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

<p><a name="_Hlk23424265">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 </a>records of body length at first sexual maturity from FishBase, we quantified variation in the magnitude and direction of SSD in &gt;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.</p>