Imagine what it must be like to catch prey like a kingsnake; you have to bring your most sensitive tissues in close contact with other animals that will perform all possible attempts to escape. Now, assume you are a kingsnake that has recently fed but encounters another feeding opportunity; are you up to the task on a full stomach? Can a constricting snake even produce pressures high enough to incapacitate a second (third, fourth, etc) prey item if it already has prey in its stomach? What about prey items of different sizes? Size matters in many aspects of predator-prey dynamics, but does it affect constriction performance? These questions were the central focus of my recent investigation, published in the June issue of Journal of Zoology. How do prey size and repeated prey encounters affect constriction performance?
Constriction behavior involves exerting forces around prey that the prey experience as pressure. A kingsnake anchors its teeth to its prey and quickly encompasses the prey with damagingly high constriction pressures produced by their powerful axial muscles. Almost all of the work that has been done on constriction performance has focused on aspects of snake size or morphology. However, there are numerous portions of constriction that are likely heavily affected by aspects of the prey that snakes encounter, yet these important variables have gone untested.
In my first experiment, I randomized 20 kingsnakes (Lampropeltis getula) and fed them two repeated meals of either small (5% relative prey mass, RPM) or large (15% RPM) prey. In these trials, meals were pre-killed mice or rats purchased from a commercial supplier. Many snakes are known to feed on massive prey and while 15% RPM may not sound like a lot, it is the equivalent to an average human male (90 kg) eating 120 hamburgers at one time! Imagine how you would perform with that much added mass currently inside your stomach! During each feeding event, I recorded the maximum constriction pressure that snakes exerted on their prey as well as the size of the coil that each snake used. Contrary to historical predictions, prey size did not affect constriction performance. However, when feeding multiple times, snakes that had large prey already residing in their stomach encountered dramatic reductions in performance (reduction of 60% in coil size and 51% in peak constriction pressure).
In my second experiment, I offered kingsnakes (n=10) six repeated prey opportunities with prey of 7% RPM. Each snake only accepted five prey items and showed marked reductions in performance as they continued to feed. As snakes ingested more prey, they started using less and less of their body in their coils, and peak constriction pressures dropped following the same general pattern as coil length. By their final feedings, snakes were using coils that were 45.7% shorter and exerted peak pressures that were 50.1% lower.
Often in studies of performance, a reduction in performance is typically assumed to have a direct fitness consequence for the animals involved. However in this predator-prey scenario, I don’t think that is the case at all. While the snakes showed marked decreases in their performance, the pressures exerted on prey were still quite high. In order to quickly incapacitate mammalian prey, snakes must exert pressures high enough to interfere with breathing or circulatory mechanisms. Impairing either in mammalian prey will dispatch them quickly, although impairing circulatory function works faster than suffocation. Either way, mammals are very bad at surviving a constriction coil. In this case, snakes showed reductions in constriction pressures that they exerted as they continued to eat large or more prey. While their performance was reduced, they are likely still able to kill mammalian prey rapidly via circulatory arrest.