The mating behaviour of the earthworm Lumbricus terrestris (Oligochaeta: Lumbricidae)

A field observation suggested that the mating of the earthworm Lumbricus terrestris L. might involve a pre-copulation behaviour sequence during which prospective partners visit each other's burrows. This was studied in laboratory investigations, where pairs of Finnish, field collected L. terrestris were observed directly and behaviour of groups of four or nine previously unmated L. terrestris of English origin was studied by video-recording. In total, 15 full mating sequences were recorded. Mate searching involved trail-following on the soil surface. This was followed by a series of, usually reciprocated, burrow visits. The number of visits by an individual worm showed a high degree of variability (n = 1-17). A burrow visit typically consisted of anterior segments insertion, for a period of 30 to 50 seconds, but also deeper burrow-penetrations, which sometimes lasted several minutes, were recorded. Resident worms, when visited, either withdrew below ground completely or remained at the surface, with the first few anterior segments in view. Visiting worms normally retained their posterior segments in their own burrows. Visits were fewer when burrow openings were more widely separated. Partners often maintained close contact while moving back and forth between the burrow openings and the pre-copulation phase appeared as specific courtship behaviour. Uninterrupted, the pre-copulation behaviour sequences lasted from 11 to 22 minutes in video-taped matings and approximately 90 minutes in the single directly-observed mating. After a pre-copulation sequence, pairs adopted a static 's'-shaped copulation position of close ventral contact, during which both individuals bent their anterior segments away from the partner's body. Copulations lasted from 69 to 200 minutes (median length for video-taped matings 135 minutes). Other individuals often touched the copulating pair. Where this occurred, matings were shorter compared to those with no interference. We present a scheme for the mating of L. terrestris, discuss its different phases and propose a few lines for further studies.