Dogs vary in size far more than most other animals. Different breeds range from teacup dogs that only weigh a few pounds to massive 200-pound canines. While there is natural variation from dog to dog (and often between male and female dogs of a particular breed), dog breeds come with a defined weight range.
What Is a Dog Breed?
A dog breed is a distinct type of dog that has predictable physical and temperament characteristics that can be consistently reproduced in that dog's offspring. When two dogs of the same breed mate, the puppies should largely look and act as they do.
A dog of a particular breed has a traceable ancestry referred to as a pedigree. A pedigreed or purebred dog is one produced by mating a male and female dog of the same breed. Registering the litter produced from such breeding authenticates breed status of those puppies by placing them on record in a dog registry association. There are over 400 distinct dog breeds recognized around the world.
How Do Breeds Develop?
Dogs have been associated with humans for at least 15,000 years, with recent genetic research pointing to as early as 100,000 years ago. Some of the breeds known today have been around for 3,000 years or longer. For instance, the Alaskan Malamute and Saluki have changed very little over the centuries.
Humans intervened in the evolution of the dog with selective breeding to improve traits, such as herding and scenting ability. Rather than natural selection and survival of the fittest, the appearance and behavior of dogs evolved to suit the needs and whims of humans.
Spontaneous genetic mutations also happen at a relatively constant rate, and most are not advantageous. Nature would weed these individuals out of the gene pool. But dog breeders might find the mutations interesting and use the dogs with them to introduce changes to existing dog breeds or to develop a new breed. Welcome mutations include body shape and size, ear placement, tail carriage, scenting and sighting ability, or coat type and color.
Selective breeding has refined breeds tremendously over the past 300 years. Breeders still experiment today by creating hybrids and designer dogs. But despite the great variety in size and shape, all dogs are easily recognizable as canines.
Interestingly, research has pointed to a single gene variant that is most responsible for the difference in size of dog breeds. It produces variations of insulin-like growth factor, a hormone that influences cell growth. Large breeds are more likely to have the usual variant, and small breeds are more likely to have the small variant. This might be why breeders have been so successful in developing large and small breeds.
Large Dog Breeds
A giantism mutation created breeds, such as the Great Dane and St. Bernard. These mastiff-type breeds not only are larger than most dogs, but they also tend to be more heavily muscled and cobby (having a compact, short-bodied structure). By comparison, sighthound breeds, such as greyhounds and Scottish deerhounds, are no less muscled but appear lither. And there is a wide range between the two extremes.
Large dogs were often bred to be herding and guard dogs. Many are noted for being very loyal to their human family. Bigger breeds need more space. So if you live in an apartment, you will need to schedule exercise time for your big dog. They also eat more food, which can become costly. Plus, large breeds have access to countertops and high shelves, so you'll need to think more about dog-proofing your living space. And a big dog means more hair. While some large breeds don't shed as much as others, there will still be more dog hair to clean up simply due to the size. You, or the groomer, will also have to spend more time caring for the coat of a big dog.
Moreover, you will need to take extra care in feeding a large breed puppy, so the dog doesn't grow too fast, as this can lead to bone and joint problems. Large breeds are more at risk for hip dysplasia and elbow dysplasia, which is inherited. Reputable breeders should not breed affected dogs.
Deep-chested large breeds are prone to bloat and torsion, in which the stomach fills with air and twists when the dog eats too much or too fast. This is a veterinary emergency. You must give big dogs meals of limited amounts rather than allowing them to free-feed to avoid this condition.
Small Dog Breeds
Small breeds are those where the typical adult weighs 21 pounds or less. The American Kennel Club has the Toy Group classification for breeds weighing up to 10 pounds.
Small breeds developed when average-size dogs were miniaturized with selective breeding. Sometimes this was from breeding the smallest dogs of a single breed. And in other cases, they were crossed with breeds that were already much smaller. The whippet, for example, looks like a scaled-down greyhound, while the poodle comes in three sizes including the tiny toy poodle. It might be hard to believe, but the pug is a mastiff-type dog, and so is the Chihuahua, often with a similar attitude as their larger counterparts.
Some small dog breeds are simply short. Dwarfism (achondroplasia) results in shortened, somewhat curved leg bones but leaves the body proportional. Examples include dachshunds, basset hounds, and corgis.
Because small dogs were developed from a wide range of breeds, you will find breeds that retain their hunting instincts (such as terriers) and guarding instincts (with plenty of barking). Some have very high energy levels and need lots of exercise and mental stimulation despite their size.
On average, small dogs live longer than big breeds. But some small breeds are prone to dislocating kneecaps (patellar luxation). Plus, breeds, such as the dachshund and basset hound, are prone to intervertebral disk disease, affecting their spinal column and causing pain, weakness, or even paralysis.
In general, small dogs are less expensive to feed and easy to transport. You also usually won't face size restrictions for them in leasing an apartment or staying at a hotel as you might for a big dog. But socialization is very important for small breed puppies, as they can become fearful, defensive, and aggressive. Early positive introductions to several people and other dogs can help.