Dogs are long-sighted, but birds can spot the details

PUBLISHED : Sunday, 13 November, 2011, 12:00am
UPDATED : Sunday, 13 November, 2011, 12:00am


If you have had your pet for a while, you probably know its personality and how it's feeling. But do you know how well your companion sees its environment?

Veterinarian Hans Westermeyer, from the Peace Avenue Veterinary Clinic in Mong Kok, offers insights into your pet's eyesight and its perception of the world.

'There's a variation across the animal kingdom, with dogs and cats having poorer eyesight than humans,' says Westermeyer, a veterinary specialist in ophthalmology.

'If we were to talk in megapixels, dogs and cats have about five to 10 times less [than humans], with less variability to define details.'

He provides another analogy. If two dots were on a wall, canines and felines would have to be five to 10 times closer than the average person to see it. At the opposite end of the spectrum, birds have four to five times better eyesight in resolving details.

'Birds beat us hands down,' remarks Westermeyer, who holds a diploma from the American College of Veterinary Ophthalmology. Raptors such as hawks, falcons and owls, have superb eyesight, needed to hunt from the air, adds Westermeyer.

Even though a dog can't see fine details, the vet says the average Fido can spot objects at night better. He explains two reasons for this. First, dogs have more rods - which perceive small amounts of light - in their eyes than humans. 'But the trade-off is that their rods aren't as good at perceiving detail.'

The second reason is the dog's tapetum, a structure that acts like a mirror to reflect light. 'If you shine a light into a dog's eye, you get that yellow glow, while cats have a greenish-yellow glow,' he says. 'First, light hits the retina, but then any light that misses the retina bounces back on the tapetum, so there's a second chance to capture it.'

Another feature is that a dog can see moving objects at great distances. He recounts a study of Labrador Retrievers that were tested to see a person 100 metres away spot a bird decal. He says: 'If you think about how small that is, it's quite remarkable.'

Feline eyes are even better than dogs' in seeing at night. A cat's unique pupil can close to a tiny slit in bright daylight, or open fully in low lighting for maximum illumination.

A common problem with cats' eyes is the feline herpes virus. 'It's very similar to human herpes, but you can't contract it from a cat,' Westermeyer says.

This condition can also lead to corneal ulcers, weepy or puffy eyes, coughing, sneezing and upper respiratory problems. 'About 95 per cent of cats have been exposed to feline herpes virus as kittens. The virus hides in the brain, and whenever the cat becomes stressed it manifests,' he says. 'It's harder to find a cat that doesn't have herpes. But just like some people, some cats will never have any problems.'

In the canine world, the most common eye problems are cataracts. According to Westermeyer, dogs develop cataracts mainly due to genetics, unlike humans, who usually suffer them as a result of old age.

He says: 'Some breeds get cataracts as part of their genetic code, like miniature Schnauzers, which have them at birth. Poodles can get cataracts at two to three years of age.'

Another misconception is that cats and dogs are colour-blind. 'They don't see as much colour, but they do see a fair amount,' the vet explains.

Cones in the eye are responsible for colour vision with the more types of cones accounting for greater variations in colour. Humans have yellow, blue and green cones but dogs and cats only have blue and yellow, so these animals are missing the red-green spectrum.

At the opposite end of the rainbow, birds have five types of cones.

'So you would expect birds to see much richer colours and more ultraviolet lights. This probably helps them see better feather colours in their mates as well as their prey,' says Westermeyer.