How snakes evolved the ability to swallow oversized prey

Evolutionary Adaptations in Snakes

One of the most fascinating aspects of snakes is their ability to swallow prey that appears (to us) to be impossibly large in relation to the dimensions of their heads. Other snake-like organisms such as caecilians, glass lizards, and sirens lack this ability, raising logical questions about how and why this adaptation evolved exclusively in snakes. Physiologically, there are specific morphological and behavioral mechanisms responsible for this feeding behavior: the snake's quadrate bone connecting the jaws allows distention of the entire joint; the two sides of the lower jawbone are connected in the middle by muscle instead of inflexible bone, and the snake's teeth are recurved to hold prey in place during swallowing and to allow the snake to "walk" its jaws over prey items one side at a time. The absence of similar features in other related species is likely attributable to the fact that these specialized evolutionary adaptations were largely driven by the unique hunting and killing mechanisms that removed the usual limitations of the size of prey in relation to the size of the predator's body and head.

Morphological and Behavioral Mechanisms

To accomplish the feat of swallowing prey items substantially larger than the snake's head, the snake has evolved three specific morphological and behavioral adaptations. Snakes have evolved the ability to open their mouths much wider than their heads by virtue of the anatomical mechanism of a specialized quadrate bone attaching the upper and lower jaw (Carr, 1993; Gould, 1991; Gould, 1995). More particularly, this allows them to unhinge their jaws at the connection to open their mouths almost a full 180°. Generally, the snake uses this ability to swallow any prey that is larger than it would be capable of swallowing without this adaptation.

Similarly, the snake's lower jaw is not solid bone; rather, the left and right halves are connected at the chin by a muscle instead of solid bone (Carr, 1993; Gould, 1991; Gould, 1996). This allows the lower jaw to stretch laterally extending the snake's ability to accommodate large prey even more than the double-hinged connection between its upper and lower jaw. This particular adaptation has a dual function in that also allows the snake to "jaw walk" over its prey by repeatedly advancing one side of its jaw at a time over the prey in the swallowing process. Finally, the snake also possesses many recurved teeth (Carr, 1993; Gould, 1991). In addition to facilitating the initial kill, these adaptations also provide a very secure purchase on prey and make jaw-walking physically possible.

Explaining the Differences between Snakes and other Similar Species