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 re peated mutual burrow visits between neighbors. This study focuses on body s ize as one possible trait that earthworms may assess during these burrow vi sits. Size-related mate choice is predicted to result in size-assortative m ating, which we found in one field sample (n = 90 pairs), but not in a seco nd (n = 102). We discovered that when mates separate, one of them can be pu lled out of its burrow. This was more likely for small individuals or those mating across wide distances. In a subsequent greenhouse experiment, we al lowed focal individuals to mate with two neighbors of different sizes. Rela tive size affected neither mating rate nor primary preference, but focals m ated sooner with the same-sized neighbor than with a differently sized one. Small focals visited large neighbors more often than small ones. We conclu de that size influences mate choice as well as the outcome of mating and di scuss how the "tug-of-war" that ends a mating contributes to this result. P recopulatory visits may involve assessment as well as enticement to lure th e partner closer to the individual's own burrow, in order to minimize the r isk when mating with a partner that is large or far away.