Burrowing behaviour of radio-labelled earthworms revealed by analysis of 3D-trajectories in artificial soil cores

A new apparatus called 'gamma column' developed by Capowiez et al. (2001), enables the continuous movements of radio-labelled earthworms to be tracked in artificial soil cores (3D-space), The present study aimed at analysing the patterns of creation and use of burrow systems by earthworms. Each individual was radio-labelled by injecting a small source of Co-60 (13muCi) into its coelomic cavity and then incubating them separately in artificial soil cores for 21 days. The continuous movements of earthworms were classified as three kinds of movements ("to dig"; "to move forwards"; "to move backwards"). Break points that defined the limits between these movements were either the inversion of the displacement of the earthworm or the start of a new burrowing phase. Trajectories of the individuals (one of Lumbricus terrestris and one of Nicodrilus giardi) were analysed and qualitative differences were found: L. terrestris built only one vertical burrow in punctuated bouts of activity and, between these short digging events, oscillated many times up and down in its burrow. N. giardi displayed distinct phases of digging, when it built a more complex burrow system with several branches, and phases of use when it oscillated equally from the top to the bottom of the core. Even though it was based on only one observation per species, this preliminary work provides a promising description of the dynamics of burrowing behaviour and tests their hypothetical species-specificity under artificial conditions.