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First published online May 5, 2005
Journal of Experimental Biology 208, 1803-1815 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01610

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The ecology of visual pigment tuning in an Australian marsupial: the honey possum Tarsipes rostratus

Petroc Sumner^1,*, Catherine A. Arrese² and Julian C. Partridge³

¹ Department of Visual Neuroscience, Division of Neuroscience, Faculty of Medicine, Imperial College London, St Dunstan's Road, London W6 8RP, UK
² School of Animal Biology, University of Western Australia, Crawley, WA 6009, Australia
³ School of Biological Sciences, University of Bristol, Bristol, BS8 1UG, UK

View larger version (157K): [in a new window]   Fig. 1. Banksia attenuata (3 m high) showing inflorescences ranging from immature (green) though mature (yellow) to senescent (brown/grey). Also shown is shrub of Eremaea beaufortioides (orange flowers).

View larger version (25K): [in a new window]   Fig. 2. Calculated signal-to-noise ratios achieved by different hypothetical L cone tuning (see Materials and methods) for the visual task of detecting flowers amongst their visual backgrounds (largely leaves and bark of vegetation). For both immediate and more general backgrounds the L cone tuning of the honey possum offers higher signal-to-noise ratios than the L cone tunings of other marsupials (e.g. quokka), but the best L cone would have a max more long-wave than that found in any marsupials.

View larger version (25K): [in a new window]   Fig. 3. Calculated noise-scaled chromatic distances achieved by different hypothetical L cone tuning (see Materials and methods) for the visual task of discriminating target flowers from non-target flowers. In all illumination conditions, the L cone tuning of the honey possum is more advantageous than the L cone tunings of other marsupials (e.g. quokka), but better still would be L cone tunings more long-wave than those found in any other marsupials.

View larger version (29K): [in a new window]   Fig. 4. Calculated noise-scaled chromatic distances achieved by different hypothetical L cone tuning (see Materials and methods) for the visual task of discriminating food rich (mature) from other (immature and senescent) inflorescences of Banksia attenuata, the most important measured food resource for honey possums. In this case, incrementally shifting the honey possum's L cone tuning to longer wavelengths would offer no clear advantage. For the conditions of sun and cloud, which represent larger portions of time than the dusk conditions, and for which we have many more measurements, a local optimum in L cone max is evident close to the value found in the honey possum.

View larger version (25K): [in a new window]   Fig. 5. Radiance spectra of Banksia attenuata, compared to spectral sensitivity of honey possum cones. (A) Three examples of mature, food bearing, inflorescences alongside the chromatically nearest immature inflorescence (see Materials and methods). The maturity of the mature samples increases from top to bottom panels. The spectra are normalised for luminance (i.e. the absorption of honey possum L cones is equated). (B) Relative sensitivity of honey possum S cone (grey), M cone (grey), L cone (solid black line), and a hypothetical L cone (L') with max value at 580 nm (broken black line). The sensitivity curves have been adjusted for lens absorbance and self screening (see Materials and methods). The tuning of the honey possum L cone is approximately aligned with the local peak of the immature radiance spectra - characteristic of chlorophyll. If the L cone max was incrementally shifted from 557 nm to longer wavelengths, the contrast between the mature and immature spectra would reduce at first, and then increase again for much longer wavelength max values. Note that with L and M cone max values at 557 nm and 505 nm, the L:M absorption ratio is higher for immature than mature inflorences - the opposite relationship to that for human colour vision with max values at 565 nm and 535 nm.

View larger version (12K): [in a new window]   Fig. 6. Chromaticity diagrams depicting mature and immature Banksia attenuata flowers. Chromaticity contrast (relative to mean immature chromaticity) is plotted in Maxwellian coordinates (see Kelber et al., 2003). (A) Chromaticity for extant honey possums; (B) chromaticity for hypothetical honey possums with L cone max at 580 nm (max values beyond this being implausible for rhodopsins). It can be seen that this long-wave shift makes the immature and mature chromaticities less distinct from each other. Note that the mature samples lie on the left (M cone side) of the immature samples, whereas for human vision, they would lie on the right (L cone side).