Through the eyes of animals: What does the world look like to them?
What colours do they see?
How well do they see in the dark?
What can they see that we can't?
In this week's blog, Elly dives into the fascinating world of animal eyesight. Inspired by questions she often gets asked on safari, she goes on to reveal a few interesting facts about the animals you might just have at home...
Have you ever wondered what the world looks like through the eyes of a cat, dog or even giraffe? Or why someone who has a keen attention to detail is known as "eagle-eyed"?
It is difficult to imagine how another human views and interprets the world around us, let alone attempting to conjure up an accurate representation of what another animal might see.
Everything about animals evolves to make them the perfect fit for their particular niche in their environment. This includes what and how well they can see. So, how does human vision compare to that of the animal's we see regularly on safari?
Not all eyes are created equal...
I don't mean to sound arrogant, but our eyesight as humans is some of the best in the animal kingdom. I know many people (including my parents!) would argue differently, currently sporting the latest in short- or long-sighted eyewear in an effort to improve their dwindling visual capabilities, but trust me when I say our general view of the world is pretty darn good.
There are two types of important cells (also known as photoreceptors) in the eye which determine what we can and cannot see: rods and cones. Put simply, rods determine how well we can see in low light whereas cones are reponsible for our colour vision.
Humans have three types of cone cells in our eyes which allow us to distinguish between red, blue and green. Working together, they help us to see an estimated range of a million different colours! However, when the light fades, it doesn't matter how many carrots you eat - your vision is going to be pretty average. This is partly because, unlike most animals, we lack something called a tapetum lucidum.
Although it may sound like a Harry Potter spell, these fancy Latin words are actually the name given to the reflective layer on the back of an animal's eye. It helps them to see better in low light conditions and is responsible for the 'eye shine' that we use to find nocturnal creatures on game drives at night, as the light bounces back off this special tissue.
If you have ever taken a flash photo of your pet in the dark, those demonic red eyes that your flash creates are exactly what I'm talking about here. (Note: this is not the same as the red-eye effect that you get in photos of humans. That is caused by the light reflecting off the layer behind our retina that is rich in blood vessels).
So, alas, we can categorically say that humans were not built for excessive night-time activity. Lions and leopards, on the other hand, have night vision that is about 7-8 times better than ours. Where we have a ratio of about four rods to every cone cell in our eyes, cats have about 25 rods to every cone cell. But Elly, what does this mean?!
If you recall what I explained earlier, rod cells help us to see in low light. They do not help us with colour vision, however, and the cone cells that help us see colour require light to do so. This is why everything we see at night (without the help of lights) is basically a black and white image.
So, not only do cats have more night vision cells and a tapetum lucidum to bounce more light around inside their eyes, but they can also widen their large pupils much more than we can to let even more light in. Given they are most active at dawn and dusk when light levels are low, this helps these cunning predators to paint a very clear picture of their landscape.
However, this superhero-level night vision comes at a cost. With far fewer cone cells than humans, cats and dogs have poor colour vision. Some scientists believe their visual range is limited to blues and greys, whereas others say they are simply unable to distinguish between red, orange, yellow and green objects. Undoubtedly, these predators see a very dulled down version of the bright and colourful world that we humans see.
A bird's eye view...
Let's quickly talk about birds - eagles, to be specific. It would be remiss of me to write an article about animal vision without wowing you with a tale or two about these incredible aerial hunters. Have you ever wondered how an eagle is able to spot a rabbit from the sky from almost three kilometres away? Let me enlighten you...
Eagles have an acute sense of vision. Although their eyes are almost the same size as ours, their eyesight is thought to be 4-8 times stronger than the average human. As LiveScience says, "If you swapped your eyes for an eagle's, you could see an ant crawling on the ground from the roof of a 10-story building."
Thanks to some very densely packed cone cells, eagles have an enhanced ability to resolve fine details. They also have a deeper fovea, which is a cone-rich structure in the back of the eye. Ours is shaped like a shell or bowl whereas that of the eagle is like a pit, which has the effect of magnifying the centre of their field of view. This helps them to spot small objects or prey from high up in the sky.
There is also striking variation of colours that we see in the plumage of many birds. It makes sense, then, that birds are able to perceive these colours, otherwise all birds would be drab and brown. It turns out, birds can see all of these colours and more!
Eagles, like all birds, have superior colour vision. Not only do they see colours as more vivid than we do, but they can also effectively differentiate between more shades and see ultraviolet light too. This is an entire spectrum of light that the average human cannot see and is thought to have evolved to help them follow the UV-reflective urine trails of small prey.
What the world looks like to an eagle, we may never know. However, there's no need to feel inferior as the eagle's admirable eyesight has evolved to suit their high-flying lifestyle. Humans simply have no need for this level of visual acuity and our 20/20 vision suits us just fine!
Why are my cat's pupils vertical slits whereas a lion's pupils are round?
I'd like to introduce you to my family cat, Harley (pictured below). Despite being a grumpy, old sod most of the time, he reluctantly agreed to take part in my next demonstration.
If you look closely at his eyes, you will notice that the pupils are shaped like vertical slits. Compare this with the eyes of a big cat, like the lion pictured above, and you can see that their pupils are round and circular in shape.
The vertical pupils on your household moggie create an increased depth of field on vertical contours. This means that when the cat focuses on a object, vertical things like blades of grass in front of, or behind, that object stay in focus more than horizontal things do.
This allows your cat to precisely determine how close or far away this object (or prey) is, and therefore ambush it with far greater accuracy. If you are a predator that is low to the ground and hunting a small target in long grass then you can imagine this is a super benefit to have.
Lions, on the other hand, stand almost a metre taller than old Harley and can therefore see over the tops of the grass. They are also typically hunting much larger targets and do not need to get quite as close to their prey before they ambush. If they make an error in their strike, they often make up for it by using their speed and muscle strength in pursuit of their prey.
Other predators that have round pupils are leopards, hyenas and wild dogs. All of these carnivores are known as "active foragers", or those who chase down their prey. It is true that lions and leopards often employ ambush tactics while hunting, but this usually acts to close the gap between them and their prey before a pursuit takes place, therefore increasing their chance of success.
So, what about their prey?
Those animals who are further down the food chain need to give themselves the best possible chance of surviving a potential ambush. For humans, with eyes on the front of our heads, our field of view is about 180 degrees. Prey species have eyes that are positioned on the sides of their heads, giving them an impressive 290 degree field of view and helping them to spot danger that may be sneaking up behind them.
Although it is sometimes difficult to tell, these animals also often have horizontal pupils. And yes, you guessed it - this has the same effect as the vertical pupils of a house cat but this time on a horizontal contour. Amazingly, when the animal drops its head to the ground to graze (like the bushbuck pictured above), the eyes swivel in their sockets so that the pupil remains parallel to the ground. This allows them to retain a clear picture of their surroundings and spot potential danger.
So, just by looking closely at an animal's eye, you can tell a lot about whether an animal is the hunter or the hunted, how tall it stands and what lifestyle it may lead. We may never know exactly what the world looks like to these wonderful animals, but it is certainly mind-boggling to think about!
Article written by Elly Gearing with photos courtesy of Nerise Bekker and Elly Gearing.