On my trip to Kentucky I had the pleasure of meeting James Krupa, a biologist and natural historian with wide research interests. One of his specialties is a bizarre group of reptiles: egg-eating snakes. Actually, there are two groups: five species of African egg-eaters in the genus Dasypeltis, and the single species of Indian egg-eater, Elachistodon westermanni. These snakes not only can eat eggs, which they swallow whole, but that is all they eat.
Krupa kindly showed me these snakes as well as their skeletons and remnants of the eggs they’d nommed. As you might expect, they have numerous adaptations for dealing with this difficult food. First of all, they’re toothless, since teeth would impede the swallowing of eggs. Here’s a skeleton of one; note that where the teeth usually are, there’s nothing.
Because they’ve evolved toothlessness, they have a problem: how do they defend themselves? Krupa suggested that selection has acted to make them mimics of other, poisonous snakes. This is Dasypeltis atra, the montane egg-eating snake, which resembles a mamba. Isn’t it lovely?
They’ve also evolved a threat display that involves squirming around when cornered and rubbing their scales together. This produces a hissing sound. Here’s a video of a baby Dasypeltis atra showing that behavior:
The eating of an egg by one of these snakes is truly a wonder of nature. They swallow the thing whole, and it’s often several times larger than their diameter (see video below). When the egg is some way down the throat, they contract their muscles and move from side to side. This presses the eggshell against special protrusions on the snake’s vertebrae, which pierce the egg. This photo of a skeleton shows the egg-piercing protrusions (circled) that have evolved on some of the vertebrae:
Their tracheas are specially flattened so that they’re not occluded when swallowing an egg. This allows the snake to breathe during the time-consuming process of swallowing.
After they pierce the egg and squeeze out every drop of the contents, they regurgitate the shell. Here’s Krupa holding the remnants of a large egg that’s been nommed. Note the neat piercings down the center, where it’s come against the vertebrae:
And a video of the whole remarkable process, which I didn’t see. I believe this is the Southern brown egg eater, Dasypeltis inornata:
It’s easy to envision the evolution of this ability. Some snakes, like the rat snake, already eat smaller eggs as part of their diet. If you lose your teeth, and evolve some protrusions on your spine, you suddenly gain access to a rich source of nutrition unavailable to other snakes.