In a webinar presented by Dr Michael Davis on Exercise in environmental extremes, and again in a Hydro Training Video where I discussed a research paper on the Physiological changes in swimming dogs, it was highlighted over and over again that on a physiological level, there are significant differences between species. Dogs, horses, people (and most likely cats!) have unique responses in certain situations, conditions, and environments. Not just behaviourally, but all the way down to a cellular level.
We need to really consider and have a basic understanding of these differences – and I say basic, because we don’t have enough knowledge and data yet to truly understand these differences on a deeper level – when we want to treat these different species, especially when it comes to exercise and working with the athlete, as well as when we are working with a patient that has a compromised cardiovascular system.
Differences Known
Some of the differences that we do know about, include heat dissipation and tolerance, pH responses in the body, blood distribution, blood stores, oxygen availability, heart rate responses, respiration rate responses… These are all physiological responses that have differing and often opposite mechanisms in each species.
Heat Dissipation
There are significant differences in how humans, horses, and dogs dissipate heat from their bodies into the environment. For humans and horses, heat is dissipated through the skin. We can see this when using thermography, as well as observing our response to exercise. Sweat glands across the skin are highly capillarised, heat travels from the muscle, through the blood stream, to our skin and is dissipated into the environment thanks to the evaporation of sweat. The disadvantage of this, is that there needs to be sufficient movement of the surrounding air over the skin to allow effective evaporation. And that is where the canine has the advantage.
For dogs, the primary area of heat dissipation that they can control or influence is through their face and tongue (Davis, 2020; Krönert & Breazile, 1976). And because they can control the movement of air across this dissipation surface through their respiration, they have a large advantage in terms of cooling themselves, as they are less dependent on the movement of their body through the air, or the environmental availability of moving air. The disadvantage is that their dissipation surface is much much much smaller. Dogs also dissipate heat through their paws.
Heat Dissipation through Respiration
There are a few noteworthy physiological consequences of heat dissipation through the tongue and with the aid of the respiratory tract. Firstly, because the dog is not sweating, there is no loss of electrolytes in this cooling process. Which leads to the osmolarity of body fluids and a resultant increase in thirst.
As a result of the dog panting during exercise, they will become hypocapnic as their CO2 levels drop by approximately 20% (Musch et al., 1985). Humans and horses will experience an increase in CO2 production during submaximal exercise (Davis, 2020). As far as we know, signs of metabolic activity such as an increase in temperature and CO2 coupled with a decreased pH will enhance the delivery of oxygen to active tissue as haemoglobin loses some affinity for holding onto oxygen in all species.
In dogs we see something very interesting that is contrary to this understanding. As they exercise in higher altitudes, their pH rises and their CO2 levels drop, which should (according to our understanding), lead to increased oxygen saturation in the blood stream. Instead we see the opposite, a decreased oxygen saturation and blue mucous membranes (Davies, 2020)
Blood Distribution
To facilitate this difference in cooling, the blood distribution in the body needs to be optimized. We can see this difference between species again, as in horses during moderate exercise, the blood flow to the skin increases by 60-70%, while in a dog the blood flow to the skin decreases by 75% and increases to the tongue by 200% or more (Davies, 2020; Musch et al, 1987).
There is an additional difference to the blood distribution and availability to the body between dogs and horses – horses have a store of blood in the spleen that is released into the body through splenic contraction during exercise. This increases the amount of blood in circulation by about 30%, making a greater amount of oxygen available to the muscles for optimal function. Dog’s just don’t have this reserve.
When it comes to blood supply to the muscle during maximal exercise, dogs will increase the distribution of blood to the muscles by 4-5 times, while horses will increase the blood distributed to the muscles by 15 times. (Davies, 2020; Hodgson et al., 1993)
Heart Rate
In people and other small species, the body responds to changes in temperature with a change in heart rate – a decrease in environmental temperature leads to a decreased heart rate, and increase in temperature leads to an increase in heart rate (Davies, 2020).
Cardiovascular Capacity
Cardiovascular capacity is essential for optimal function, health and athletic ability, and is the most important factor when it comes to the ability to dissipate heat. But again, we respond differently to a fitness program to improve our cardiovascular capacity. Horses will respond to an appropriate fitness program with an amazing 57% increase in their cardiac capacity, while dogs will respond with a 27% increase in their capacity (Davies, 2020).
Unknown Differences
And now, shall we enter the realm of the unknown?
With the developments made through research that have highlighted the above differences between species, more questions have been raised than answered, and with the differences known come mechanisms and process still unknown, and many scenarios and situations still untested.
My true purpose in highlighting these differences is to add a note of caution and awareness to our thinking. We have all been trained differently and have different backgrounds, some of us have been competing with dogs in sports all our lives – we would be cautious to advise equine clients based on our experience in the canine industry. The same thing applies to those of us who have been surrounded by horses our whole lives and now find ourselves working with small animals, often in hydrotherapy situations where environmental factors are closely coupled to exercise. We need to know what is and is not safe for our patients, and this knowledge can’t be extrapolated from our experience and history with a different species.
If you haven’t yet, please do watch the webinar recording by Dr Michael Davis, on Exercise in Extreme Environments. It is more than fascinating!
And if you would like to watch the Hydro Training video where I discuss the research paper Physiological Effects of Water Temperature on Swimming Toy Breed Dogs, it shares some interesting findings also!
I would love to hear from you – please leave your comments on how your experience has given you an advantage in understanding the needs of a patient, as well as where they have left you in the dark!!
References
Davis, M.S. (2020) From the Mountains to the Deserts: Exercise in Environmental Extremes. Webinar presented for [Onlinepethealth], 29 January 2020.
Krönert H, Pleschka K. Lingual blood flow and its hypothalamic control in the dog during panting. Pflugers Arch. 1976 Nov 30;367(1):25-31. doi: 10.1007/BF00583652. PMID: 1034283.
Musch TI, Friedman DB, Haidet GC, Stray-Gundersen J, Waldrop TG, Ordway GA. Arterial blood gases and acid-base status of dogs during graded dynamic exercise. J Appl Physiol (1985). 1986 Nov;61(5):1914-9. doi: 10.1152/jappl.1986.61.5.1914. PMID: 3096950.
Musch TI, Friedman DB, Pitetti KH, Haidet GC, Stray-Gundersen J, Mitchell JH, Ordway GA. Regional distribution of blood flow of dogs during graded dynamic exercise. J Appl Physiol (1985). 1987 Dec;63(6):2269-77. doi: 10.1152/jappl.1987.63.6.2269. PMID: 3436863.
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