We often think of visual acuity when we think of sight. Fine for humans, but there’s more to vision than how clearly an animal can see.
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Vision in Animals: Adapting Sight for the Environment
As with many other traits, vision in animals is specially adapted for survival. Animals that are preyed upon, like deer, typically have monocular vision. Typically positioned on either side of their head, their eyes have two fields of view. Their brains combine these fields so they can watch out for predators.
Predators, on the other hand, have binocular vision and forward-facing eyes. Our field of vision is overlapped by the brain to provide depth perception. This makes it easier to spot prey and judge distances for hunting.
A great example can be seen in cats, who have a wide binocular vision overlap of 140°--compared to a human’s 120°. Alongside keen motion detection and excellent peripheral vision, binocular vision helps cats calculate precise jumps and capture small prey.
Motion detection is important for canines as well. Dogs' ability to process more images per second than humans, combined with their excellent low-light vision, contributes to their effectiveness as hunters and watchdogs.
When looking at vision types, birds are especially interesting. They have two retinal focal points in each eye. This adaptation allows them to see forward and sideways simultaneously. This leads to effective navigation and swift reaction time while flying.
Spectral Differences: Light and Color Vision in Animals
Photoreceptors are cells that transform light energy into electrical signals that the brain interprets.
Cone photoreceptors provide high definition day-light color vision in animals whereas rod photoreceptors provide low definition night vision and sensitivity to movement. Predators tend to have a higher number of cone photoreceptors. prey species have a higher amount of rod photoreceptors.
And, generally speaking, vertebrates can either see in color, or they can see in low light levels. Not both.
That’s where New World monkeys and some lemurs have developed an advantage. They exhibit polymorphic trichromatism. Where some females can see red, green and blue, the rest of the species can see when light is low or fruits are camouflaged. This split means cooperative foraging is much more successful.
Trichromatic vision is handy, but birds take it a step further with tetrachromatic vision; they see light in the ultraviolet (UV) range. This adaptation allows them to see plumage and plants that reflect UV light. Eating, mating, and communication are all aided with this broader light perception.
Visual Acuity: Differences across the Animal Kingdom
To study visual acuity, scientists measure how many pairs of black and white parallel lines a species can discern before they see a smear of gray.
At most humans can resolve about 60 cycles. And, at 10 cycles, a person is considered legally blind. Interestingly, dogs and cats can resolve around 10 to 15 cycles, so next time you’re showing your furbaby something, remember they’re legally blind.
By contrast, eagles, vultures and falcons can discern 140 cycles, allowing them to spot a rabbit while flying thousands of feet above the ground. But, most birds–and fish–see fewer than 30 cycles. And most insects can only see one cycle within one degree of the field of vision.
While allowing humans to read, drive, and recognize faces, some animals take advantage of visual acuity for predatory purposes. For instance, birds are believed to be able to see spider webs from six feet away. That means, they can avoid flying into them. These insect death traps are thought to be imperceptible to spider prey, though, which is how spiders are able to snag many of their meals.
Vision, it seems, is just one more way nature helps the fittest survive.
Until next time, stay curious.