Do costs of reproduction differ between the sexes of dioecious plants?

Costs of reproduction arise from fitness-based trade-offs between current and future reproduction. Because the average fitness of females and males is constrained to be equal, and because costs of reproduction are paid in units of fitness, the extent to which reproductive costs can differ between the sexes has been called into question.

Using expressions that incorporate the trade-off between current and future reproductive fitness, I examine whether costs of reproduction can in principle differ between the sexes. These expressions clarify the types of evidence that could be used to infer divergence in costs of reproduction between the sexes.

Costs of reproduction can differ between the sexes only insofar as females and males can diverge in their expected future reproductive potential. If the two sexes differ in their future reproductive potential and one sex prioritizes future opportunities over current output, that sex should experience lower costs of reproduction.

The expected future reproductive potential of plants is driven by their schedules of survival and reproduction, which are difficult to study for long-lived plants. However, it may be possible to infer differences in the costs of reproduction between females and males by combining two pieces of evidence: sex differences in (1) the magnitude of trade-offs between reproduction and growth or survival; and (2) the propensity to produce offspring after previous bouts of reproduction (e.g. via differences in post-reproductive growth or survival). Evidence for (1) and (2) exists widely, but they have rarely been studied together in dioecious populations, leaving little solid evidence regarding how often costs of reproduction differ between females and males.