Here we consider the cats' and dogs' special senses, and we'll highlight some of the differences between the ways we and our companion animals perceive the world. This allows us to better understand their behavior and therefore provide better care.
We'll start with the cat and her sense of vision. The cat's eyes are specialized to detect movement, a requirement for a predator. The cat has a very large eye relative to body size. The eyes are set well forward on the head, and the cornea is curved and protrudes slightly from the skull. These characteristics of the eye give cats both binocular and lateral vision.
The cat's field of binocular vision is approximately 120 degrees. Depth perception within the binocular vision is very important for judging distances to the prey, facilitating chasing, and catching it. Cats have additional lateral vision of approximately 80 degrees on each side, resulting in a total range of vision of about 280 degrees. As a predator, the cat needs a wide field of vision to locate prey.
In comparison, humans have a total range of vision average only 100 degrees.
Unlike human eyes which work best in daylight, the cat's eyes are specialized to function extremely well under conditions of dim light, again reflecting its historical role as a nocturnal predator. Cat eyes are able to function in approximately 1/6th the amount of the light needed for human vision. But they must also be able to function well in daylight, and one adaptation of the cat's eye that allows this is the pupil which can open very widely in low light levels, but can also contract to a very small size in bright light. While in humans this process is controlled by a circular ciliary muscle, in cats there are two ciliary muscles that give the cat its characteristic elliptical pupil that becomes slit-like in bright light conditions.
The cat eye also has a layer of specialized reflective cells called the tapetum lucidum located behind the retina. These cells reflect unabsorbed light back toward the retina, thus providing a second opportunity for light absorption. Together these adaptations allow the cat to see well in low illumination.
But it's important to emphasize that cats do need some illumination to see. They cannot see in total darkness, which is a common misconception.
Cats are also said to be colorblind, but under bright light they are capable of detecting some color, usually only faint hues or a washed out version of what we see. This is believed to be due to the fact that it's more important for predators to detect movement than to see the colors of vegetation.
When we consider the cat's hearing, again we think about the role as a hunter. Cats use their sense of hearing to locate and capture prey. They can detect sounds within a very wide range and at extremely high frequencies. The cat's ability to detect sounds of high frequency is an adaptive mechanism that aids in capturing small prey, like rodents, because they use high frequency sounds to communicate.
Some structural adaptations to the ear include a large pinna (or outer ear flap) that aids in collecting and locating sound waves. Small muscles that control the pinna enable a cat's ears to rotate up to 180 degrees.
Another fascinating aspect of cat hearing is that cats are able to discriminate between two sources of sounds very efficiently. For example, at a distance of three feet, cats can distinguish between sounds coming from two sources that are only three inches apart. This is a very important ability for a predator that tracks down the movement of prey in dim light.
Odors play an important role in the behavior of cats. This is illustrated by the sheer size of the olfactory epithelium and the large number of scent receptors compared to humans.
Cats also have the sniff mechanism which aids in maximizing scenting ability. The sniff is actually a disruption of normal breathing and consists of a series of very rapid, short inhalations and exhalations. Sniffing forces air into a bony structure that traps the inhaled air. Instead of being further inhaled into the lungs, the air rests in this space to allow scent molecules to interact with scent receptors in the nose.
The cat also has an additional olfactory organ called the vomeronasal organ, also commonly referred to as the Jacobson's organ. This organ, found in the roof of the mouth, contains receptor cells that connect to the region of the brain associated with sexual, feeding, and social behaviors. It's believed that the vomeronasal organ is involved in courtship and sexual behavior, as well as the ability of a cat to recognize other cats and people.
Now we'll consider the dog. Like cats, dogs also have vision specialized for maximum efficiency under low illumination because the wolf progenitor hunted primarily at twilight. Dogs are three times as capable as humans at detecting low intensities of light. They also have diminished color vision, as discussed for the cat.
The dog's field of vision is heavily affected by breed and skull type. Because of the placement of their eyes on the skull, most dogs have good lateral vision.
You can see here that eye placement on the head according to breed has quite an impact.
The total field of view for the average dog is about 60 to 70 degrees greater than that of humans. Although this is advantageous for the dog to scan the horizon and locate moving objects, there is reduced binocular vision as compared to humans.
The dog's ability to hear is extremely well developed. We know that dogs can detect sounds at much higher frequencies than humans. Dogs can also hear over a greater distance about four times that of humans. Most dogs have mobile ears which can pivot to locate sounds. Some breeds of dogs have less mobile ears which impeded their ability to hear as well as to communicate.
Now it's often said that dogs perceive their world in nose pictures. All dogs have a sense of smell far superior to that of humans, as you can see here by comparing the size of the olfactory epithelium and the number of scent receptors. This amazing ability is useful to the dog in many ways, from tracking food to communicating with other dogs to finding the perfect spot for rolling.
A number of things make a dog's scenting ability superior. The shape and length of a dog's muzzle is uniquely suited to extreme smelling ability. Starting at the nostrils, their shape is unique to allow expiration out of the sides of the nose so that fewer scent molecules on the ground or in the air are disturbed.
A dog's nose is normally cool and moist. The moisture secreted by mucus glands in the nasal cavity captures and dissolves molecules in the air and brings them into contact with the specialized olfactory epithelium.
Some dogs have longer ears, which may also play a role in helping to funnel scent molecules into the nose.
And like cats, dogs certainly use the sniff mechanism for maximum detection of odors.
They also have a nasal pocket that permits odor molecules to accumulate and interact with olfactory receptors.
It is the dog's amazing scenting ability that lends him to important working roles in today's society.