Dichromatic colour vision in wallabies as characterised by three behavioural paradigms.

Wiebke Ebeling,Jan M Hemmi

doi:10.1371/journal.pone.0086531

Wiebke Ebeling, Jan M Hemmi

Open Access

https://doi.org/10.1371/journal.pone.0086531

Copy DOI

Journal: PloS one	Publication Date: Jan 29, 2014
Citations: 25	License type: CC BY 4.0

Affiliation: University of Western Australia, Curtin University

Abstract

Despite lacking genetic evidence of a third cone opsin in the retina of any Australian marsupial, most species tested so far appear to be trichromatic. In the light of this, we have re-examined colour vision of the tammar wallaby which had previously been identified as a dichromat. Three different psychophysical tests, based on an operant conditioning paradigm, were used to confirm that colour perception in the wallaby can be predicted and conclusively explained by the existence of only two cone types. Firstly, colour-mixing experiments revealed a Confusion Point between the three primary colours of a LCD monitor that can be predicted by the cone excitation ratio of the short- and middle-wavelength sensitive cones. Secondly, the wavelength discrimination ability in the wallaby, when tested with monochromatic stimuli, was found to be limited to a narrow range between 440 nm and 500 nm. Lastly, an experiment designed to test the wallaby’s ability to discriminate monochromatic lights from a white light provided clear evidence for a Neutral Point around 485 nm where discrimination consistently failed. Relative colour discrimination seemed clearly preferred but it was possible to train a wallaby to perform absolute colour discriminations. The results confirm the tammar wallaby as a dichromat, and so far the only behaviourally confirmed dichromat among the Australian marsupials.

Highlights

Marsupials have presented us with an unusual case of diversity in mammalian colour vision, apparently having evolved both dichromatic and trichromatic species but lacking evidence of a third photopigment in the latter
Experiment 1: LCD Colour-Mixing The cone excitation ratio (CER) of the reference stimulus was calculated from its RGB values and the monitor calibration (Figure 2), and a range of suitable test pairings were determined to include this reference CER
The results from Experiment 3– Neutral Point offer the strongest evidence in that we clearly identified a narrow range of wavelengths where white light is not distinguished from a monochromatic colour by the wallaby

Summary

Introduction

Marsupials have presented us with an unusual case of diversity in mammalian colour vision, apparently having evolved both dichromatic and trichromatic species but lacking evidence of a third photopigment in the latter. A number of more recent publications, have suggested the presence of a third cone type in the retinae of several Australian marsupials, namely the fat-tailed dunnart [7,8,9], honey possum [7], [8], bandicoot or quenda [10], and quokka [10]. Of these species, the dunnart was the only one tested in behavioural experiments and demonstrated to be able of making trichromatic colour discriminations [11]. Though, we are still lacking evidence of the gene that gives rise to this third cone type [9], [12,13,14,15]

Methods

Results

Discussion

Conclusion