The Colourful World of the Honey Possum
The honey possum is a key pollinator for a many flowering plants in Austrailia. Emma-Lynn Donadieu reports on Imperial research revealing how well adapted this tiny marsupial's sense of colour is to this role.
Looking up, he scans the top of the Banskia until he finds what he is looking for: a blurry yellow cone. A quick check for predators, and he dashes nimbly up through the branches of the bushy shrub towards his next meal. Gripping a branch with his tail, he hangs upside-down as he manipulates the cone of flowers with his tiny paws. With his long tongue, he licks up the delicious golden nectar.
Honey possums, tiny marsupials found only in Western Australia, have a special relationship with Banksia, acting as one of its prime pollinators. Shrub and marsupial have special adaptations to ensure a successful relationship: Banksia produces flowers near the ground to encourage small mammals like the honey possum to feed; honey possums have thin snouts and a long tongue to access the nectar. Recent research by Dr Petroc Sumner, a scientist at Imperial, suggests that honey possums have even evolved specially adapted colour vision to help them discriminate nectar carrying mature yellow Banksia flowers, from immature green ones.
Colour vision depends on the ability to differentiate between different wavelengths of light. This task is carried out by cones - tiny cells in the retina at the back of the eye. Different cones carry different light sensitive protein pigments called Opsins, making them sensitive to different wavelengths of light. The signals they produce enable the brain to identify colour. Humans and most old world primates have three different kinds of Opsin allowing them to discriminate between greens, blues, and reds. Most other mammals have only two kinds, making them red-green colour-blind.
Austrailan researcher Catherine Aresse recently discovered that, like us, honey possums and some other marsupials have three different kinds of Opsin. She speculated that their enhanced colour vision might help them in their search for food by allowing them to locate mature Banksia flowers more easily.
To test Aresse's speculatation, Sumner and a colleague measured the light spectrum reflected from Banksia leaves and flowers in the honey possum's natural environment: Mount Lesueur National Park in Western Australia. Their equipment for collecting this data, nicknamed the 'possum-eye-view' consisted of a camera perched on a tiny tripod at honey possum eye level, linked by an optic cable to a spectrometer and laptop. They then ran models using this data to determine the optimum colour vision honey possums should have to give them the best chance of detecting the difference between mature and immature Banksia flowers.
When they looked at the tuning of honey possum Opsins, they found that the long wave pigments are tuned to give the precisely the colour discrimination ability needed for this task. In fact, honey possums could have better overall colour discrimination ability if their cones had evolved to be sensitive to even longer wavelengths. However, this would reduce their ability to discriminate mature from immature Banksia flowers. Sumner and colleagues suggest this reflects a strong selective pressure in favour of this ability. By being able to discriminate mature Banksia flowers from the ground, honey possums could save themselves an unnecessary climb through vegetation in full view of predators whilst they search for food.
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