Many studies have documented that genetics explains part of the variation among dogs in hip scores. This is usually 15-30% of the total variation, but in some cases it can be as much as 60%.
This means that environmental factors must account for the remainder of the variation; i.e., as little as 40%, but usually 70-85%.
When breeders are focusing on selection to improve hips, they are trying to chip away at genetics, which usually matters much less than environmental factors.
“Environment” as used here encompasses all factors that are “not genetic”, which can be anything from type of exercise to food consumption.
One way to tease out the effects of these environmental factors is to isolate one, manipulate it, and observe the effect on the trait of interest, in this case, the hip score. There are hardly any studies of this sort on hip dysplasia dogs, but there is one on rats that is key to understanding the role of traction in the whelping box in the risk of developing hip dysplasia.
I have explained to you the importance of a tiny ligament (teres ligament) in the hip socket that keeps the head of the femur snugly in the hip socket in newborn puppies. ([Do your puppies have enough traction in the whelping box?](https://www.instituteofcaninebiology.org/blog/do-your-puppies-have-enough-traction-in-the-whelping-box)) In that post, I showed you how that ligament is stressed when the puppy’s legs are in the “extended and adducted” position (straight behind the puppy and pulled together). (Review that explanation if you don’t remember how this works – it’s important). This position of the legs can result in hip dysplasia in human infants, as when the baby is swaddled up tightly like a burrito. How can we prove that the same mechanism applies in animals besides humans?
You could do an experiment with animals that mimics the “burrito swaddle” by putting the legs in the extended/adducted position and looking at the effect on hip conformation.
Here’s a study that did exactly that. This experiment was on rats, but similar studies have been done with rabbits and other mammals. The experiment was simple. The legs of baby rats were taped together (“swaddled”) and evaluated after 5 days and 10 days, and compared to a control group (no tape).
The prevalence of developmental dysplasia was highest in the pups that were swaddled for the entire 10 days (36 of 44 pups), and in most of these the hips were dislocated. Dysplasia (as subluxation) was less frequent (21 of 44) in the pups swaddled for only the first 5 days or the second 5 days.
Think about this. These rat pups differed only in whether their legs were taped together or not. So it is the position of the legs – extended and adducted – that resulted in hip dysplasia. In the pups with legs taped for 10 days, the teres ligament was simply gone. In pups with legs taped for only 5 days, the teres ligament was present but damaged.
The first question you are asking is whether we know if this applies to dogs. A similar experiment has been done on puppies, but one leg was put in a cast to keep it in the extended position, and this also resulted in dysplasia in the hip on that side.
How is this relevant to dogs? Look at the many photos and videos of very young puppies nursing and crawling around in whelping boxes with poor traction. The back legs are extended and adducted, over and over, as the pup tries to push itself forward. Over the course of the three to four weeks before it starts to walk, a puppy could do this thousands of times, putting stress on the teres ligament every time.
From the experiment on rats and others that have been done on other newborn mammals, we should expect this leg movement that we usually call “crawling” will result in dysplastic hips.
Notice that although all of the rat pups in a treatment group were treated the same way, not all of them developed hip dysplasia (e.g. 36 of 44 in the 10 day group). The reason for this would have to be revealed in additional studies, but it demonstrates that we should not expect clear, consistent differences among the animals in each group.
The purpose of the Traction Mat is specifically prevent this leg position of extended and adducted. It does not provide general traction like a rough surface would, but instead provides a vertical surface the pup can push against instead of slipping over the surface of the floor.
If you understand this, you will realize that we will never solve the hip dysplasia problem by trying to choose parents with better hips and removing the others from the breeding stock. This also means, however, that if the teres ligament is damaged early in a pup’s life, other factors – both environmental, like exercise and overweight, and genetic, like large body size – can determine the whether dysplasia is modest or severe.
If you want to produce dogs with sound hips, you must protect the teres ligament from the damage that occurs when the puppy extends and adducts the back legs to try to move itself forward.
In fact, we cannot eliminate hip dysplasia in dogs without doing this, because the risk starts at the moment of birth, and the damage is done in the first few weeks after whelping. We know from the rat study that we can’t expect every puppy, even in the same litter, to have excellent hips. But we should be able to reduce the incidence and severity of hip dysplasia by addressing the basic traction problem.