Pedal grasping in an arboreal rodent relates to above‐branch behavior on slender substrates

Abstract

In order to exploit the three-dimensional, discontinuous and unstable arboreal milieu, most arboreal mammals employ efficient foot (pedal) grasping that establishes firm contact with the substrate and enables secure and safe locomotion and postures. Such pedal grasp can be performed in variable ways in relation to substrate characteristics. In order to investigate the interplay between pedal grasping modes, arboreal locomotor and postural behavior, and substrate size and inclination in arboreal mammals, we filmed and quantitatively analyzed these behaviors in captive African woodland dormice. Our observations revealed that African woodland dormice exhibited a flexible locomotor and postural repertoire, mainly on small and horizontal substrates. Hallucal grasping was the main pedal grasping mode and associated with walk, clamber and stand on mainly small-to medium-sized horizontal substrates, whereas claws were commonly used during vertical climb and cling on medium-sized to large vertical substrates. These results clearly demonstrate the importance of hallucal grasping for balancing above, and not below, small-and medium-sized arboreal substrates of moderate inclinations. Therefore, we assert that efficient pedal grasping and its associated positional behavior may have been fundamental for the differentiation and successful radiation of virtually all early mammals that sought refuge in tree canopies, enabling efficient utilization of the small branch niche. Evidence suggests that extant arboreal rodents offer useful functional morphology evidence of the transitional stages that lead to arboreal specialists, euprimates included, and research toward this direction has much to offer to our understanding of evolutionary adaptations exhibited by small-bodied arboreal mammals.

Volumetric spatial behaviour in rats reveals the anisotropic organisation of navigation

“…This is surprising given that (a) it would presumably be quite difficult for the rats to maintain a horizontal foraging strategy in the tilted lattice where all of the bars ran diagonally and (b) in the tilted lattice movement along all three dimensions of the maze was easier than vertical movements in the aligned lattice and all three were equal in terms of energy expenditure. Previous research has similarly reported a horizontal bias in the movements of rats climbing cubic lattices (Grobéty and Schenk 1992 ; Jovalekic et al 2011 ), captive tamarins and marmosets climbing branches (Chamove and Goldsborough 2004 ), marmosets climbing cubic lattices (Schenk et al 1995 ), captive African woodland dormice climbing unstructured branches (Youlatos et al 2015 ) and wild arboreal dormice climbing trees (Bright and Morris 1991 ). In contrast, Flores-Abreu et al ( 2014 ) found that rats moved vertically more than horizontally in a cubic lattice, although this was likely because the animals were making direct paths to reward sites which were horizontally closer on average to their starting positions than vertically.…”

“…One possibility for these differences is that it was less easy for rats to rest in the maze. Climbing rodents generally prefer horizontal branches over oblique and vertical ones for travelling, foraging and nesting (Meserve 1977 ; Urbani and Youlatos 2013 ; Youlatos et al 2015 ). We never tested if rats preferred one lattice orientation over the other, but together, these results suggest that with the pervasive slope in the tilted lattice rats may have needed to rest more frequently.…”

Arboreality in acacia rats (Thallomys paedulcus; Rodentia, Muridae): gaits and gait metrics

“…In order to reap the advantages of arboreality, rodents (as other arboreal mammals) must safely and skilfully negotiate substrates of variable size, inclination, length and flexibility, in the complex, three‐dimensional and discontinuous habitat of the different forest layers (Cartmill, ; Hildebrand, ). Towards this end, the morphology and locomotor and postural behaviour are accordingly specialized (Cartmill, ; Essner, ; Kirk et al ., ; Samuels & Van Valkenburgh, ; Schmidt & Fischer, ; Camargo et al ., , ; Youlatos et al ., ). Regarding locomotion, gaits can function as a mechanism that promotes stable and safe displacement along arboreal substrates (Cartmill, Lemelin & Schmitt, , ; Lammers & Gauntner, ; Lemelin & Cartmill, ; Lammers & Zurcher, ,b).…”

Mechanics of Arboreal Locomotion in Swinhoe’s Striped Squirrels: A Potential Model for Early Euarchontoglires

“…Considering the likely small body size as suggested by stem placental fossils (Ji et al, 2002;Luo et al, 2011;Youlatos et al, 2015) and early members of the Euarchontoglires (e.g., plesiadapiforms, Silcox and López-Torres, 2017), small body size should be one of the main considerations when identifying potential modern analogs for early primates and further early Euarchontoglires (Nyakatura, 2019). This is especially important considering the size threshold for effective asymmetrical locomotion proposed by Chadwell and Young (2015).…”