Subcutaneous compliance and gravitational adaptation in snakes

Previous studies have implicated morphological adaptations as important counter-measures to gravitational stresses on the circulatory systems of arboreal or climbing snakes. Such features include tight skin and relatively non-compliant tissue compartments that oppose edema formation, but these traits have not been previously studied quantitatively. To provide information on this subject, interstitial fluid pressures were measured in eleven species of snakes using slit-end catheters positioned in subcutaneous tissue located at the base of the tail. Interstitial pressures in all species were typically 0 to +2 mm Hg in normally hydrated tissue, but varied widely when snakes were active or when the tail was curved at the site of measurement. Local compliance of the free fluid space was determined from measurements of pressure while saline was infused via the catheter tip. Such measurements varied from 0.18 microliters/mm Hg in Philodryas baroni, an arboreal species, to 2.3 microliters/mm Hg in Crotalus adamanteus, a ground-dwelling, terrestrial species. In general, compliance of the subcutaneous tissue space was greatest (P < 0.05) in non-climbing and aquatic species of snakes that do not face problems of gravitational edema in dependent tissues. Presumably, the compliance measurements reflect adaptive structural differences related to requirements for counteracting gravitational stresses in the various species.