An evolutionary explanation of female-biased sexual size dimorphism in North Sea plaice, Pleuronectes platessa L.

Sexual size dimorphism (SSD) is caused by differences in selection
pressures and life-history trade-offs faced by males and females.
Proximate causes of SSD may involve sex-specific mortality, energy
acquisition, and energy expenditure for maintenance, reproductive tissues,
and reproductive behavior. Using a quantitative, individual-based,
eco-genetic model parameterized for North Sea plaice, we explore the
importance of these mechanisms for female-biased SSD, under which males
are smaller and reach sexual maturity earlier than females (common among
fish, but also arising in arthropods and mammals). We consider two
mechanisms potentially serving as ultimate causes: (a) Male investments in
male reproductive behavior might evolve to detract energy resources that
would otherwise be available for somatic growth, and (b) diminishing
returns on male reproductive investments might evolve to reduce energy
acquisition. In general, both of these can bring about smaller male body
sizes. We report the following findings. First, higher investments in male
reproductive behavior alone cannot explain the North Sea plaice SSD. This
is because such higher reproductive investments require increased energy
acquisition, which would cause a delay in maturation, leading to
male-biased SSD contrary to observations. When accounting for the observed
differential (lower) male mortality, maturation is postponed even further,
leading to even larger males. Second, diminishing returns on male
reproductive investments alone can qualitatively account for the North Sea
plaice SSD, even though the quantitative match is imperfect. Third, both
mechanisms can be reconciled with, and thus provide a mechanistic basis
for, the previously advanced Ghiselin–Reiss hypothesis, according to which
smaller males will evolve if their reproductive success is dominated by
scramble competition for fertilizing females, as males would consequently
invest more in reproduction than growth, potentially implying lower
survival rates, and thus relaxing male–male competition. Fourth, a good
quantitative fit with the North Sea plaice SSD is achieved by combining
both mechanisms while accounting for sex-specific costs males incur during
their spawning season. Fifth, evolution caused by fishing is likely to
have modified the North Sea plaice SSD.