I'm sure we've all had the unfortunate experience of grossly overindulging at the dinner table. However, surely none of us has faced the sorts of dietary challenges confronted by ophiophagous snakes - those that consume other snakes as prey. Putting potential issues of cannibalism aside, what is particularly striking about some of these snakes is that they can feed on prey as long as or even longer than they are. It probably doesn't take an insightful reader long to recognize that this poses a mechanical problem or, more specifically,a packing problem. If a snake's stomach ends 2/3 of the way down its body,where does it put a prey item whose total length extends beyond this?
To address this question, Kate Jackson, Nate Kley and Beth Brainerd have been working to understand what happens internally when snakes eat other snakes of equivalent or larger size. In a recent study, they used four king snakes as predators and six corn snakes as prey. Length ratios (prey/predator)ranged from 0.93 to 1.39 among the feeding events observed. A combination of standard video, X-ray video and still X-rays was employed to understand the process of feeding in these animals. Several dissections were also used to determine the position of prey in the predator's gastrointestinal tract, as well as to compare predator digestive tracts before and after ingestion.
Jackson and colleagues use four phases to describe the process of ophiophagy in kingsnakes: (1) Capture, during which the predator often immediately takes the prey into its jaws. (2) Constriction, in which the predator coils its body around the prey to subdue it. This lasts seven to eight hours for initial feedings but is substantially shorter in duration in subsequent feedings. During this phase predators also coordinate head and body movements to reposition the head of the prey near their mouths. (3) Ingestion involves getting the prey into the mouth, always head-first, and then transporting it down the gastrointestinal tract. X-ray video shows that the head of the prey is transported to a position near the predator's cloaca. At this point, further prey transport by the predator (involving jaw ratcheting and occasional movements of the body) forces the prey's body into bends that allow for its length to be accommodated. Dissection reveals that prey do not enter the intestine during ingestion, rather, the stomach and esophagus distend posteriorly, accounting for the positioning of the prey nearly down to the cloaca. (4) Digestion takes place over the following two weeks, after which no remains of the prey can be seen via X-ray. Amazing footage of these phases can be viewed by clicking on the supplementary material link in Jackson et al. (2004) at http://www.sciencedirect.com/science.
Thus it is the combination of prey bending and gastrointestinal tract stretching that allows snakes to eat other snakes of similar or longer length. It should also be noted that most feeding events in this study ultimately ended with regurgitation so, apparently, the snakes can appreciate how we feel following an overindulgent dinner after all.
