Honey possums are unassuming little creatures. But their reputation for being rather boring changed when Catherine Arrese at the University of Western Australia reported that the tiny marsupials have three cone visual pigments instead of the more common two; their colour vision is better than that of most mammals. Honey possums dine on the nectar of bright Banksia flowers, so Arrese suggested that their enhanced colour vision might come in handy when they locate their meals. Petroc Sumner at Imperial College realised that he could easily test her prediction with the methods he was using to analyse primate colour vision. Julian Partridge, reading Arrese's paper in his office at Bristol University, was thinking exactly the same thing. Sumner and Partridge both contacted Arrese, and soon found themselves on a plane headed for the Australian bush to test her idea (p. 1803).
In Mount Lesueur National Park, Sumner and Partridge measured the light spectrum reflected from flowers and leaves using a teleradiometer nicknamed the “possum's-eye-view” because it was perched on a tiny tripod at honey possum eye level. Then they modelled possum cones' responses to these light spectra, and how easy it would be for possums to discriminate between flowers and neighbouring leaves. They found that honey possums' ability to detect flowers against their natural background was better than the ability of other local marsupials. But the possums' discrimination ability would be even better if they shifted their cones' sensitivity to longer wavelengths.
So why haven't the honey possums' cones tuned in to longer wavelengths? Sumner and Partridge suggest a possible ecological explanation: the honey possums' cones are in fact tuned to the best possible wavelength to detect the all-important subtle shift from an unripe green Banksia flower to a ripe yellow flower. If they adjusted their cones' sensitivity, possums might lose the ability to discriminate between an inedible flower and one that will provide dinner.
- © The Company of Biologists Limited 2005