Precopulatory mate assessment in relation to body size in the earthworm Lumbricus terrestris:: avoidance of dangerous liaisons?

In the earthworm Lumbricus terrestris L., mating occurs on the soil surface, but partners remain anchored in their burrow and mating is preceded by repeated mutual burrow visits between neighbors. This study focuses on body size as one possible trait that earthworms may assess during these burrow visits. Size-related mate choice is predicted to result in size-assortative mating, which we found in one field sample (n = 90 pairs), but not in a second (n = 102). We discovered that when mates separate, one of them can be pulled out of its burrow. This was more likely for small individuals or those mating across wide distances. In a subsequent greenhouse experiment, we allowed focal individuals to mate with two neighbors of different sizes. Relative size affected neither mating rate nor primary preference, but focals mated sooner with the same-sized neighbor than with a differently sized one. Small focals visited large neighbors more often than small ones. We conclude that size influences mate choice as well as the outcome of mating and discuss how the tug-of-war that ends a mating contributes to this result. Precopulatory visits may involve assessment as well as enticement to lure the partner closer to the individual's own burrow, in order to minimize the risk when mating with a partner that is large or far away.