Abstract
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.
Highlights
Sex allocation in simultaneous hermaphrodites is defined as the proportion of resources allocated to the male relative to the female functions
We examined the effects of MGS in the male function (MGSm) and MGSf on the fixed and variable costs of sex allocation in a simultaneous hermaphrodite, the sessilian barnacle Balanus rostratus
To investigate the effects of MGSm, MGSf, and that of body size, we used linear mixed-effects models (LMMs, R package lmerTest; Kuznetsova, Brockhoff, & Christensen, 2017) in which each reproductive investment and sex allocation was treated as a response variable (N = 164)
Summary
Sex allocation in simultaneous hermaphrodites is defined as the proportion of resources allocated to the male relative to the female functions. The fixed costs include producing and maintaining reproductive organs such as genitalia that are not consumed in each reproductive event, and the variable costs correspond to the resources allocated to produce the gametes (Heath, 1977; Schärer, 2009) These two costs will likely have different responses to environmental factors such as the two aspects of MGS; for instance, the variable cost is affected by the MGSs but not the fixed cost. The “fixed” cost, such as the penis, varies according to the physical environments such as wave exposures in animals such as barnacles (Neufeld & Palmer, 2008) These are seldom distinguished in empirical studies (Lorenzi, Sella, Schleicherova, & Ramella, 2005; Schärer, 2009). We examined the effects of MGSm and MGSf on the fixed and variable costs of sex allocation in a simultaneous hermaphrodite, the sessilian barnacle Balanus rostratus
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