Ecological Selection and the Evolution of Body Size and Sexual Size Dimorphism in the Galapagos Flightless Cormorant

Abstract

Several hypotheses that involve either sexual selection (intra- and intersexual) or disruptive ecological selection (e.g., niche divergence, reproductive role division) or both have been advanced as adaptive explanations for the evolution and maintenance of sexual size dimorphism (SSD). However, ever since Darwin, the prevalent explanation in species with male-biased SSD has been intrasexual selection favoring larger size in males in the competition for mates. Here, I show that in the Galapagos Flightless cormorant (Phalacrocorax harrisi; Phalacrocoracidae), the sexes differ significantly in body mass and in all five external morphometric traits measured, with body mass being the most dimorphic trait followed by bill width and bill depth. Correlations between morphometric traits and between morphometric traits and body mass differed by trait and by sex. Comparative analyses including 16 other species within the family showed that the Flightless cormorant is the largest phalacrocoracid. Several factors are theoretically likely to favor the evolution of larger size and greater SSD in species where the male is larger than the female. However, sexual selection favoring larger size in males through competition for mates or by mate choice all seem unlikely explanations for SSD in the Flightless cormorant. Here, I argue that the main driving force for the evolution of the species’ larger size is disruptive ecological selection involving selection for larger body size in males as an adaptation for larger prey and genetic correlation between the sexes for body size explain the increased size and larger sexual dimorphism of the species. Comparative analyses also showed that the Flightless cormorant has a significantly greater sexual dimorphism in both body mass and bill depth, but dimorphism in bill length was similar to that of other Phalacrocoracidae. Thus, the Flightless cormorant is the most sexually dimorphic of the Phalacrocoracidae. The degree of sexual dimorphism in all traits correlated positively and strongly with mean body mass of each sex and with the mean of both sexes combined. However, the slope of reduced major axis (RMA) regressions of male traits as function of female traits, except for bill depth, did not depart significantly from geometric isometry (β = 1.0), showing that Phalacrocoracidae do not follow Rensch’s rule. Thus, among phalacrocoracids, variation in body size fully explains variation in the degree of interspecific variation in SSD and sexual dimorphism in other traits. This means that a remarkable SSD and sexual dimorphism in other traits in the Flightless cormorant relative to other phalacrocoracids can simply be attributable to the species’ largest size; thus, the magnification of sexual dimorphism, including SSD, is an effect of disruptive ecological selection favoring larger size in males and consequently a lower rate of increase in female size as a correlated response similar to that in other phalacrocoracids resulting from genetic correlation between the sexes for body size. I also suggest that the remarkable larger size (gigantism) and remarkable sexual dimorphism of the Flightless cormorant are both novel character states that have evolved in situ following colonization of the Galapagos Islands.