Lifespan Is Usually Related to Animal Size
Scientists worked out a long time ago that the lifespan of animals is mostly related to their size. Bigger animals usually live longer than smaller animals.
See also: Dog Coat Color Predicts Lifespan in Study on a Beloved Breed
The main reason people used to think big animals live longer is because of something called metabolic rates. A metabolic rate is like how much petrol a car uses — cars that use up their petrol more slowly can drive for longer, a bit like animals with lower metabolic rates. Smaller animals usually have higher metabolic rates, which lead to shorter lifespans, like a car that uses up its petrol very quickly.
The problem is that this doesn’t work for all animals. Some parrots have high metabolic rates but can live for more than 80 years! A metabolic rate is related to heart rate, and some parrots have a heart rate of 600 beats per minute. (Your heart beats around 70 to 100 times per minute.)
And dogs are one of the animals that do not follow this rule.
Why Do Dogs Break This Rule?
Dogs don’t follow the rules on larger animals living longer. A 150-pound Great Dane is lucky to reach seven years, but an 8-pound Chihuahua can live for 10 years or more. We still don’t really know why this happens. But if you want a dog that lives longer, you should choose a small breed.
Small chihuahuas can live for over 10 years.
People now think it might depend on the “evolutionary pressures” that each animal faces. An evolutionary pressure includes things like other animals that want to kill you for food. Because they are so big, large animals like elephants and whales are less likely to be attacked in the wild than small animals like guppies or mice.
Large animals like elephants and whales are less likely to be attacked in the wild than small animals.
Sizing up the question
It is a complex question with no easy answer. As a rule of thumb (there are outliers of course), lifespan is usually related to size in the animal kingdom. Larger animals such as elephants have remarkably long lifespans of up 70 years (or even centuries in the case of tortoises!). While small creatures such as mice will only live to about 1-2 years. Dogs fall somewhere within this range and have a lifespan of between 8-15 years, depending on breed and other factors. Dogs being a smaller size than the average human adult means that they have a shorter lifespan than we do.
Why do dogs have shorter lifespans than humans?
Lifespan in general is determined by trade-offs between survival and reproduction. Wolves, the ancestors of dogs, can live 15-20 years, roughly twice as long as comparable-sized dogs. They start breeding in the wild no younger than 2 years old. They need to form pairs and establish a territory before breeding. Older wolves will often have help raising their pups from older juveniles who have not managed to mate or find territories. In contrast, most dogs can breed from 6 months to 12 months of age, and they don’t benefit from having territories, pair bonds, or packs. Whereas wolves breed until they die, dog breeders will usually retire older females. So the whole life history of dogs is shifted to more of a “live fast, die young” style compared to wolves. Advertisement Advertisement Advertisement Advertisement
On top of that, artificial selection and inbreeding have created huge problems for dogs. Here’s what dogs look like when they’re not bred to conform to human expectations.
Striving to breed to an idealized “type” while ignoring basic physiological necessities doesn’t create a robust organism. That’s how we get tortured monstrosities like English bulldogs, which can barely breathe without snorting and whose pups must be cut out of the mother’s womb because she can no longer deliver them. Even seemingly harmless traits often bring a higher probability of serious health problems. White fur, for example, is often accompanied by neurological deficits ranging from subtle behavioral abnormalities to deafness or even early death. Generally speaking, working dogs have sustained longer lifespans because they’re required to be physically fit to do their jobs. Show dogs are mostly just required to meet peculiar aesthetic requirements and be easily managed. The lethargy resulting from chronic health problems is actually a positive for champions who dominate the gene pool, even if it shortens their lives. (It’s a bit like foot-binding, whose victims were prized as wives for their passive and mild behavior, which resulted from being crippled and in constant pain.) Advertisement Advertisement Advertisement Advertisement
Loss of genetic diversity also shortens lifespans. In a healthy population, essentially all individuals have several defective genes, but each defective gene is rare in the population as a whole. Each individual holds two copies of each gene, so in a randomly mating population, it’s rare for an individual to have two defective copies. Usually, as long as the individual has at least one good copy, it will be fine. Health problems only arise when an individual has two defective copies. But when the population experiences a genetic bottleneck—that is, only a few individuals get to breed—any defects they have will spread to a large proportion of the population. That means that when these individuals mate, a large proportion of their offspring will carry two copies of the genetic defect and therefore be unhealthy. Advertisement Advertisement
Unfortunately, for the last century or so, dog breeders have actively pursued a misguided strategy of purifying breeds by demonizing cross-breeding and allowing only “champions” to breed. Healthwise, this is terrible. You can’t eliminate all the subtle genetic problems that plague breeds by selective breeding. They arise faster than you can purge them. While some of the more severe defects have been reduced by conscientious breeders who test their dogs before breeding, this selective breeding further narrows the gene pool and has thus promoted many more defective genes that cause mild reductions in health and lifespan. We can’t test for these defects, and consequently they are now prevalent throughout most nonworking breeds.