Sexual size dimorphism in swine denies Rensch"s rule

Sexual dimorphism, defined as a phenotypic difference between males and females of a species, is a common phenomenon in animals. Rensch"s rule describes the pattern of sexual size dimorphism, claiming that larger species generally exhibit higher male to female body size ratios. Offering domesticated animals excellent opportunities for testing predictions of functional explanations of Rensch"s theory, we have tested in this paper whether the morphological size of domestic pig breeds follows this rule. We analysed the literature data on adult body size (live weight and withers height) of males and females in 130 contemporary domestic swine breeds and 4 wild Sus species. The analysis confirmed that the pattern of sexual size dimorphism in domestic swine does not conform to Rensch"s rule. It is proposed that this is due to the fact that males and females have been subjected not solely to a sexual selection regimen, but also to environmental factors, interspecific competition with other domestic species, an increase of intersexual food competition, poor feeding resources, and reproductive functional constraints. Considering all of the breeds studied, it is also likely that different counteracting selective pressures exist worldwide