy and Behavior Affect Prey Size and Feeding Performance

Quantifying the performance of animals is a powerful methodology for determining the functional consequences of morphological variation. For example, snakes consume prey whole, and variation in the anatomy of their trophic apparatus directly affects gape and limits maximal prey size. However, for the foraging ecology of snakes and other systems, scant data exist regarding how often maximal capacities are taxed in nature. Hence, we quantified: (1) maximal gape, (2) the size of prey relative to maximal gape, and (3) how the type and relative size of prey affected behavior and prey handling times (HTs) for two species of natricine snakes that primarily eat soft- (Regina septemvittata) or hard-shelled (Liodytes alleni) crayfish. Liodytes alleni had significantly larger maximal gape than R. septemvittata with equal snout–vent length. The percentages of large prey (>60% maximal gape area) consumed in the field were low in both R. septemvittata (22%) and L. alleni (2%). However, R. septemvittata, especially juveniles, ate relatively larger prey than L. alleni. Strategies for dealing with the seasonal scarcity of small crayfish differed as juvenile R. septemvittata commonly removed and ate only chelipeds from crayfish too large to swallow whole, whereas juvenile L. alleni ate many small odonate nymphs. During laboratory trials, unlike R. septemvittata, L. alleni usually used its body to restrain prey with behaviors that depended on relative prey size and prey hardness. Liodytes alleni consumed soft-shelled crayfish significantly faster than R. septemvittata and significantly faster than hard-shelled crayfish. Several of the differences in gape, prey size, and prey HTs and behavior between the crayfish-eating snakes resemble those between two phylogenetically distant species of homalopsid snakes that consume either hard- or soft-shelled crabs. In both groups of crustacean-eating snakes, the decreased capture success in captivity and the rare consumption of relatively large hard-shelled crustaceans in the field suggest that the ability to capture this type of prey constrains prey size more commonly than maximal gape. Based on data integrating snake size and gape with the relative mass of intact prey, the predicted potential feeding performance R. septemvittata consuming intact prey exceeded that of the other three species.