Tamarillos: what a great fruit for inept, part-time gardeners! Stupendously quick to grow – to a couple of metres in not much over twelve months. Producing a crop in less than two years and in a shady spot too, tucked in amongst monstera deliciosa, naranjillas and a dwarf Cavendish banana in the lee of the neighbour’s tall pine tree, with filtered light in summer and just a touch of winter sun. The egg sized fruits are quite pleasant to eat: flesh with the texture of a honeydew melon and with an overtone of passionfruit. I like the big bright green tropical leaves and the fact that fruit flies seem to leave them alone. But best of all, Matimba (as our youngest named the baby tree when it went in) didn’t seem to be pestered by possums. Since the fruits dangle from slender pendulous branches I wondered at first if the critters couldn’t make it to where the action was. Then I thought perhaps they hadn’t spotted them yet, remembering how my figs and beans went untouched for a year or two.
And then recently I spotted a green but nibbled fruit under the tree. Obviously the contents weren’t to the liking of the thief – not quite ripe enough, perhaps. It might well have been an optimistic bird that did the dirty work. But given that tamarillo fruits quite distinctly change in colour as they ripen – gold in the case of Matimba and red in the case of her as yet non-fruiting little sister Molly – this evidence of mid-snack mind-change made me wonder: “Do possums have colour vision?
I realised only recently that primate colour vision is actually pretty unusual amongst mammals, whose ancestors swapped technicolour for better night vision while hiding amongst the shadows, waiting for dinosaurs to leave the party. Humans, primates and monkeys have a kind of gerry-rigged third cone that gives us an in on the neat seed dispersal system fruit-bearing plants sorted out with birds and their dinosaurian tetrachromatic eyes. Like parrots, we can spot a ripe fruit against a canopy of green leaves (although we don’t get to see the UV spectrum, which is disappointing). Okay, there are other explanations for primate colour vision – like spotting tasty red-hued fresh leaves – but I’m sticking with this one for now. Interestingly, colour-blind humans, primates and monkeys (particularly males) are still unusually common. It seems that colour-blind individuals are great at seeing through camouflage, so a sprinkling of dichromatic members of the group serving as predator-spotters does a mob of monkeys or apes no harm at all.
Pulling out of the fascinating vortex of animal colour vision research and returning to my original question, what about possums? With the ubiquity of brushtailed possums in suburban houses and gardens in Australia, surely someone would have a definitive answer on this one.
It turns out that marsupial colour vision has been a hotbed of academic research over the last fifteen years. Until the early noughties it was assumed that marsupials, like most placental mammals, were dichromats, with pretty limited colour vision. But then researchers identified that some marsupials, like the fat-tailed dunnart, the honey posssum and the quokka, were trichromats (as indeed were the ancestors of platypus and echidnas, the monotremes). Some marsupials, like the poor old tammar wallaby, do seem to have the same rather average colour vision as the placentals.
Brush-tailed possum vision is so cutting edge that Lisa Vlahos’ PhD on it, completed in 2013, hasn’t even been published in science journals yet. But, based on the annual reports at the Vision Centre at ANU (sadly I haven’t been able to access her endearingly named PhD thesis “Possum Magic”) it look like brushtails can see part of the UV spectrum, but can’t distinguish between white and green light: more like dogs than chickens, they’re red-green colour blind.
Which might explain my chewed and rejected green tamarillo fruit. Or not. But it was fun finding out anyway!