The Basics of Thermoregulation in Animals

Written by Holly Pollard-Wright DVM, CCRP
Owner/CEO of VETERINARY KINETICS REHAB Inc

One of the critical functions of the nervous system is the regulation of body temperature. Body temperature regulation is essential to maintaining homeostasis, the state of steady internal, physical, and chemical conditions supported by living systems in normal function.1 Endothermic vertebrates are animals with a spinal backbone (consisting of a vertebrate) that can produce and regulate their body temperature independent of the environment. In contrast, ectotherms are animals dependent on external sources of body heat in which body temperature fluctuates according to the environment. Aerobic capacities to support sustained activity are thought to be a principal factor in the evolution of endothermy, the physiological generation, and body temperature regulation by metabolic means.2  Endothermic vertebrates’ resting and maximal oxygen consumption levels exceed ectotherms by an average of five to tenfold.2  Because of this, endotherms have a much broader range of activity that can be sustained by augmented aerobic metabolism, a chemical process in which oxygen is used to make energy from carbohydrates or sugars.2  In contrast, ectotherms during activity are more reliant upon and limited by anaerobic metabolism, a process where energy is created through the combustion of carbohydrates in the absence of oxygen. Birds and mammals can regulate their internal body temperature within a narrow range that is higher than the surroundings.3  Body temperature varies by individual, age, activity, and time of day and is dependent on a balance between heat inputs and heat outputs from the internal and external environment.4  

During activity at a relatively high body temperature, an animal can only initiate movement if it has concomitantly evolved the ability to warm itself.5  An elevated body temperature represents the means to enable the optimization of cellular processes, which evolved as a secondary consequence of the higher metabolic rates needed for sustained activity (e.g., flight) or occupation of new ecological niches (e.g., cold climates).3 In this process, biochemical restructuring for activity at high tissue temperatures evolved. This process extended the ability of an animal to be maximally active beyond the short time otherwise required to “overheat.”5  To sustain high activity rates for durations that exceed several minutes, large and highly active animals such as birds, mammals, many insects, and some present-day reptiles inevitably heat up during strenuous activity.5  The animals that evolved a pre-activity warm-up by shivering and basking became preadapted for homeothermy.5  Mammalian homeothermy is thermoregulation that maintains a stable internal body temperature regardless of external influence. It is thought to be an acquired evolutionary two-step process.6 In the first step, mammals were enabled to invade a niche nocturnal (i.e., active at night) without increasing resting metabolic rates. In the second step, mammals were allowed to invade a niche diurnal (i.e., active during the day) that involved the acquisition of higher body temperatures and metabolic rates.6 

Dogs and Thermoregulation

The maintenance of body temperature is a highly regulated process in which body temperature is determined by the difference between heat gain and heat loss. When body temperature is decreased, this can slow metabolic pathways to the point that vital bodily functions are affected. In contrast, prolonged or excessive increases in body temperature can cause proteins essential for living things to not function due to “denaturing,” where that protein’s shape will come undone. Thermoregulation is how body temperature is maintained with tightly controlled self-regulation independent of external temperatures. The thermoneutral zone (TNZ) is the normal range of temperatures at which dogs and other species can maintain their body temperatures without expending energy to increase heat production or heat loss. In dogs, the TNZ ranges from 20°C (68°F) to 30°C (86°F); when temperatures outside of this range occur, the dog has to expend energy to maintain its body temperature.7  Dogs have minimal sweating capacity, and thus sweating is not a significant heat regulation mechanism.8  The dogs’ sweat glands are primarily localized to their feet, and heat loss through sweating from the feet and nose is minimal. Because of this, dogs rely mainly on evaporative mechanisms through panting to thermoregulate.8  In dogs, heat tolerance is influenced by acclimatization to the environment, physical fitness, and hydration status. The dog’s endogenous adaptive mechanisms to combat heat stress include heat acclimation and the rapid heat shock response.9  Acclimation is a time-dependent process, leading to a dynamic expansion of the body temperature regulatory range. It includes equilibrium shifts related to the temperature threshold for heat dissipation, a type of heat transfer, and thermal injury.9  Heat acclimation (HA) is the process of intentional and consistent exercise in the heat that results in positive physiological adaptations.10  In this process, adaptive physiological and behavioral changes are induced, improving the individual’s ability to cope with extreme environmental heat and enhancing exercise performance in heat and thermoneutral conditions.9,10   The heat shock response is a rapid molecular cytoprotective mechanism that involves the production of heat shock proteins (HSPs), made when cells are briefly exposed to temperatures above their normal growth temperature.9


