Different effects of mating group size as male and as female on sex allocation in a simultaneous hermaphrodite

Sex allocation theory predicts that the optimal sexual resource allocation
of simultaneous hermaphrodites is affected by mating group size (MGS).
Although the original concept assumes that the MGS does not differ between
male and female functions, the MGS in the male function (MGSm; i.e., the
number of sperm recipients the focal individual can deliver its sperm to
plus one) and that in the female function (MGSf; the number of sperm
donors plus one) do not always coincide and may differently affect the
optimal sex allocation. Moreover, reproductive costs can be split into
“variable” (e.g., sperm and eggs) and “fixed” (e.g., genitalia) costs, but
these have been seldom distinguished in empirical studies. We examined the
effects of MGSm and MGSf on the fixed and variable reproductive
investments in the sessilian barnacle Balanus rostratus. The results
showed that MGSm had a positive effect on sex allocation, whereas MGSf had
a nearly significant negative effect. Moreover, the “fixed” cost varied
with body size and both aspects of MGS. We argue that the two aspects of
MGS should be distinguished for organisms with unilateral mating.