LOCOMOTION IN DROMICIOPS-AUSTRALIS (MARSUPIALIA, MICROBIOTHERIIDAE)

Cinematography was used to record locomotion in two adult male Dromici ops 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. F ilm records were analysed to determine locomotor velocity, stride leng th and gait. Locomotor speed and stride length were not affected by su bstratum, but gait was. Truly symmetrical gaits were used by both anim als across a range of speeds on the narrowest dowel. These were charac terised by diagnoal couplets of support and a diagonal sequence of lim b activition. During locomotion on the other substrata, gaits characte rised by slight asymmetry were generally used. The most common of thes e was one in which diagonal support couplets predominated and each hin dlimb was activated slightly ahead of the contralateral forelimb. At h igher speeds on the 39-mm dowel the animals sometimes used the half-bo und and transverse gallop. The duration of the locomotor cycle decreas ed exponentially with increasing speed and seemed not to be influenced by substratum diameter, once speed was taken into account. Stride len gth increased exponentially with speed and also appeared independent o f substratum. With symmetrical gaits, the relative phase lag of foreli mbs 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 seque nce run. Two of the symmetrical gaits used by Dromiciops are similar t o those used by arboreal didelphids and phalangeroids and by most prim ates, but are rarely used by other mammals. These gaits appear especia lly suited to locomotion on narrow branches, suggesting that this spec ies may utilise such substrata to a significant extent in nature. The asymmetrical gaits used by Dromiciops, the half-bound and transverse g allop, are used by various other quadrupedal marsupials, although not commonly by arboreal didelphids. It is conjectured that the symmetrica l marsupials, although not commonly by arboreal didelphids and phalang eroids were present in ancestral marsupials and that the latter forms also used the half-bound or transverse gallop. Absolute speed at the t ransition from symmetrical to asymmetrical gaits ('trot-gallop transit ion') exceeded that predicted by allometric equations derived from dat a for terrestrial placental mammals. Relative speed (as measured by th e square root of the Froude number) was also higher at this transition than in placentals.