Twilight of the Chickens

Jeez, chickens go to bed early. I’m outside getting the washing, and ok, it’s heading towards dusk, but not only can I see the location of my smalls, visibility’s so good I can even spot and dodge the brush turkey doings as I go. But the chooks are already tucked up in their palatial quarters, or in the case of Snowball (pictured above), having a nap in an elevated position while waiting to be eaten.

So, twilight is more complicated than you might think, and I’m not talking about the teen vampire series. Apparently it comes in three types. As the sun first sinks, there’s civil twilight. Technically, that’s when the centre of the sun is 6 degrees below the horizon – in good weather you can still see the things around you (say, your knickers on the line, or an inconvenient pile of guano). Then, after about 20 minutes, more (as you get closer to the poles) or less (closer to the equator), you have nautical twilight. The sun is 12 degrees below the horizon now, and if you’re a sailor, you can take a bearing on the stars with the horizon still visible. If you forgot to pick sweet potato greens before sundown, you’re rummaging around in the cupboard for a torch.

After nautical twilight comes astronomical twilight, with the sun 18 degrees below the horizon. To an untutored eye it might appear as if night has finally arrived, but impatient astronomers wanting to check out nebulae will be still pacing up and down waiting for full dark. Of course, you won’t get the full sequence come the summer in Trondheim (civil twilight from sunset to sunrise), Glasgow (nautical twilight for most of the darker hours) or even London (astronomical twilight all night long, even without throwing in the orange glow of light pollution).

I find this orderly taxonomy of darkening moments curiously soothing, an effect only slightly diminished on reading that nerdy acronyms like EENT (end evening nautical twilight) aren’t just used by meteorologists and astronomers to document the passing days and track the movements of the stars, but also in military campaigns to synchronise watches.

But all this is from a human point of view. For chickens, it’s different.

If possums have got pretty dud typical mammalian dichromate vision, chickens are rocking their cones. Not just three sets of cones like us, but five, including one that enables them to see ultraviolet light and a double cone for detecting motion. And “cellular sunglasses”: an oil-drop to filter particular wavelengths of light.

But wait! There’s more! Chickens also have, in essence, a third eye. Okay, not as visible as parietal eyes of Tuataras and other less famous reptiles (and related organs in the eyes of other tetrapods – like the receptor that looks like a little blue pimple between this critter’s eyes).

But still, a pineal gland perched up just under skull that receives enough light to regulate sleep and trigger annual reproductive cycles. Extremely cool. Perhaps too cool for some. While researching this post, I noticed, right underneath a webpage spelling out the multidimensional excellence of chicken vision, an advertisement for eye surgery.”Replace tired and baggy eyes with a younger look!”. Presumably the reader, ruminating dolefully on the superiority of the avian retina and the failings of human sight, is primed for this kind of thing.

But perhaps we humans shouldn’t be so grim about our drab colour vision, our tediously symmetrical pair of eyes. At the very least the time our mammalian ancestors spent cowering in a burrow while the dinosaurs strode the earth gave us respectable night vision. We can revel in our fine array of twilights while the shutters come down with a clang at the end of the day for our long time companions.

Okay, The Twilight of the Chickens may not have the apocalyptic ending of the Ragnarøkkr, the Twilight of the Gods. The rivalry between Treasure and Shyla over who gets the highest perch in the upcycled coop doesn’t have the same Wagnerian grandeur as Odin’s battle to the death with the wolf Fenrir. But pleasingly, even the Norse myths have a place for chooks: the end of days is heralded by the crowing of a crimson rooster, a golden rooster, and a rust red rooster. I must tell Andy Ninja.

Posted by

mccnmatt

Posted on

March 27, 2014

Posted under

Animal pests, Animal senses, Australian natives, Fruit, fruit for shade, Fruits for shade, Gardening, Native animals, natural history, Plants and what I know about them, Possums

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Do possums see in technicolour?

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!