Behavioural determination of visual function in the koala (Phascolarctos cinereus)
Schmid, LM, Schmid, KL & Brown, B 1991, Wildlife Research, vol. 18, no. 3, pp. 367-374.
Reduction of vision in koalas was found here to result in significant behavioural changes. Maintaining a minimum level of vision appears necessary for koalas to be able to locate and climb trees, as with severely obstructed vision, captive koalas were not able to reach or climb a nearby tree.
This experiment involved ten male adult koalas housed at Lone Pine Koala Sanctuary. Fabric hoods were designed that systematically reduced the vision of koalas to four different extents. The behaviour of each of the ten koalas wearing each of the four hoods was examined and compared to that without wearing a hood. Reduced vision as a result of the hood was found to have the following effects on koala behaviour: (1) decreasing the number of steps taken per second; (2) increasing the height of the first leglift; (3) increasing the average height of the leglift; (4) decreasing the maximum height of tree climbing; and (5) decreasing the speed of climbing up the tree. Specifically, for (1), number of steps taken per second generally decreased with increasing hood density but, interestingly, step number per second with ‘hood 1’, the lightest material, was higher than step number per second with no hood. Additionally, for (4) and (5), koalas climbed highest and fastest when wearing hood 1.
Given the general trend observed for behaviours to shift proportionally to the degree of vision lost, it was unexpected that the koalas’ speed on the ground, climbing speed and climbing height peaked when hood 1 was worn. This may be because koalas felt threatened while wearing the hood, resulting in aversive behaviour. It should be noted that, in many ways, the conditions of this experiment did not simulate the natural conditions experienced by wild koalas, and the results must be interpreted with caution.
Due to the experimental limitations of this study, the authors are unable to evaluate whether koalas in the wild are able to adapt to reduced vision. The fact that koalas were observed to adjust their behavior in response to obscured vision, however, is important as ocular chlamydial infection is known to cause bilateral or unilateral loss of vision in wild koalas. Such behavioural changes may contribute to increased mortality in these koalas.
Summarised by Zilong Du
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