The relationship between humans and dogs is one of the greatest examples of symbiosis between two different species.
It is a mutually beneficial relationship, taken to the extreme, to the point that humans have bred dogs to enhance the traits that interested them. A greater sense of smell in tracking dogs such as the Bloodhound, the ability to herd and control flocks in sheepdogs such as the Border Collie, the powerful speed and agility of the Spanish Greyhound…
One of the characteristics that has been sought in some breeds may be more surprising: a smaller size.
There are considered to be three main reasons why owners may have developed smaller dogs:
1. Hunting and pest control. A large dog is perfect for big game hunting, but you don’t need 30 kilos of muscle when your goal is to eliminate rodents from your barn. A perfect example of this is the Schnauzer. The initial breed is the so-called Standard Schnauzer, from which, through crossbreeding, the Giant Schnauzer (for herding and protection) was first obtained, and later the Miniature Schnauzer (for pest control).
Another example is the Miniature Bull Terrier. Physically, they are identical to the Bull Terrier, except for their size. Their purpose was to hunt rats and mice on the ground.
2. Companion animals. As the work done by dogs was replaced or lost relevance, their role as companion animals became more important.
In addition, with populations moving to cities and living in smaller spaces, smaller breeds were better suited to these new conditions.
Sometimes these fads posed a real problem. Returning to the miniature Bull Terrier, when breeding increasingly smaller breeds became popular, some tried to turn it into a toy dog, which led to serious health problems.
Fortunately, standards have long been established to ensure the welfare of animals, and the current breed remains healthy thanks to responsible breeders.
3. Legal restrictions in the Middle Ages. Yes, it is what it sounds like. In many European societies in the Middle Ages, you could not own a ‘hunting dog’ unless you had the appropriate hunting permits. Permits that few people, such as nobles, could obtain. In the case of large dogs, the way to prevent them from being ‘hunting dogs’ was to inflict a series of injuries that would render them incapable of hunting. But another option that many people found was simply to avoid large dogs. No one would see your Pomeranian and accuse you of having a hunting dog.
Today, point two is undoubtedly the dominant one. In cities, dogs belonging to so-called miniature breeds account for almost half of all animals.
How do you get small dogs?
To obtain these small dogs, crossbreeding was key.
You had litters of dogs and crossed the smallest ones, the trait you were interested in, hoping that the new offspring would be of that size or even smaller.
This selective breeding required time and a certain amount of luck for the offspring to accumulate the genetic variants that were of interest. This process has been going on for tens, hundreds of years. Another advantage of the process is that dogs have more numerous offspring than we do, and they can breed earlier.
The 15th-century farmer was doing genetic engineering without knowing it.
Another way was to cross the animal directly with another small breed. In the case of the Schnauzer mentioned above, crosses with Affenpinschers were mainly used to obtain the miniature variety.
Another curious detail: if we compare the annual mutations in the human species with those in the canine species, dogs have a higher mutation rate than we do. Their genome is more adaptable than ours and undergoes greater changes.
Because in the end, as always, genetics is the key to these differences in size.
The genetics of dog size
Twenty genes have been identified so far that regulate the animal’s body size through different pathways.
The best known and most studied gene, and certainly the most important for dog size, is the IGF1 (Insulin-like Growth Factor 1) gene. This gene is thought to control up to 15% of the variation between different dog breeds.
This gene encodes the information to produce a hormone in response to growth hormone (GH). Among other functions, this hormone promotes cell and tissue growth, which is particularly important during the animal’s youth, the stage of greatest growth.
It has been found that small species have fewer copies of this gene and/or less active variants, leading to lower hormone production.
Seventy-five per cent of dogs weighing less than 15 kilograms had two copies of the C variant of the IGF1 gene. Conversely, 75% of dogs weighing more than 22 kilograms had two copies of the T variant. The effect was so marked that the researchers directly called them small and large alleles, respectively.
Other studies have also found a correlation between low levels of the hormone in the blood and smaller sizes.
These variants are not believed to be new mutations. Ancient wolf populations already possessed them and may have used them to adapt to changes in their ecosystem.
However, only some of the animals would have these genetic variants similar to the C allele, and to a lesser extent. It was dog breeders, through selective breeding, who unwittingly put it back into circulation.
At the chromosomal level, it appears that the ‘headquarters’ of size are located in a region of chromosome 15, where the IGF1 gene is located.
Small dogs have this genetic region very similar to each other, even if they are of different breeds.
Another curious fact is that Rottweilers had a region similar to miniature breeds even though they do not belong to that group. This gives us an idea of the level of complexity involved in the factors that define an animal’s size.
Other genes known to be involved are the GHR gene, which codes for the growth hormone receptor, and the IGF1R gene, the receptor for the IGF1 hormone itself.
It is likely that the genes that influence human size have the same effect in dogs, or some analogue thereof.