Gliding is a crucial adaptation to arboreal habitats in several groups of mammals.Along with certain advantages it imposes limitations on the quadrupedal running since it affects the locomotor apparatus. To estimate the impact on quadrupedal running in gliders, the feathertail glider (Acrobates pygmaeus) was chosen considering that the small size allows minor morphological modifications for aerial locomotion.The gaits were studied on horizontal flat substrate which made it possible to compare running technique of the glider with that of terrestrial mammals. In all analyzed plots the footfall sequence was found to be asymmetrical; in most cases the bound was used, in contrast, the gallop occurred only occasionally. The half-bound with the fore lead, the most usual asymmetrical gait in quadrupedal marsupials, was much less common in A. pygmaeus than the bound; the rare among mammals half-bound with the hind lead was also found. The bound was not only the most common gait but also the steadiest one; therefore we can conclude that A. pygmaeus uses all other asymmetrical gaits as transitional forms associated with changes in speed, direction, etc. The bound with extended suspension is probably preferred by A. pygmaeus because it most closely resembles gliding by posture. | INTRODUCTIONGliding is an efficient way of locomotion for inhabitants of the complex three-dimensional arboreal environment. It allows the cost of transport to be reduced compared to terrestrial locomotion (Alexander, 1982;Scholey, 1986) and besides, increases the chance to avoid predators (Byrnes & Spence, 2011;Emmons & Gentry, 1983). However, gliding entails a loss of altitude and the need to compensate for this by climbing tree trunks (Panyutina, 2020;Scholey, 1986). Thus, the quadrupedal performance (at least climbing) has a crucial influence on total energy expenditures for locomotion (Dial, 2003) and either promotes the survival and positive selection of relevant forms, or, conversely, leaves no chance to win in the evolutionary race.The proportions of the locomotor apparatus of gliding mammals differ from those of their nonaerial relatives (Essner, 2007;Peterka, 1936;Thorington & Heaney, 1981). There are no clear biomechanical explanations as to the negative impact of changes in the skeletal proportions and the locomotor muscles design in gliders on the efficiency of their quadrupedal running, whereas, direct energy measurements show the existence of such an impact.Gliders spend more energy on quadrupedal running than nongliders (Flaherty et al., 2010;Shaw, 2004). However, studies on this subject are still fragmentary, and little is known about how the acquisition of gliding affects running technique. Of interest is not only the structural modification of the locomotor apparatus in gliders, but also a degree of modification of their type of quadrupedal locomotion, in case it in fact differs.In this context, it is especially intriguing to study such alterations in the smallest gliders, in which the development of aerial locomotion can be achie...

A complete kinematic analysis of the trunk and limbs was performed for the smallest gliding mammal, the feathertail glider (Acrobates pygmaeus). To compare the running technique of the glider with that of terrestrial mammals, only movement on horizontal substrate was studied. Kinematic analysis was performed separately for bound with extended suspension and bound with both suspensions. Terrestrial locomotion of the feathertail glider was classified as primitive ricochet, which is distinguished from true gallop by the absence of a delay in protraction of the hindlimbs after their take‐off. The kinematic analysis showed significant impacts of gliding adaptation on the quadrupedal running, the most striking of which are the extremely sprawling position of the limbs, in particular, the wide stance of the forelimbs, and consequently, the absence of crossing of the fore‐ and hindlimbs in gathered stage of the cycle. The degree of limbs' sprawling in the feathertail glider is closer to the reptilian condition than to the mammalian parasagittal state. The sprawling condition of the feathertail glider is secondary and therefore is designated as “deparasagittalization.”