On being thick-skinned: dermal shields in large mammalian herbivores

The skin is a complex organ with many functions. Skin thickness is unevenly distributed over the body of all mammals. Distributions of skin thickness in several metatherian and eutherian herbivores were measured and are illustrated; distributions reported in other studies are discussed. While the distribution of the thinnest areas of skin is much the same in all species, areas of pronounced thickening differed, in their location, between species. They also tended to characterize adult males (rather than females or juveniles). Previously advanced explanations for these distributions are reviewed. The most universally plausible hypothesis is that they act as shields against blows received in intraspecific aggression. The species-specific patterns of distribution of skin-thickening are compared with species' weaponry and fighting style. The repeated matching of distributions of probable blows and thickened skin in metatheria and eutheria, and in species that strike with feet, teeth or cranial weapons (each type of weapon and fighting style having evolved several times amongst the reviewed species), supports the hypothesis strongly.