Abstract
The arboreal habitat is a complex mesh of intertwining elements, which make locomotion challenging. Nevertheless, mammals have managed to cope with its complexity. During above-branch arboreal locomotion, the necessity for safety and stability is greatly stressed. Moreover, as early eutherian mammals exhibit postcranial correlates for arboreality, it is important to assess the behavioural mechanisms that are related to competent arboreal locomotion. However, there is a lack of research in small mammals. Therefore, we examined eight small-bodied mammals: Acrobates pygmaeus (~12 g), Micromys minutus (~10 g), Apodemus agrarius (~20 g), Apodemus flavicollis (~30 g), Thallomys paedulcus (~70 g), Myodes glareolus (~20 g), Muscardinus avellanarius (~20 g), and Glis glis (~130 g). All specimens were filmed walking on various arboreal substrate diameters (2mm, 5mm, 10mm, 25mm) and inclinations (45° descending, horizontal, 45° ascending) at 240 fps in a specially designed terrarium. Our analyses ...
The arboreal habitat is a complex mesh of intertwining elements, which make locomotion challenging. Nevertheless, mammals have managed to cope with its complexity. During above-branch arboreal locomotion, the necessity for safety and stability is greatly stressed. Moreover, as early eutherian mammals exhibit postcranial correlates for arboreality, it is important to assess the behavioural mechanisms that are related to competent arboreal locomotion. However, there is a lack of research in small mammals. Therefore, we examined eight small-bodied mammals: Acrobates pygmaeus (~12 g), Micromys minutus (~10 g), Apodemus agrarius (~20 g), Apodemus flavicollis (~30 g), Thallomys paedulcus (~70 g), Myodes glareolus (~20 g), Muscardinus avellanarius (~20 g), and Glis glis (~130 g). All specimens were filmed walking on various arboreal substrate diameters (2mm, 5mm, 10mm, 25mm) and inclinations (45° descending, horizontal, 45° ascending) at 240 fps in a specially designed terrarium. Our analyses concerned basic gait parameters such as diagonality, duty factor, duty factor index, velocity, stride length and stride frequency. Most examined small clawed scansorial mammals primarily opted for lateral-sequence gaits during arboreal locomotion. Acrobates pygmaeus was a notable exception, displaying diagonal-sequence gaits convergently to primates. Most small mammals reduced diagonality, the relative swing phases of the limbs, and velocity on narrower substrates to increase stability, and promote continuous navigation. On wider substrates, most small mammals increased velocity, and relative swing phases, and in some cases the frequency of asymmetrical gaits, assisting in swift movements, obstacle avoidance, and energetic optimization, and possibly minimising exposure to predators. The effect of inclination on gait parameters and metrics was dissimilar between examined species, implying that there is probably much underlying complexity, and various mechanisms that affect it. In terms of velocity regulation, almost all small mammals increased velocity primarily by stride frequency, rather than stride length, which also promotes arboreal safety, by reducing body oscillations. Moreover, some examined parameters, such as duty factor and stride frequency, predited the degree of arboreality consistently. Increased duty factor and decreased stride frequency in more terrestrial species accommodated stability by reducing touchdown torques, increasing simultaneous contact points and contact periods with substrates for which they are not well adapted. On the other hand, decreased duty factor and increased stride frequency on more arboreal species facilitates obstacle avoidance and manoeuvrability. Diagonality, which has often been linked to arboreality, was not a good predictor of the level of arboreality, and does not seem to be necessary for successful arboreal locomotion, at least in small mammals. In conclusion, despite some limited dissimilarities, small arboreal and scansorial mammals display behaviours that ultimately contribute to the successful exploitation of the arboreal milieu. Body size and morphological similarities between extant arboreal/scansorial small mammals and early eutherian ancestors (e.g. Eomaia and Juramaia) suggest that these early forms would have behaved in similar ways, displaying lateral-sequence gaits, relatively high duty factors, and increasing velocity mainly by increasing stride frequency. These adaptations, which would have assisted in the efficient use of both terrestrial and arboreal substrates, very likely contributed to their successful dominance and subsequent radiations.
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