Social recognition in Annelids and the evoluion of social recognition and cognitive abilities by sexual selection

Social recognition has rarely received attention in the studies on annelids. This is not surprising since the biology of behavioral interactions in annelids is mostly unexplored. Only few pheromones have been identified, which function as cues in mate recognition and gamete release. Many annelids use chemical and visual cues to locate partners and classify them according to mating status, body size, or oocyte ripeness. In some hermaphroditic polychaete worms and leeches the ability to recognize the quality of potential partners seem to be very refined, especially in relation to the ability to assess the number of competitors over mat- ing. These examples suggest that sexual selection might have favored individual ability to assess conspecific numerosity accurately and vary their male and female resource allocation (sex allocation) accordingly. Finally, annelids can estimate whether they are related to their potential partners and whether they belong to the same or a different population, which again result in adjustments of their repro- ductive allocation. We suggest that sexual selection is likely to be responsible for the evolution of the ability to assess mate quality and social group size because sex allocation adjustments are favored by sexual selection.