On the fit of skins with a particular focus on the biomechanics of loose skins of hagfishes

There is a considerable diversity in how skins fit. Here, we review the function of both tight and loose skins and note that the latter are poorly understood. Analysis of loose skin examples suggest five functional categories: (I) freedom of movement, (II) surface area enhancement, (III) increased structural extensibility, (IV) lubrication, and (V) maladaptive examples arising through sexual or artificial selection. We investigate the skins of hagfishes as a model for understanding loose skin function by examining its structure using histology, standardized puncture resistance testing using the ASTM F1306 protocol, and the effect of internal pressure using a simple inflated balloon model. Skins of hagfishes are composed of multiple layers of cross-helically wound connective tissue fibers of a 45 degrees angle to the longitudinal axis, resulting in a skin that functions as fabric cut on the bias. Hagfish skins are relatively yielding; however, skin looseness adds a structural extensibility that may allow hagfishes to compensate for low puncture resistance. Physical balloon models, with stiff cores that limit length changes, show that only low pressures allow short loop radii without local buckling. Hagfishes represent ideal organisms for studying loose skin function because their skins seem to fit in all functionally adaptive categories.