In this case, the larger one takes the female role, the small one the male role. Less energy is required to produce sperm than to produce eggs.

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This pattern of mating behavior has been reported for freshwater snails such as (De Witt, 1991).

There are only a few reports on testing the size advantage model in simultaneous hermaphrodites (Angeloni et al., 2002; De Witt, 1996; Schärer et al., 2001; St. De Witt (1996) reported that the female role snail of was larger than was the male role in a copulating pair in nature as well as in the laboratory.

Although the topic of preferred gender role is one of the central research subjects, this suggestion has never been tested in mating behavior experiments that controlled for body size differences between paired snails.

In the present study, we conducted experiments on mating behavior in order to determine whether the size advantage model can explain the relation between the body size and gender role in a simultaneous hermaphroditic freshwater snail, , which continues to grow even after sexual maturation.

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.

Smaller individuals tended to play the “male” role (sperm donor), and larger snails the “female” (sperm recipient).

Next, we analyzed the mating behaviors involved in gender-role decision in snail pairs of three different body-size combinations, using “large” and “small” snails.

However, De Witt (1996) did not report actual body size distributions within the observed populations.

The distribution of body sizes could be such that by chance there are frequent encounters between individuals with large differences in their body sizes.

A hypothesis was constructed based on the size advantage model: small snails should prefer to act as the male, and large snails as the female.