The kinematics of locomotion in caecilians: effects of substrate and body shape.

Caecilians are limbless amphibians that have radiated extensively in the tropics, and have evolved distinct cranial and postcranial specializations associated with a burrowing lifestyle. Some species are recognized as being surface active, whereas others are dedicated burrowers. Previous authors have demonstrated that some caecilians use a hydrostatic mechanism to generate burrowing forces which is dependent on the existence of skin-vertebral independence. It has been hypothesized that skin-vertebral independence may be lost in extremely elongated species, thus affecting their ability to burrow. Here, we use X-ray video to study the kinematics of locomotion in five species of caecilian differing in their degree of body elongation. Animals were filmed moving in or across different substrates imposing different functional demands on the locomotor system. Our data demonstrate that all species have the ability to perform internal concertina locomotion, but indicate differences between species in the kinematics of locomotion with more elongate species showing a smaller degree of skin-vertebral independence. In all species, locomotion was dependent on the substrate and species switched from using lateral undulation on the surface substrates to the use of whole body or internal concertina in wide and narrow tunnels, respectively. When burrowing in soil, all species used a combination of whole-body and internal concertina locomotion. Additional studies on the ability of different species to generate forces are needed to test whether the reduced skin-vertebral independence in elongate forms has resulted in a decreased ability to generate burrows.