Locomotion in Dromiciops-Australis (Marsupialia, Microbiotheriidae)

Abstract

Cinematography was used to record locomotion in two adult male Dromiciops australis. Both animals were run on five horizontal dowels varying in diameter from 6.3 to 39 mm and on a horizontal board 89 mm wide. Film records were analysed to determine locomotor velocity, stride length and gait. Locomotor speed and stride length were not affected by substratum, but gait was. Truly symmetrical gaits were used by both animals across a range of speeds on the narrowest dowel. These were characterised by diagnoal couplets of support and a diagonal sequence of limb activition. During locomotion on the other substrata, gaits characterised by slight asymmetry were generally used. The most common of these was one in which diagonal support couplets predominated and each hindlimb was activated slightly ahead of the contralateral forelimb. At higher speeds on the 39-mm dowel the animals sometimes used the half-bound and transverse gallop. The duration of the locomotor cycle decreased exponentially with increasing speed and seemed not to be influenced by substratum diameter, once speed was taken into account. Stride length increased exponentially with speed and also appeared independent of substratum. With symmetrical gaits, the relative phase lag of forelimbs with respect to their ipsilateral hindlimbs changed with speed so that a moderate walkig trot was replaced either by a fast waling trot or fast diagonal sequence walk and ultimately by a slow diagonal sequence run. Two of the symmetrical gaits used by Dromiciops are similar to those used by arboreal didelphids and phalangeroids and by most primates, but are rarely used by other mammals. These gaits appear especially suited to locomotion on narrow branches, suggesting that this species may utilise such substrata to a significant extent in nature. The asymmetrical gaits used by Dromiciops, the half-bound and transverse gallop, are used by various other quadrupedal marsupials, although not commonly by arboreal didelphids. It is conjectured that the symmetrical marsupials, although not commonly by arboreal didelphids and phalangeroids were present in ancestral marsupials and that the latter forms also used the half-bound or transverse gallop. Absolute speed at the transition from symmetrical to asymmetrical gaits ('trot-gallop transition') exceeded that predicted by allometric equations derived from data for terrestrial placental mammals. Relative speed (as measured by the square root of the Froude number) was also higher at this transition than in placentals.