Why are dogs so different?
Many domestic animal species have been selectively bred for thousands of years now, yet dogs seem unique in the number of different shapes and sizes they appear in. How do dogs manage to take so many different forms? Is there something unusual about the canine genome that allows this? Could other domestic species, if bred over time for a wider array of purposes, also theoretically assume such widely divergent forms?
This is a good question. I'm now sure we have a good answer, but there are a number of theories. First, it's fairly easy to understand how artificial selection helps us develop the same species into different breeds based on the traits we desire most. If one group of people needs help chasing down rabbits, say, they will keep breeding their fastest pups to each other until after multiple generations they have created a dog that is much leaner and faster than his cousins. Meanwhile, other groups of people are choosing dogs based on their own needs, such as to hunt larger prey, herd sheep, pull carts or guard homesteads. The same goes for other domestic species, when people choose breeding animals based on the traits they value: the production of richer milk vs. more meat vs. a finer wool, for example.
A number of factors may have influenced our ability to produce more changes in dogs over the millennia:
DOGS have a shorter generation interval than horses or other live stock Dogs were domesticated roughly 10,000 years earlier than horses were, and we have bred many more generations of dogs than we have horses. Another boost comes from the fact that dogs come in litters rather than the horse’s much slower rate of one baby a year. Each individual pup born presents a new opportunity for genetic change that may be selected for further propagation.
Dogs are omnivores Their varied diet allows them to continue to eat after stacking up multiple changes in the head and teeth. Horses, in contrast, need to maintain good teeth in decent alignment because grass is one of the toughest things to eat. Any significant changes to a horse’s head are likely to lead to a decline in function that will not be selected for propagation to future generations.
Dogs are used for a widervariety of purposes This tends to drive selection in different directions as owners choose the best among the candidates for these uses. Horses have always been bred for just two basic jobs, pulling and riding. These purposes require a certain baseline athletic ability. That, combined with the diet and teeth needed to support it, has added a degree of conservatism into the changes that are attractive to owners. With horses, the original package is pretty close to what was needed and desired all along, so selection rarely disrupts this in radical ways. In contrast, the original form of the dog is rarely the final target, so selection tends to opt for change over time.
In any population, an individual will occasionally be born with a significant genetic mutation. I knew a horse at a local barn, years ago, who had a spine that was dramatically shorter than normal. This led to discomfort for both horse and rider, and it was
need to worry about? What signs should I watch out for? How often should an older dog be tested for age-related diseases?
What an observant pair you and your neighbor make! Our farm dogs are lucky to have a bit more freedom than many house pets, but the downside is that we might miss some early warning signs of chronic disease---like subtle changes in thirst and urination---that are easier to notice in dogs with a more urban lifestyle.
Risk factors for diabetes mellitus include obesity, age, genetics, some medications and episodes of pancreatitis. Still, regardless of how many risk factors your dog might have, your questions are excellent. Signs of diabetes that are often seen by owners do include increased thirst and urination, but you may also notice lethargy, weakness of the rear limbs (which is especially noticeable when the dog is jumping), and increased appetite that is often accompanied by weight loss (rather than the weight gain you might expect).
But diabetes is not the only disease that can cause increased thirst and urination. Your veterinarian might also look for kidney disease, Addison’s disease (hypoadrenocorticism), Cushing’s disease (hyperadrenocorticism), thyroid disease and other problems.
How often you need to screen an older dog for diabetes and other agerelated diseases will vary, depending on his overall health and other risk factors. In general, though, I recommend twice annual physical examinations for older dogs and cats. At these visits you can discuss your dog’s health risks with your veterinarian, and together you can decide how often to do some bloodwork to screen for age-related diseases.
I hope that you and your dog have many healthy years ahead! Keen observation at home, combined with regular visits with your small animal veterinarian, will help to ensure that future.