Flatworms flatten to size up each other

Abstract

When copulations are costly, fertilization is reciprocal and fecundity is positively related to size, hermaphrodites are expected to favour large partners, leading to size-assortative mating. Size-related mate choice has, however, never been observed in hermaphrodites. In the flatworm Dugesia gonocephala copulations cost time and are reciprocal, and size is a positive predictor of female fecundity. Every copulation is preceded by a phase in which one partner glides on top of the other and both spread out and flatten, suggesting that partners assess each other's size. A total of 124 copulating pairs collected on four different dates, proved that mating is size-assorted in the field. In experiments with groups, more copulations took place between equally sized individuals than were expected when matings were random. In experiments with pairs, partners of different size exhibited twice as many mating attempts for the first copulation than did partners of the same size. We conclude that D. gonocephala employs a unique kind of pre-copulatory 'flattening' behaviour as a mechanism to signal as well as to assess relative size. This does not only confirm that hermaphrodites can mate assortatively when certain assumptions are met, it also proves that even lower invertebrates can show active mate choice.