Interpretations of hermaphroditism have been influenced by the long-held idea that organisms can be classified in a teleological series ranging from simple to complex, with humans placed at the top. Darwin considered that hermaphroditic animals, those which have both male and female sex organs, cannot be subject to sexual selection because the "union of sexes" makes them unable to perceive the attractiveness or rivalry of their partners, and moreover, because of their incapacity to exercise any kind of preference or choice. Until very recently, this view was generally accepted and justified on the grounds that the sensorial abilities of these animals are very limited and imperfect, basically because of their position at the lower level of the animal scale. In this paper we review new evidence that contradicts this vision. Indeed, recent studies suggest that earthworms are able to detect the degree of relatedness, the quality and mating status of their partners, and they are able to fine-tune control of transferred ejaculate volume and cocoon production. Overall, the picture that emerges is that earthworms are phenotypically very flexible, evaluation of partners is subject to intense selection, and sperm competition is important in shaping their mating behavior. Consequently, earthworms constitute an excellent model for studying sexual selection in simultaneous hermaphrodites. Interestingly, these results are more consistent with the latter observations of Darwin on earthworms than with his earlier conclusions on hermaphroditic animals. In his last book, written in 1881, Darwin affirmed, among other things, that despite being low in the scale of organization, earthworms possess some degree of intelligence. In the same book, Darwin postulated the role of earthworm behavior in soil formation, thus establishing the basis of modern soil ecology. Since earthworms exert considerable influence on the structure and function of terrestrial ecosystems, mating strategies that affect aggregations, dispersal or colonization may have important consequences on ecosystem functioning. Hence, earthworms are excellent models for integrating evolutionary and ecosystem ecology. (C) 2013 Elsevier B.V. All rights reserved.
