Selection for increased male size predicts variation in sexual size dimorphism among fish species

Variation in the degree of sexual size dimorphism (SSD) among taxa is
generally considered to arise from differences in the relative intensity
of male-male competition and fecundity selection. One might predict,
therefore, that SSD will vary systematically with: 1) the intensity of
sexual selection for increased male size, and 2) the intensity of
fecundity selection for increased female size. To test these two
fundamental hypotheses, we conducted a phylogenetic comparative analysis
of SSD in fish. Specifically, using records of body length at first sexual
maturity from FishBase, we quantified variation in the magnitude and
direction of SSD in >600 diverse freshwater and marine fish
species, from sticklebacks to sharks. Although female-biased SSD was
common, and thought to be driven primarily by fecundity selection,
variation in SSD was not dependent on either the allometric scaling of
reproductive-energy output or fecundity in female fish. Instead,
systematic patterns based on habitat and life history characteristics
associated with varying degrees of male-male competition and paternal
care, strongly suggest that adaptive variation in SSD is driven by the
intensity of sexual selection for increased male size.