Foot postures and grasping of free-ranging Sunda colugos (Galeopterus variegatus) in West Java, Indonesia

Abstract

Colugos (order Dermoptera) are medium-sized nocturnal arboreal eutherian euarchontan mammals, which glide, climb vertically and hang from different arboreal substrates in the rainforests of Southeast Asia. Their close phylogenetic position to either Primates (as Primatomorpha) or Scandentia (as Sundatheria) renders them significant for understanding evolutionary-adaptive trends of key features within the Euarchonta (primates, scandentia, dermoptera, and plesiadapiformes). In this context, we studied foot positional, postural, and grasping patterns in relation to substrate use in free-ranging Sunda colugos (Galeopterus variegatus) in west Java, Indonesia. On large, strongly inclined substrates, colugos primarily used claw climbing and clinging activities, with a dorsiflexed, abducted, and everted foot using an abducted clawed grasp and abducted hallux. On small, less inclined substrates, colugos habitually used suspensory locomotion and postures with pedal plantarflexion, adduction, inversion, and an adducted clawed grasp and adducted hallux. The morphofunctional similarity of the colugo foot with that of early euarchontans suggests comparable behaviors at the base of euarchontan evolution. Furthermore, the functional-adaptive significance of these behaviors is further investigated under different phylogenetic scenarios, linking dermopterans to either scandentians or primates. Our findings underscore the importance of similar behavioral studies for examining functional-adaptive evolutionary scenarios.