Australia: The Land Where Time Began

A biography of the Australian continent 

Kangaroos - hearing and vision

Vision and hearing are the most important sensory systems in kangaroos and wallabies, though they vary between species to suit the lifestyle of the particular species. The eyes are high on the skull of large kangaroos and wallabies, for a wide field of vision, and the visual field of the 2 eyes overlaps by about 25 %, and their visual capacities are similar to those of rabbits and ungulates, rather than other animals such as cats and humans. The peripheral field of vision of the tammar wallaby is about 324o, of which 50o is binocular vision. The visual field of the rabbit is 360o, of which 24o is binocular vision, in the cat the visual field is 186o, but with 98o overlap. Like the rabbit, the tammar wallaby's peripheral vision allows it to see movement in almost every direction, but unlike the rabbit, it can see its hands precisely because of the greater amount of binocular vision (Wimbourne et al., 1999).

The concentration of light sensitive cells varies across the retina, from very high on the central horizontal plane of the retina, the visual streak, to much fewer in the peripheral parts of the retina. The part of the visual streak where the forward binocular vision focuses has the highest density of photoreceptor cells on the entire retina (Hemmi & Grunert, 1999). The densest concentration of ganglia associated with the nerve signals from the retina is also on the visual streak. The visual acuity of the tammar is sharpest along the visual streak, each ganglion cell receiving signals from no more than a few photoreceptor cells, whereas the ganglion cells receiving input from the periphery receive it from many photoreceptors, the result being that detection of movement is highest in the periphery of the retina.

The normal field of vision of the tammar and the larger kangaroos is from the horizon to the area immediately around them, and this is believed to be the reason for the arrangement in which the most acute part of the retina is along the horizontal streak (Tyndale-Biscoe, 2005). The visual streak is lacking in the arboreal species, such as tree kangaroos, and the densest concentration of both photoreceptors and ganglions is in the centre of the retina in these (Dunlop et al., 1988).

A study of the visual acuity of the tammar has found it to be higher than in most other small mammals, but slightly less than in the cat and a great deal less than in humans (Hemmi & Mark, 1988). The visual acuity of the tammar is believed to be as a result of its hand-eye coordination. Studies have found that the tammar wallaby has colour vision (Hemmi, 1999), especially at frequencies of 420-500 nm, the blue to green part of the spectrum, the visual acuity at this range being so great that it can discriminate 2 monochromatic colours differing by as little as 20nm. It has 2 types of photoreceptor cone, situated in visual streak, for blue and for green, so it cannot discriminate colour in the yellow to red range.  The presence of colour vision in a macropod led to questions about the possibility of colour vision in other macropod species, such as the presence of yellow-sensitive cones in rock-wallabies, species of which are often of colours from russet to yellow, to allow them to recognise such colours (Tyndale-Biscoe, 2005).

The large ears of kangaroos and wallabies can each rotate 180o, so between them they can scan the full 360o. Studies using sound receivers near the ear drums of tammars have found that the acoustic pressure increased 25-fold in the optimum range from 3-8 kHz when the ears were pointed towards a sound source, and to a lesser amount at frequencies higher or lower than this range. When the ear was pointed away from the sound source the level of sound detected dropped sharply, allowing the tammar to pinpoint the location of the sound source very precisely by moving the pinna. As a result of the ears being moved independently of each other the tammar can detect sounds from different parts of the environment at different frequencies and intensity.

The ears of tree kangaroos, unlike those of large kangaroos and wallabies, are short and rounded and non-mobile. It is believed this is the result of no longer needing to be alert to the presence of terrestrial predators in the environment all around them since the move to the trees (Tyndale-Biscoe, 2005).

Sources & Further reading

Tyndale-Biscoe, Hugh, 2005, Life of Marsupials, CSIRO Publishing.


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                                                                                           Author: M.H.Monroe  Email:     Sources & Further reading