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Fact #1- a double coat is comprised of two layers - a topcoat, which is longer, sleeker "guard" hairs that act as a sort of shield against exposure to the elements. and an undercoat, which is shorter, softer, denser hairs that act as insulation. double coats are found on breeds of dogs such as collies, huskies, and labs. conversely a single coat, which lacks the thick undercoat layer, would be the type found on a dog like a pitbull.

(this collie is shedding its undercoat - note the lighter, softer hair pulling away from the longer darker hair. source.)

Fact #2 - dogs are mammals which obey the laws of thermodynamics, meaning they are constantly producing body heat. a dog's average body temperature is normally right around the 100f (38c), and they are constantly radiating heat. this is important.

in the winter, a double coat works fabulously for keeping a dog warm - the topcoat repels moisture and blocks wind, while the undercoat traps body heat and holds it close to the skin. there's a reason why cold-weather animals like wolves and foxes and bison all have thick double coats - they work great at trapping body heat!

but then, the summer. part of the myth that gets so often cited is "the undercoat traps cool air and keeps the dog cooler" to which we have to ask - what cool air? if it's 90f outside and my dog is radiating 100f heat, there’s no cool air to be trapped. and even if there were, air trapped in the undercoat would be absorbing the body heat from the dog's skin and quickly turning into hot air, which then further heats the dog. this is basic heat exchange physics - hotter temps will transfer to cooler temps until the temps have equalized. the phrasing of "traps warmth in winter and traps cold in summer" sounds nice and neat, but it's ignoring the fact that your dog is constantly producing body heat. this theory works fine on non-living objects, like a thermos full of cold water. it does not work on an object that is radiating 100f heat.

IF double coats truly kept dogs cooler in hot temperatures, then we would expect to see scientific studies backing this up, right? studies that show dogs with double coats have no problems keeping cool in hot weather? well, the studies we've done say the opposite. studies show that dogs with thick double coats are MORE likely to overheat than dogs with short single coats.

here's a list of sources saying exactly this, from the blog post i linked earlier:

"Contrary to cold environments, increased insulation of fur is not beneficial to maintaining heat balance in warm environments." - Seasonal Changes in Heat Balance of Dogs Acclimatized to Outdoor Climate, Yasuyuki Sugano

“Predisposing factors for heatstroke -thick haircoat" - Short technical Report on Thermoregulation in Dogs and the Pathophysiology of Hyperthermia, Jerilee A. Zezula, DVM

“Predisposing Factors for Heatstroke – thick haircoat" - Heatstroke – thermoregulation, pathophysiology and predisposing factors, Carey Hemmelgarn DVM ChristI Gannon DVM DacVecc

“Factors that inhibit heat dissipation: Thick, dense coat – increased insulating effect” - Hyperthermia and Heatstroke in the Canine, Lori E. Gordon, DVM

“Well-insulated breeds with thick fur and fat may struggle to maintain a normal body temperature,especially during hot summer days and in a warm ambient environment.“ - Hyperthermia during anaesthesia, Author : Clara Rigotti, Marieke De Vries 2010

“Well-insulated breeds, such as Chow Chows and St. Bernards, may struggle to maintain normal core body temperature, making them more prone to hyperthermia, especially in a warm, stressful environment” - Hypothermia in a chow chow under general anaesthetic, H.Jones, K.Robson ,2022

“Predisposing factors that decrease heat dissipation: Hair coat – Thicker coats decrease radiation and convection” - Heatstroke in small animal medicine: a clinical practice review, Scott I. Johnson, DVM, Maureen McMichael, DVM, DACVECC and George White, DVM

”a thick insulating pelage is a disadvantage in situations where energy supply is unlimited and expenditure is constrained by the capacity to dissipate body heat.This is because the pelage insulation becomes the primary constraint on heat loss. “ - Maximal heat dissipation capacity and hyperthermia risk: Neglected key factors in the ecology of endotherms, Journal of Animal Ecology  79(4):726-46 J. Speakman, E. Krol

ALL of these studies are saying the same thing - animals with thicker coats cannot dissipate heat easily, and are at a higher risk for overheating and heat stroke.

so now here's tumblr's favorite part - nuance. does this mean that every dog with a double coat is completely intolerant of heat, and you should just shave them all bald instead?

no.

the double coat DOES offer some advantages in hot weather - primarily relating to the topcoat. it blocks UV radiation from reaching the dogs skin, and keeps the heat of the sun off of direct contact with the skin. this is extremely useful! my old dog was a greyhound, who had a short, black single coat. he was miserably intolerant of heat because even mild sunshine would rapidly heat his surface temperature. the topcoat is VERY useful in this regard, and shaving a dog is ridding them of this natural barrier.

however. the undercoat is almost entirely detrimental in hot weather. again - it's entire purpose is trapping heat, and your dog is constantly radiating 100f heat.

look to nature - what happens when those fluffy, thick-furred wolves and foxes and bison hit summer? they shed out almost their entire undercoat. they get downright nakey.

(ex: arctic foxes shed dramatically - all that white fluff is the winter coat. if it kept them cooler in the summer, they would have evolved to keep it. source.)

the ideal way for a double-coated dog to deal with heat is not to shave off the whole coat, but instead to groom and bathe and brush the absolute crap out of their coat, and remove as much undercoat as possible. when the undercoat is reduced, it becomes looser, and allows air to flow throughout the coat. that FLOWING air is the key to staying cool! when the air is able to CIRCULATE through the coat, it wicks away the dog's body heat and disperses it outside the coat. THAT is the action that is keeping them cool - not “trapping cold air” because trapped air will always absorb body heat. it’s got to MOVE. instead of acting like a snug winter sweater, the double coat can now act like a loose, flowing shirt.

but! if that undercoat is not thoroughly and continuously groomed and bathed and brushed, and allowed to stay heavy and thick and block airflow, then you're right back to having a dog that's broiling itself with its own body heat.

(you gotta get that undercoat out if you want any chance of not overheating. source.)

so, in conclusion: double coats are great for keeping warm in cold weather. they are NOT great for keeping cool in warm weather, and in fact can be actively harmful UNLESS you take great care to brush thoroughly to remove undercoat and allow for airflow. IF that is done, then a double-coated dog should be either just as good as, or only slightly worse off, than a dog with less hair. however, the blanket statement of "double coats trap cool air and keep dogs cooler in summer" is factually untrue because the entire point is to NOT trap any air to stay cool. in fact, the thing that keeps a double-coated dog the coolest is removing as much undercoat as possible - AKA, making it as close to single-coated as possible.

again, i recommend reading this blog post for more in-depth discussion and more sources.