Many of the principles behind jaw design are intuitive. For instance, you can form a good impression of a bird's diet from the size and shape of its beak, and everyone knows to keep their fingers away from an angry parrot. However, caecilian head design presents a puzzle: their jaw structure is consistent with high force development, suggesting specialisation for the consumption of hard food items such as seeds or snails, but they are predators that survive on a soft diet of earthworms and chewy subterranean arthropods. As caecilians don't appear to need such impressive jaws for crunching their food, how do these burrowing legless amphibians use this forceful bite?
John Measey and Anthony Herrel tackled this question by studying feeding caecilians with conventional and X-ray video, and direct measurements of bite forces. They confirm that the amphibian's bite forces can indeed be high, reaching 1.4-1.6 times the amphibian's body weight. The team explain that the caecilian's bite is powered with muscles situated behind rather than around the head, so that the muscles are positioned to produce the maximum bite force while maintaining a streamlined head that is suitable for burrowing through narrow channels.
However, it is the video observations that are the most fascinating and compelling aspect of this study: on biting its prey, a caecilian vigorously spins or corkscrews along its long axis. When applied to an earthworm, this has the effect of ripping it to pieces, and earthworm fragments recovered from caecilian guts look like pieces of twisted rope. Just like a crocodile's `death roll', the caecilian spin can reduce prey items to swallowable pieces using relatively simple jaws, and without grasping limbs. Also, the amphibian's maximum gape size does not limit the size of prey they can tackle, allowing caecilians to consume relatively large victims without leaving a large bump in the gut that would certainly hinder burrowing.
So these observations begin to make some sense: caecilians can grip large, slippery earthworms using a forceful bite, and then twist them into mouth-sized pieces. However, Measey and Herrel also noticed that caecilians continue spinning even when their meal is almost over, or the prey is small enough to eat whole. Are the caecilians being `stupid', continuing spinning when the work of dismembering their meal is complete; are they constrained to a stereotyped feeding behaviour? Or are the caecilians `sizing-up' as well as `chopping' with their bite-twist behaviour? Given that it is dark underground, Herrel and Measey suggest that it could be difficult for caecilians to gauge their victim's size without giving them a good twist. So the answer is probably a bit of both.
- © The Company of Biologists Limited 2006