Morphological peculiarities in the integument of enigmatic anomalurid gliders (Anomaluridae, Rodentia)

Scaly-tailed squirrels, the most poorly known group of gliding mammals, hold the record for variety of remarkable integument peculiarities. One of the most striking of these features is the scales on the tail, which apparently allow them to reduce energy costs when positioning themselves on a tree trunk. No less interesting is a peculiar spur that supports the flying membrane: the unciform element ('spur'). Despite the peculiarity of such elements, their nature has not yet been studied. Using anatomical, histological methods and scanning electron microscopy we studied the structure of the skin and its derivatives in five of the six species from both genera of extant gliding scaly-tailed squirrels (Anomaluridae, Rodentia): Idiurus macrotis, Idiurus zenkeri, Anomalurus beecrofti, Anomalurus pusillus and Anomalurus derbianus. In addition to the common mammalian skin structures, such as hair, vibrissae, sebaceous glands, meibomian glands of eyelids and eccrine sweat glands of the palmar and plantar pads, these animals have unique species-specific skin derivatives (the tail scaly organ and its specific glands, vibrissae of the withers, patagium and its hair brush) that play a significant role in their adaptation to gliding and to their environment in general. The structure of the elbow spur is also described and hypotheses on its evolutionary origin from the tendon of the triceps muscle are presented.