Mechanical properties of snake skin vary longitudinally, following large prey ingestion and among species

The ability for snakes to ingest large prey (macrostomy) is a widespread, derived trait that involves distending the skin during ingestion and metabolic upregulation during digestion. The material behavior of the skin must accommodate significant stretch associated with a large prey bolus, but data remain sparse for how the material properties of snake skin vary: longitudinally within an individual, after ingesting large prey and among species. To test whether these three factors affected the mechanical properties of snake skin, we quantified uniaxial stresses and strains in circumferential loops of skin from the neck, mid-body and tail of fasted and recently fed Boa constrictor. We also tested skin from several pre-cloacal longitudinal positions in fasted snakes that included two non-macrostomates (Afrotyphlops lineolatus, Anilius scytale) and a highly specialized macrostomate species that eats only bird eggs (Dasypeltis gansi). For B. constrictor, the anterior-most skin failed at higher strains for fed (mean±s.e.m. 2.17±0.10) compared with unfed individuals (1.80±0.04), and maximal stiffness (Young's modulus) had a significant increase posteriorly. The values of Young's modulus for the anterior-most skin of D. gansi (0.050±0.014 MPa) were by far the lowest observed both within that species and among all species. The material properties of skin of the two non-macrostomate species had little longitudinal variation. Hence, the extent of longitudinal variation in skin properties is both species dependent and affected by feeding. The more distensible skin in macrostomates relative to the non-macrostomate species tested suggests that more compliant anterior skin is a derived trait that facilitates macrostomy.