An evolutionary explanation of female-biased sexual size dimorphism in
North Sea plaice, Pleuronectes platessa L.
Abstract
Sexual size dimorphism (SSD) is caused by differences in selection
pressures and life-history tradeoffs faced by males and females.
Proximate causes of SSD may involve sex-specific mortality, energy
acqui-sition, 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: (1)
male investments into male repro-ductive behavior might detract energy
resources that would otherwise be available for somatic growth, and (2)
diminishing returns on male reproductive investments might lead to
reduced energy acquisition. In general, both of these can bring about
smaller male body sizes. We report the following findings. First, higher
investments into 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 account-ing 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 qualitative-ly 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 into reproduction than growth,
potentially implying lower survival rates relaxing male-male
competition. Fourth, a good quantitative fit is achieved by combining
both mechanisms while accounting for costs males incur during spawning.