The Wonder of Bone.

We recently attended an incredible talk by Dr Martin Fischer on the wonders of Dogs in Motion. This frankly blew our tiny minds. So we’d like to share some of the incredible work being done on bones and explain how this links with canine heatstroke.

Dog breeds and bones

It all comes down to the skull. Firstly, a bit of background on how selectively breeding dogs for skull shape impacts the skeleton as a whole.

Dogs with a shortened nose (brachycephalic breeds) often also have a shorter, squarer pelvis. This has a cascade effect. It impacts how they walk, which in turn impacts the forces on their leg bones, which impacts the shape of the long bones. Tall, slender dog breeds tend to walk with their legs underneath their body, like a model on a cat walk stepping one foot in front of the other. This is the most efficient method of moving if you are a dog. As soon as the dog becomes broader, the chest becomes more barrel shaped, the dog walks with the feet either directly underneath the shoulder joint, or, in very barrel chested dogs they may even walk with the feet wider than their body, like a cowboy. This is not an efficient way of moving, but may be necessary for fighting. Traditionally, selective breeding aimed to develop a dog with a purpose. Sight hounds were bred for speed to catch prey, bull breeds were developed for their jaw strength for fighting.

Two very different dogs

This altered skeleton structure also causes the legs to move differently. In a Greyhound or Whippet, the legs simply swing back and forth, like a pendulum. The bones grow in an elliptical shape as this provides the best strength to withstand the force of trotting, running, jumping and landing. This again is streamlined and efficient. In a barrel chested breed, the legs don’t just swing back and forth, they twist and rotate to provide the same stride length. This twisting and rotating causes the bone to grow in a perfect circle, which is less aerodynamic and reflects the less streamlined motion. This means a barrel chested dog, which includes almost all brachycephalic breeds, has to work harder to move the same distance as a non barrel chested breed. This increased work, requires increased effort, which generates more body heat from muscle activity. They also tire faster, meaning exhaustion occurs more quickly.

What do bones have to do with heatstroke?

Dogs only sweat on their paw pads, so they rely on their breathing for temperature regulation. A dog’s long muzzle (nose) is a key method of cooling. Inside these long muzzles, are the nasal turbinates (see below), as series of scroll like bones that are coated in tissue similar to the surface of the lips and gums. These nasal turbinates hugely increase the surface area within the muzzle. This means a huge area of wet tissue is available for air to pass over every time the dog inhales. This functions to trap particles and potentially dangerous microbes like bacteria within the mucous in the nostrils to reduce the risk of respiratory infections. But most importantly, passing cool air over a wet tissue, allows evaporation and cooling.

Dog skull cross section

Dogs with a short muzzle, have a smaller area available for air to cool within the nasal turbinates. If the dog with a short muzzle also suffers from brachycephalic obstructive airway disorder (BOAS), they also have smaller nostrils, so the air cannot flow into the nasal turbinates as quickly. They then have narrowed air passages in the throat and whole windpipe, meaning both inhalation and exhalation is slower, and requires more effort. Remember these dogs are also using more effort and energy to move their limbs than a normal dog, add to that increased heat generated by the respiratory muscles having to work harder too.

Nose length comparison
Compare the length of the muzzle on the two skulls. The brachycephlic breed on the left has a much shorter muzzle, so significantly less surface area of nasal turbinates for cooling.

So, not only do brachycephalic breeds over-heat more quickly because simply walking and trotting they use more energy than a longer legged, longer nosed dog, but they also have a significantly less effective cooling mechanism because of their short nose. At the talk by Dr Fischer we were told that in Germany, Bulldog owners are advised to actively cool their dog as soon as ambient temperature exceeds 25oC. French Bulldog owners are advised to actively cool their dog above 28oC.

Altering bone, alters a dog’s risk of developing heat stroke.

Bone and insulation

The wonders of bone continue. People often think of a bone as a permanent, fairly fixed body structure. You suffer growing pains during adolescence whilst your skeleton grows and develops, then you reach your adult size and that’s about it. However, bone is continually growing, developing and changing right up until you reach middle age. Bone reacts to the stresses placed upon it. If you do a lot of high impact activity like aerobics, running, jumping or cross fitness, your bone responds to this by strengthening and changing in structure. If you are, as Dr Fischer put it, a “sofa pumpkin” (aka a couch potato, but we MUCH prefer this term!) and subject your bone to very little strain and load, bone will become less dense as you age.

sofa dog

How does this affect body insulation? Active bone, releases a hormone called osteocalcin. This hormone alters how cells respond to insulin in both mice, and humans. A failure to respond to insulin causes type 2 diabetes mellitus. In humans this condition is associated with a sedentary lifestyle, excess intake of carbohydrates and obesity. Mice with no osteocalcin are more likely to develop both type 2 diabetes and obesity. Mice with high levels of osteocalcin are almost resistant to developing type 2 diabetes and obesity. The link between a sedentary lifestyle and an increased risk of both obesity and diabetes mellitus type 2 is partly due to bone.

So, a dog doing very little high intensity exercise that involves stressing and loading the bone (just walking and mainly being a ‘sofa pumpkin’) is at greater risk of becoming obese and developing type 2 diabetes. The obesity is a big problem. An increased layer of fat under the skin acts as insulation, so the dog loses less heat into the surrounding air. This means the dog needs less energy to maintain body temperature (dogs typically use around 70% of their energy intake from food just to maintain their body temperature, this increases if the dog is in a cold environment). So if their diet remains the same, they will be consuming too many calories for simply maintaining their body, so the excess energy will be stored as, you guessed it, more fat. This becomes a vicious cycle.

Sofa Pumpkin

Whilst dogs lose heat through their respiratory tract, in particular using their nasal turbinates, they also lose heat through conduction and convection into the air and the floor in contact with their skin. If they are overweight the extra fat beneath the skin reduces this heat loss. These dogs are then heavily reliant on using their respiratory tract to cool, so you might notice they pant more than a less overweight dog, at lower ambient temperatures. Obese dogs are at increased risk of heatstroke, and are more likely to die from the condition.

How do bones impact activity?

Anyone who has ever experienced osteoarthritis or a fractured bone will know the answer to this question all too well. A painful joint or bone is horribly unpleasant, so we avoid using it. Not using a bone, especially a bone in the leg, will impact loading on that bone. This impacts bone strength, the release of osteocalcin, and also the health of cartilage that lines our bones in joints. Cartilage relies on loading to supply nutrients to the deeper layers, no nutrients, the cartilage thins and calcifies into new, abnormal bone. This contributes to osteoarthritis development.

So the less you use your bones, the weaker they become, and the more likely it is that you will develop painful joint disease. This makes you less inclined to use your bones, because they are painful. Which starts the cycle all over again.

So what should we do?

Body composition and skeletal health is hugely important for your dog’s overall health and welfare. Musculoskeletal disorders, including osteoarthritis, are among the top three causes of death in dogs, alongside cancer and neurological  problems.

Look critically at your dog. Can you see their ribs or can you feel them? If not, your dog could probably lose some weight. This will also allow them to be more flexible and active. Being more active, will prolong their bone and joint health. Ultimately, this will reduce their risk of heatstroke.

If the wonder of bones has gripped you as much as it did us, we thoroughly recommend Dr Fischer’s book, Dogs in Motion. A sneak preview in the video below.

Dog’s in motion (opens video in new window)

 

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