Ecology, Humour

Eye shine — you shine but I don’t

I spend a lot of my time with individuals who see the world through very different eyes.

For a start their eyes shine at night, not with avarice or the holy spirit but with any light that they catch in their eye —

Eyeshine in Welsh Mountain Sheep

Eyeshine in Welsh Mountain Sheep

You see they are a prey species and they stand out all night in the darkest fields uneasily looking out for wolves and rustlers so they need to see in the dark.  One of the adaptions that many nocturnal mammals have made is to acquire a tapetum lucidum, a biological mirror behind their translucent retina, so that light stimulates the retina as it falls upon it and stimulates the retina’s photosensitive cells again as it bounces off the mirror layer heading back the way it came — this helps them to see in the dark.

So when you go out in the field at night with your torch and all the sheep turn to look at you because they think you are something spooky, all their eyes light up with intense pale green light, all directed at you, which is definitely spooky.

Dogs have a tapetum lucidum too — this one shines bright green.

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There’s a sheep behind him.  Foxes have eyes that glow green, different species have variation in their tapetum lucidum and glow differently — hunters who went out lamping for rabbits and foxes (I think it’s illegal so they don’t do it anymore) will tell you they can tell what they are shooting by the shade of the eyeshine, as they charge around in a truck with a lamp on top picking up eye glow and shooting things — very fortunately humans do not have a tapetum lucidum, otherwise more of them would get shot.

Cats are famous for their glowing eyes and that is where I got into trouble.  I spent a happy evening flashing and snapping at our cats, trying to demonstrate their eyeshine and  their strange lozenge shaped pupils that constrict down to a tight vertical slit in bright light — you see one of the problems for these creatures, who are adapted for the dark, is managing bright light.  Mainly they shut their eyes.

Cat dealing with bright light

Cat dealing with bright light


Minutes after this unsuccessful photo-shoot, Midnight (our short haired black cat) started doing something very strange and alarming, kicking his right foot out then grabbing at his mouth with both his paws as if trying to pull something out of his mouth — he did this repeatedly making a peculiar slavvery noise. There wasn’t anything in his mouth or throat, he wasn’t salivating or retching and there was no sign of a bite or sting on his lips.  The other cat and the dog looked worried and followed him round fussing as he repeated his odd stereotyped gestures, like non-verbal Tourette’s Syndrome.  OMG he’s been out and got a head injury, or a brain tumour…   Or epilepsy due to flashing lights.

There then ensued a period of research on the internet.  While the cat twitched, quietly now, on its chair by the fire, the other two animals sat upright on the floor next to him watching anxiously.

By the time my husband had got home I had cracked it — Feline Hyperaesthsia Syndrome…  Can be provoked by stress ( like being chased around the house with a flashlight).  This is a diagnosis of exclusion and mindful of vet’s bills we adopted an expectant policy — we’ll watch and expect it will get better.

It did — for twelve hours or so he looked spaced-out between twitches that gradually got less complicated and with longer gaps between them– first the kicking disappeared, than the grabbing at his mouth, then the licking of his lips gradually stopped and he had a long sleep.  Then he woke up and had a large breakfast and has been fine since.  We didn’t photograph any of this — we thought I had done enough harm.

Returning to the great mysteries of the mammalian eyes that follow me daily —

Horizontal rectangular pupil and fetching eyeshade of pale lashes

Horizontal rectangular pupil and fetching eyeshade of pale lashes

 Why do cats have vertical pupils and sheep horizontal ones?

They both need to be able to restrict the bright light of the mid-day sun.   Cats need very sharp vision, right in front of them and the potential to use a whole cross section of their lens (this has complicated optic reasons to do with putting back together the spectrum that bending light tends to produce), thus they need a vertical slit because they are predators and they pounce on little creatures right in front of them.

Sheep need a more global view of the world, they live on grassland and need to be able to spot movement all around.

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With her protruding eyes and wide pupils, she can see from right in front and to right back along her flanks.  Provided she walks in a slight zig-zag, which they do, she can see all around herself, even in bright weather when her pupils are constricted — she couldn’t do this with a vertical pupil.


Dogs have round pupils like us but can see in the dark — they have reflectors at the back of their eyes which shine but are not so sensitive to the light that they need slit pupils to protect themselves by day — I suspect this is because at night they see mainly with their noses!


2 thoughts on “Eye shine — you shine but I don’t

  1. Pingback: 20 Things You Probably Don't Know About Your Dog

  2. Pingback: 20 Things You Probably Don’t Know About Your Dog -

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