Burrowing behaviour found in various higher animal groups like Amphibia, Reptilia and Mammalia are interesting from the evolutionary point of view. A brief account would help understand the importance of this behaviour.
Burrowing has evolved many times in the animal world. Moving through soil or sediment is widespread in the animal world. The moles and other burrowers among mammals show convergent adaptation to subterranean life in their communication behaviour. Reduction or loss of eyes in them is also noticeable. Limbless amphibians called caecilians and certain reptiles like burrowing skinks are also adapted to burrowing behaviour.
In frogs, it occurs in two routes. The ‘hind feet first’ route involve a less marked transition. The ‘head first’ route involves more specialization. The caecilians like Dermophis translate itself like a worm by developing a hydrostatic skeleton and by exhibiting about complete separation between the vertebral column and skin and muscle. The spine is thrown into curves like snake during locomotion, but when it separates from the skin and muscle, the latter moves freely and provides points of contact like that of the earthworms. Several tendons also form a crossed helix.
Tuatara and some turtles make burrows. Worm-like burrowing is found in several squamates comprising lizards, snakes and amphisbaenians. Agamids like Agama etoshae, gerrhosaurids and lacertids exhibit sand-diving and move through sandy substrate. They show modifications in having cylindrical body, fringes or spines on their toes, and adaptations for breathing. Sand-diving lizards that inhabit the Namib Desert include Pachydactylus rengei, Angolosaurus skoogi, Meroles anchietae and Riopa sundevallii.
The burrowing skinks are perhaps most specialized among fossorial lizards, displaying loss of limb as found in Scleotes of Southern Africa, and the entirely limbless condition in Typhlacontias of Southern Africa. Burrowing skinks have a flattened snout, smooth overlapping scales for movement. They generally tend to lack eyes, eyelids and opening of external ear. Limbless lizards include the semi-fossorial ‘slow worms’ including Anguis fragilis.
Among the snakes, the atractaspids have evolved adaptations for burrowing into sandy soils with smooth scales and fangs that swing sideward to inject prey within their tunnels.In the group amphisbaenas, the loss of limb and morphology of head and skeleton are suited for an underground existence.Examples include the limbless Rhineura floridana, Anops kingii and Bipes bioporus, which have tiny forelimbs. Convergent burrowing mechanism in many animals follows a particular burrowing cycle, which despite divergent anatomies relies on the same type of movements in higher vertebrate groups and believed to be the result of convergent evolution, the unifying concept in biology.