[AllenG (OP)]

When I came home this evening two of my dogs were standing in the middle of the driveway.
Their eyes glowed as light from my car's headlamps reflected from the backs of their eyes.
I noticed that one dog's eyes glowed red and the other's green.

What causes the difference, and would it cause differences in the dogs' visual acuity.


Thanks,

--Allen


I'm in Georgia, USA and love y'all's podcasts.

[RD]

The green reflection is due to the tapetum lucidum...
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[Madidus_Scientia]

The one that had red eyes is an evil terminator dog.

In cats and dogs etc, the reason their eyes reflect light back is because by reflecting light back past their retina they can absorb more light and see better in the dark, with a trade-off in clarity because the reflected light is not focused.

[swelleyman]

Evil terminator...hm?  That must make the other the opposite.  Bet their personalities match their eye colour!  Its the ones with on of each colour you've got to watch. If you have pets with orange eyes please, please be nice to them, they're thinking about their treatment.

One tip, don't shine bright lights into their eyes, they don't like it!

[chris]

But no one has answered Allen's question - why one dog has red eyes and the other green...where's squarishtriangle when we need her...?

[SquarishTriangle]

I'll have a go at it...

Here's what we know. The tapetum lucidum, formed by the choroid at the back of the eye, is wedged between layers of blood vessels on either side but itself is avascular. It is responsible for reflecting light of various colours, producing the characteristic iridescence seen in flash photography of animals and in front of car headlights, and is believed to be a nocturnal adaptation by increasing stimulation of the photosensitive cells of the retina.

In dogs and cats, the tapetum is made up of cells. These cells contain crystalline rods that are arranged in such a way that they split the light that hits them into its various colour components. A similar effect is seen in herbivores, but the structure of the tapetum varies in that it is fibrous (collagenous) rather than cellular, and it is the arrangement of the collagen fibres within the structure that is responsible for splitting light. The tapetum is absent in humans and pigs.


So here's what I think might be happening:

1) The eye that appears red lacks a tapetum lucidum and the result is the typical 'red-eye' seen in humans due to the appearance of the blood vessels of choroid and the underlying the cornea.

2) The dog has different crystalline or cellular arrangements in its eyes causing the reflected light to correspond to the different wavelengths. Interestingly, tapetum appears a blue-green colour in the Dutch sheep dog but an orangy colour in the Old English sheepdog.

3) The dog is slightly bung eyed and light is hitting the structure at a slightly different angle in one eye than in the other, affecting the way the light is reflected (but being an appauling physicist, I have no idea whether or not that is valid.)

[RD]

Some dogs do not have a tapetum lucidum

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If this is true it could explain one dog having red eye (no tapetum lucidum) and the other green.

[AllenG (OP)]

Since it seems the tapetum lucidum can reflect (or is it refract) light in different colo(u)rs, say both dogs have tapetum lucidums, would both animals have the same night vision acuity? 

Does one see better with red light bouncing around in one's eye or with green light?

My brother just pointed out that the dog with the red eyes is indeed the one possessed by Satan, so he's betting that is the one who could read fine print in a cave.

[SquarishTriangle]

Ok, you have two dogs. Clearly I can't read .

I would say, disregarding all other factors, there would be little difference between the 2 dogs' vision in low light. Firstly because night vision is mainly due to detection by the colour-insensitive rods of the retina but secondly, even in mildly dim conditions, dogs are relatively insensitive to differences between green, red and orange coloured light (exhibiting what would be comparable to red-green colour blindness in humans).

[RD]

There is more green light in sunlight than red.
In low (dim) light eyes are more sensitive to blue than red.

So a dog with a tapetum which reflects green light will have better night-vision than one which reflects red. 

In fact, the sun is a yellow-green star

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A rod cell is sensitive enough to respond to a single photon of light, and is about 100 times more sensitive to a single photon than cones. Since rods require less light to function than cones, they are therefore the primary source of visual information at night (scotopic vision)  ...
   Experiments by George Wald and others showed that rods are more sensitive to the blue area of the spectrum, and are completely insensitive to wavelengths above about 640 nm (red). This fact is responsible for the Purkinje effect, in which blue colors appear more intense relative to reds in darker light, when rods take over as the cells responsible for vision.

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[AllenG (OP)]

Thank you all for the replies.