Hyaluronan and skin elasticity: Are subterranean mammals special?

It has been hypothesized that subterranean mammals have evolved increased skin elasticity to reduce friction when moving through their underground tunnel systems. This trait is commonly believed to be mediated by greatly elongated hyaluronan (HA) polymers in the dermal extracellular matrix, which have been reported from different distantly related burrowing mammals. However, replicating these findings has proven difficult, and a mechanism by which HA polymer size could modify skin elasticity has not been proposed. In fact, experimental data on skin biomechanics in burrowing mammals are currently unavailable. Here, we quantify the molecular mass of HA polymers extracted from the tissues of Ansells mole-rat (Fukomys anselli), a burrowing rodent yet unstudied in that respect, and investigate skin biomechanics in subterranean and epigeic small mammal species by means of in vivo-cutometry. We did not recover extremely elongated HA polymers in Ansells mole-rat, conflicting with published findings in congeneric species and the naked mole-rat. Polymer length in mole-rats was found to be moderately increased compared to guinea pigs across tissues, though. Our data on skin biomechanics indicate that subterranean mammal skin is not more elastic than that of epigeic forms. Interestingly, the skin of the naked mole-rat was characterized by very high stiffness. Uniaxial tensile tests demonstrated that it also exhibits exceptional tensile strength. Hence, we challenge the idea that hyaluronan or a subterranean ecology notably influences skin elasticity in small mammals and suggest that previous studies may have confused elasticity with skin looseness, a fundamentally different phenomenon.