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

Abstract

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: a) Freedom of movement, b) surface area enhancement, c) increased structural extensibility, d) lubrication, and e) 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 45o 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 as their skins seem to fit in all functionally adaptive categories.

Male spiny frogs enter the underwater battlefield with loose skin exhibiting enhanced penetration of capillaries into the epidermis

“…5 ), implying that they covaried with selective pressure from stabbing. These findings can be explained by the increased extensibility of the breeding male’s loose skin conferring an advantage against stabbing because the skin has to be stretched before being punctured [ 30 ]. Related to the engineering design of exchange surfaces, this implies a case of looseness contributing to both increasing the surface area and preventing damage.…”

“…Once caused by stabbing, even tiny open wounds with broken subepidermal capillaries may further disadvantage the animal by chemically attracting leeches [ 29 ]. On the other hand, selection for a thick epidermis in breeding males may be reduced by the increased structural extensibility of their loose skin, which must be stretched before being punctured [ 30 ]. The significance of this effect of skin looseness on amphibian life history remains unknown.…”

Underwater calling behavior in two high‐altitude frog species

“…These behaviors are not likely mutually exclusive and very loose skin can be an indicator of an aquatic lifestyle. Other possible effects of skin looseness include increased freedom of movement and structural extensibility (Uyeno & Clark, 2020). A loose sheath can allow for a large range of core motion; and puncturing loose skin requires the skin being stretched first.…”