Low reproductive output of isolated, self-fertilizing snails: inbreeding depression or absence of social facilitation?

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Inbreeding depression has been studied extensively in many animals, but the potential of `social facilitation' (a response to the presence of conspecifics in the environment) to influence reproduction has received little attention. Studies of reproductive output in freshwater hermaphrodite snails have implicated inbreeding depression, but never social facilitation, in the reduced reproductive output of isolated compared with paired/grouped snails. In this experiment, isolated, self-fertilizing Biomphalaria glabrata snails had reduced reproductive output compared with paired, predominantly cross-fertilizing snails. However, the reproductive output of control snails (paired, but prevented from cross-fertilizing) was similar to that of paired, cross-fertilizing snails, suggesting that the low reproductive output of snails in isolation was not simply due to inbreeding depression with self-fertilization. I suggest that an absence of social facilitation, caused by deprivation of social communication as a result of isolation, may play a role in reducing reproductive output of isolated B. glabrata, and that future studies of inbreeding depression using isolated and paired/grouped hermaphrodite snails should be designed to control for possible effects of social facilitation.