Locomotion, postures, substrate use, and foot grasping in the marsupial feathertail glider Acrobates pygmaeus (Diprotodontia: Acrobatidae): Insights into early euprimate evolution.

Debates on early euprimate evolution are related to the understanding of the ecological context that promoted their unique adaptations. Currently, these discussions mainly revolve around the habitual use of the small-branch niche or the frequent utilization of wider, and probably, strongly inclined substrates by euprimate ancestors. The current fossil evidence implies a diversity of arboreal quadrupedal behaviors for these early euprimates, associated with the use of various types of substrates. However, inferring the positional behavior of early euprimates based exclusively on fossils fails to unravel the positional flexibility in terms of modes and substrate use, which is important for understanding key adaptations related to limb postures. Following previous research, we studied the positional behavior, substrate use and pedal grasping modes of the marsupial feathertail gliders to investigate patterns of arboreal behavior that may be analogous to those exhibited by early euprimate ancestors. For the purposes of the current study, we observed and filmed 15 male and 20 female captive adult feathertail gliders Acrobates pygmaeus (Marsupialia: Diprotodontia: Acrobatidae) in a large enclosure in the Nocturnal Pavilion of Nowe Zoo, Poznań, Poland. Our observations demonstrated a strong preference for small and for horizontal substrates, avoidance of large and of vertical ones, a diverse positional repertoire mainly composed of quadrupedalism, clambering, climbing and gliding, the last occurring from small and oblique and vertical substrates, and the dominant use of hallucal grasping, especially on small, horizontal and oblique substrates. We thus consider that the generalized profile of A. pygmaeus could fit in a stage where the euarchontan heritage of vertical clawed activities on large substrates has decreased in favor of the use of small moderately inclined substrates efficiently negotiated by diagonal sequence quadrupedalism and handled via an apparently powerful hallucal grasp. Competent use of small substrates could have further expanded into small vertical substrates, which would progressively serve as new climbing platforms and takeoff perches for unspecialized leaping. We feel that this stage may have occurred early in euprimate evolution, as small body size likely provided the necessary behavioral flexibility to exploit various niches. Depending on alternative scenarios, it could represent that of the common ancestor of euprimates or be rooted at the base of strepsirrhine evolution. This study underscores the important of analyzing the behavior of extant models to infer the locomotor evolution of euarchontans, primates or euprimates.