When dogs’ thermal regulatory capacity is overwhelmed, they may succumb to heat-related illness (HRI). This potentially fatal disorder includes hyperthermia, abnormally high body temperature, and subsequent tissue damage.11  Although dogs show differing risks based on age, sex, and underlying health status, three primary recognized HRI triggers are11 (a). Exertional HRI after exercise in a hot environment or following intense activity. (b). Environmental HRI results from exposure to extreme environmental heat or prolonged exposure to a hot climate. (c).  Vehicular HRI results from either entrapment or travel in a hot vehicle. In dogs, heat stress or heatstroke results from a failure to dissipate accumulated heat during exposure to hot environments or during strenuous physical exercise under heat stress. It is characterized by core body temperature > 105.8 F or 41C, with central nervous system dysfunction. Notably, a single temperature reading alone is poorly diagnostic of severe illness (unless that temperature is above 45 °C (113°F).11 Instead, it is the duration of temperature elevation that results in clinical pathology.11  The most common clinical signs in dogs with heatstroke include acute collapse, tachypnea (i.e., abnormally rapid breathing), spontaneous bleeding, shock signs, and mental abnormalities, including depression, disorientation or delirium, seizures, stupor, and coma.9  Lack of acclimation to heat and physical fitness decreases the survival of heat-stroked dogs.9  In large-scale studies of canine heatstroke, several predisposing risk factors have been identified, including obesity, brachycephalic dogs, large breeds, and hereditary factors.9  Excess weight can affect a dog’s ability to regulate its temperature and cause the body to retain more heat. Brachiocephalic dogs (e.g., English bulldog) have stenotic nares, elongated soft palate, and a hypoplastic trachea. In addition, they tend to develop laryngeal edema and thus have poor, ineffective evaporative ability. Large breed dogs such as military working dogs (e.g., Belgian Malinois) are likely overrepresented as patients due to their frequent exposure to intensive training and work under harsh environmental heat stress.9 This is especially true if insufficiently trained and not acclimated to the environment they are working in, rather than due to their larger body weight. In Labrador retriever dogs, hereditary factors include Exercise Induced Collapse (EIC) syndrome, characterized by episodic limb weakness, ataxia, and collapse and might thus account for some heatstroke cases in this breed.9


Basics Safety Measures Related to Dogs Engaged in Physical Activity

  1. Ensure that the exercising dog has plenty of access to water to help avoid possible dehydration. Hydration is thought to impact the dog’s ability to thermoregulate.8  A common strategy to promote hydration in dogs is to allow them free access to water.
  2. Provide areas for the dog to retreat to cool down if necessary.1
  3. Allow a rest period before resuming exercise in a dog that shows signs of significant fatigue.1  
  4. Owners should seek immediate veterinary attention for dogs that continually pant or have increased respiratory effort that does not resolve when the dog is removed from the hot environment or exercise has ceased. Seek veterinary attention for dogs that show lethargy, stiffness, or are unwilling to move.11

References

  1. Millis, D. L., & Levine, D. (2014). Canine rehabilitation and physical therapy (2nd ed.). Saunders, Cop.
  2. Bennett, A., & Ruben, J. (1979). Endothermy and activity in vertebrates.  Science, 206(4419), 649–654.  https://doi.org/10.1126/science.493968 
  3. Tan, C. L., & Knight, Z. A. (2018). Regulation of Body Temperature by the Nervous System.  Neuron, 98(1), 31–48.  https://doi.org/10.1016/j.neuron.2018.02.022
  4. Cunningham, J. G., & Klein, B. G. (2013). Cunningham’s textbook of veterinary physiology. Elsevier/Saunders.
  5. Heinrich, B. (1977). Why Have Some Animals Evolved to Regulate a High Body Temperature?  The American Naturalist, 111(980), 623–640.  https://doi.org/10.1086/283196
  6. Crompton, A. W., Taylor, C. R., & Jagger, J. A. (1978). Evolution of homeothermy in mammals.  Nature, 272(5651), 333–336.  https://doi.org/10.1038/272333a0
  7. National Research Council, 2006. Nutrient Requirements of Dogs and Cats. National Academies Press, Washington, D.C.
  8. Otto, C. M., Hare, E., Nord, J. L., Palermo, S. M., Kelsey, K. M., Darling, T. A., Schmidt, K., & Coleman, D. (2017). Evaluation of Three Hydration Strategies in Detection Dogs Working in a Hot Environment.  Frontiers in Veterinary Science, 4.  https://doi.org/10.3389/fvets.2017.00174 
  9. Bruchim, Y., Horowitz, M., & Aroch, I. (2017). Pathophysiology of heatstroke in dogs – revisited.  Temperature, 4(4), 356–370.  https://doi.org/10.1080/23328940.2017.1367457
  10. Benjamin, C. L., Sekiguchi, Y., Fry, L. A., & Casa, D. J. (2019). Performance Changes Following Heat Acclimation and the Factors That Influence These Changes: Meta-Analysis and Meta-Regression. Frontiers in Physiology, 10.  https://doi.org/10.3389/fphys.2019.01448
  11. Hall, E. J., Carter, A. J., Bradbury, J., Barfield, D., & O’Neill, D. G. (2021). Proposing the VetCompass clinical grading tool for heat-related illness in dogs.  Scientific Reports, 11(1).  https://doi.org/10.1038/s41598-021-86235-w