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If the potential reproductive value of one gender role increases much more with body size than the other (e.g., the amount of sperm transferred per copulation, or the amount of eggs produced per litter), smaller individuals would prefer to act more frequently as a male and large enough ones as a female, or vice versa, as the size-advantage model predicts.
The distribution of body sizes could be such that by chance there are frequent encounters between individuals with large differences in their body sizes.
When a larger snail (acting as the male) approached a smaller one in an encounter, the latter was likely to show rejection behaviors (e.g., shell swinging; De Witt, 1991).
Thus, De Witt provided the first empirical indication that the size-dependent gender role can be explained by applying the size advantage model even in simultaneous hermaphrodites.
Small individuals had a significant tendency to act as the male and positively selected large snails as the female partner in both triple combinations.
However, the large individual acted as both the male and the female with nearly equal frequency.