A boa constrictor consuming a meal that was so large it had to constrict its own head. Photo credit: Jarrod Petersen.
Across the animal kingdom, there are many means for getting around. Animals scamper and saunter, but only a select few species slither and, as snakes glide sinuously, the skin on the underside of the body must be stiff to transmit the forces that propel them along. Yet boa constrictors are notorious for devouring massive meals, far larger than their heads, so the skin around the neck and head must be extremely stretchy as they engulf their dinner. So how does the neck skin of these super-diners distend so far and is the skin different from the skin in other regions of the body? Jarrod Petersen, Lucy Campbell and Thomas Roberts, from Brown University, USA, and Bruce Jayne (University of Cincinnati, USA), also wondered whether boa constrictors prepare the skin around the neck to make it stretchier in the run up to a feast.
To answer these questions, Petersen and Campbell collected samples of boa constrictor skin along the length of the body from animals that had consumed a rat, chicken or rabbit dinner in the previous 3 days. ‘Swallowing food that's many times the size of your own head is a tall order and often lasted over an hour’, says Petersen. Then the team slowly stretched each portion of skin – at roughly the speed that the skin stretches as the snakes swallow slowly – until it snapped, to compare the skin's stretchiness. Impressively, the skin around the snakes’ necks stretched 25% more than the tail skin, and when the team checked the thickness, they found the tail skin was ∼70% thicker than the neck skin. Most intriguingly, the skin around the snakes’ necks was 37% floppier in the days after dining. Thinking about possible reasons for the skin becoming looser, the team suspects that most snakes don't have time to increase the stretchiness naturally in the hours before engulfing a meal. It is more likely that the skin is damaged as the reptiles shuffle their jaws over a sizeable snack – a bit like the loose belly skin experienced by new mums after giving birth. Fortunately, the snake's skin appears to recover well in time for the next meal, so we could probably learn from the remarkable healing properties of over-stretched boa constrictor neck skin.
The team then focused on another species that swallows outsized meals, but instead of gulping down intact bodies, Gan's egg-eating snakes (Dasypeltis gansi) consume whole eggs, shell and all, in a single mouthful. ‘No other animal can open their mouth as wide relative to their size’, says Petersen, adding that the snakes look like a soccer ball stuffed inside a garden hose after consuming an egg. And when the team checked the stretchiness of the snakes’ skin, they were amazed; the neck skin was 90% stretchier than the skin along the rest of its body. They explain that this makes sense, because the egg only travels a short distance through the body until it reaches the specialised vertebrae that crack it open. ‘The stiffer skin of the body appears to hold the egg in place so it can be crushed’, says Campbell.
However, when the team measured the stretchiness of the neck skin of lined blind snakes (Afrotyphlops lineolatus) and American pipe snakes (Anilius scytale), which dine on small meals and do not expand their necks as they swallow, it was no stretchier than the skin along the rest of the body. The team suspects that stretchy neck skin is one of the key factors that allowed the ancient ancestors of boa constrictors and egg-eating snakes to evolve the ability to consume food in one large bite.
