Description and Physical Characteristics of Dogs

Dogs are generally much better at conserving heat than at cooling themselves. In sled dogs, who can survive outdoors even in bitterly cold temperatures, the fur acts as an insulating “blanket” that retains the heat generated by the dog’s high metabolism. However, in hot or humid weather, most dogs have difficulty. Dogs cannot sweat, which is an effective form of evaporative cooling. Instead, dogs lose heat primarily by panting. These rapid breaths (10 times faster than normal) are an attempt to lose heat through evaporation by moving hot, moisture-filled air in and out. During the short, shallow breaths in panting, little air can be exchanged in the lungs. In fact, dogs must stop panting periodically to take a good respiratory breath. Drinking water also helps dogs cool down, and the canine hair coat helps insulate from the sun.

Because the cooling system of dogs is relatively poor, certain summer situations can be dangerous and even life threatening. Sadly, many dogs die of heat stroke every year ( see What to Do at the Scene and How to Transport). The most common problem is associated with being shut in a parked car. Even with the windows rolled down, the inside of a parked car can quickly reach 150°F (66°C) or more in the summer, which can cause heat stroke and death in a matter of minutes. Other dangerous situations involve being penned or tied out in the sun (without access to shade) or being locked in a poorly ventilated travel crate.

Ways to keep dogs cool during hot weather include air conditioning, spray misters, shade, dips in a wading pool, or gentle spraying with a garden hose. Keeping dogs wet during the heat of the day provides a method of evaporative cooling. Plenty of cool, fresh drinking water should be available at all times.

Dogs can see movement and light much better than people. In the retina of the eye, dogs have more of a specific type of cell called a rod, which is good at collecting dim light, so they have better night vision. A reflective layer in the dog’s eye, called the tapetum lucidum, magnifies incoming light. This reflective layer lends a characteristic blue or greenish glint to dogs’ eyes when light (for example, headlights of passing cars) shines into them at night. However, dogs do not have as much visual acuity as people, meaning that they cannot distinguish fine details as well. They also cannot differentiate colors as well because they have fewer of the cells in the retina called cones, which are responsible for color vision. Contrary to popular belief, however, dogs are not completely colorblind.

Nictating membrane, dog

A unique feature of the dog eye is the nictitating membrane, which is also called the third eyelid. This additional eyelid is a whitish pink color, and it is found under the other eyelids in the inside corner (near the nose) of the eye. The third eyelid extends up when needed to protect the eyeball from scratches (for example, while traveling through brush) or in response to inflammation.

The ear canal of the dog is much deeper than that of people and creates a better funnel to carry sound to the ear drum. The average dog can hear about 4 times better than the average person, including sounds at higher frequencies than can be detected by the human ear. Dogs are also better at distinguishing the direction of a sound, which is an adaptation useful for hunting. Unfortunately, this deeper ear canal predisposes dogs to ear problems. Grease, wax, and moisture can build up in the ear, leading to inflammation and infection. Floppy ears or hair within the ears further limit ventilation, making matters worse. This is why many dogs need frequent preventive ear cleaning.

Dogs have an extraordinarily acute sense of smell; it is about a million times more sensitive than that of people. They can detect odors at extremely low levels and can distinguish odors that are subtly different. This is why dogs are able to sniff out drugs and explosives at airports, search for human victims at disaster sites (including victims deep under water), and follow the scent track of criminals.

Odor molecules dissolve in the moisture that coats the inside of the canine nose. Signals are then sent from the olfactory membranes in the nose to the olfactory center of the brain, which is 40 times bigger in dogs than in people.

Dogs also have an organ on the roof of the mouth that allows them to “taste” certain smells. As in people, taste and smell in dogs are closely linked. However, dogs gain much more information about food from smell than from taste. Dogs have only about one sixth the number of taste buds that people do, and their distinct sense of taste is actually quite poor.

The canine paw contains specialized structures that help the dog move over different surfaces. The bottom of the paw is covered by thick, resilient pads that become callused after years of steady wear in direct contact with the ground. These pads protect the paw and help provide a secure grip on many types of surfaces. The toenails help provide traction while running and are also used for digging. Canine toenails are thick, brittle structures made up of a protein called keratin (just like hair). A large blood supply runs down the middle and feeds the cuticle (or “quick”) of the growing nail. Avoiding these blood vessels when trimming toenails can be difficult, especially when the nails are dark. Regardless, keeping nails trimmed is important because nails that snag or break during running or jumping can cause considerable bleeding and pain. Broken nails should be examined by a veterinarian, who can trim away the fractured part of the nail, treat the wound to stop any bleeding, and prevent infection. Dogs have rudimentary equivalents of human thumbs called dew claws that are found on the middle side of the front paws or lower front legs. Dew claws have no function, but they commonly snag and break. Dew claws should also be trimmed periodically to prevent snagging and to keep them from curling around and growing into the foot. They are commonly removed in very young puppies or as an additional surgical procedure when dogs are spayed or neutered.

Nails, dog

Rump rubbing or “scooting” is usually associated with impacted anal glands, although it can be confused with a digestive problem. The anal glands are located in a layer of muscle at the 4 and 8 o’clock positions around the anus. These scent glands contain a foul-smelling secretion that is normally expressed during a bowel movement. The secretions often thicken, which can plug the duct, causing pressure and irritation that can lead to infection. Many dogs need to have their anal glands manually emptied by their veterinarian on a regular schedule.