Why do groomers tell you not to shear a long-haired dog in warm weather?

The main question is this: Is your dog

• Single coated with hairs that grow endlessly at a constant speed?
• Or double coated with hairs that grow to a predefined length and are shed?

Single coated breeds are often "fashion" or "toy" breeds like Poodles and Shih Tzu. They actually do need regular hair cuts because their hairs don't stop growing.

Most dog breeds are double coated, especially herding and mountain breeds and those from arctic regions, like Huskies, Shepherd dogs, Collies, Shelties, Golden Retrievers, Newfoundlands, Bernese Mountain Dogs and many more.

Double coated means that there are 2 types of hair:

1. The primary hair or "top coat" is thick, smooth, shiny and long. It protects the undercoat from dirt and water and reflects more sunlight than it absorbs. It grows at an extremely slow speed and is shed whenever it happens to reach it's full length.
2. The secondary hair or "undercoat" is thin, fuzzy and shorter than the top coat, but each follicle produces several hairs. This is the thermal insulator. It grows quickly to a predefined length, then stops growing for several months and is finally shed in bulk seasonally.

The problem is that a hair clipper cannot distinguish between top coat or undercoat, it just cuts all hairs to the same length. The top coat doesn't regrow fast, and if the dog develops a condition called Post Clipping Alopecia it doesn't regrow at all. The undercoat will regrow quickly and it will be longer than the remains of the top coat. To be absolutely clear: One single haircut changes the fur of your dog for a VERY long time, possibly for the rest of its life.

Gone is the smooth, shiny protective top coat. Gone is the soft cushioning fur that protected your dog from physical injuries and sunburn.

• As soon as the undercoat regrows, it will be longer than the remains of the top coat.
• The fuzzy undercoat absorbs more sunlight, making your dog hotter in the long run.
• It doesn't repel water anymore. On the contrary, it soaks up rain and water and keeps the dog wet longer.
• The fuzzy fur is prone to matting and picking up burrs and all sorts of dirt.
• Due to the missing protective layer, the undercoat becomes coarse like velcro.
• It can take months or years for the top coat to regrow, depending on breed and coat length. If the undercoat matts in that time or becomes coarse, you might be forced to shear the dog again, prolonging the time to full recovery of the coat and increasing the risk of permanent change or damage.

Alternative

Instead of shaving your dog, you should remove the already shed undercoat as completely as possible. Brush her fur thoroughly or let a professional groomer wash her. Groomers use cold hair dryers to blow the loose hairs out of the fur.

You should give her a cool, shaded place to rest and maybe even offer her a small basin of water to cool off in.

Then why do groomers say this rubbish about keeping cool?

To be honest, I cannot tell you for sure. It could be a case of half-knowledge being repeated without anyone doubting it.

It could also be a psychological reason. Imagine this conversation:

Owner: I want you to shave my dog, she's too hot in summer.
Groomer: That's a bad idea, her coat will be ruined.
Owner: You just shaved that poodle, you obviously had no problem ruining his coat. My own hair isn't ruined by a haircut either. My dog is too hot in summer, so I want you to shave her now.
Groomer (doesn't want to hold a lecture): Please believe me that neither you nor your dog will be comfortable with the outcome.
Owner (doesn't want to let their dog suffer): I'm certain she'll be very comfortable once she cools down a little, after all this fur is gone.

It's the same as saying "smoking is bad for you". The uncertain future negative effect doesn't feel as important as the current negative situation. The possible outcome feels unreal and intangible, while the current situation is very real and can be changed. Most people tend to ignore statistics and evidence and think "That only happens to others, I'll be fine".

On the other hand, if the groomer repeats the half-truth about fur keeping your dog cool, the owner feels like they are doing the right thing by not shaving the dog. Discussions and lectures are avoided. Most people accept the statement without questioning it or don't have the expertise to question it.

I agree 100% with Elmy

Sometimes when groomers are trained, they may not be told or may not absorb the true reason for doing certain tasks in certain ways. They may simply be told, "Don't do this".

There are plenty of folk-tales circulating in the dog world that have little or no scientific basis.

A groomer will know that it is a bad idea to trim certain breeds short. Failure to know this may even have resulted in lawsuits in the past by owners complaining about a ruined coat.

Telling an owner, that leaving the coat long will be beneficial for the dog, is also a way of deflecting argument from awkward customers.

Suggestions

You can hose a dog down if it is outside in very hot weather.

Allow the dog to find the coolest, darkest place in the house and sleep there. Tiles are better than carpets for this. Dogs can sleep a lot in hot weather and it won't harm them.

Take the dog for walks early in the day when the temperature is at its lowest.

As always, make sure that fresh cool water is available near the dog at all times.

I will provide a comparison dog - human, along with explanations.

For a human, there are 2 ways for cooling:

• lowering the temperature of the environment; possible indoors with air conditioning, not possible under the full sun;
• perspiration + evaporation; the effect is amplified by applying ventilation, even without any active cooling. It works, because the water (perspiration) absorbs heat while evaporating.

For a dog:

• lowering the temperature of the environment.

Why is there only one option for dogs? Simply because dogs cannot sweat through their skin. The only mechanisms they have for biological cooling are:

• sweating through the paws and nose; remember the wet nose?
• evaporation of the saliva; remember the dogs keeping their mouth open and the tongue hanging, breathing heavily?

If the dog has his hairs not trimmed, then the dog will only have to deal with his natural internal heat. If the hairs are trimmed / shaved, the sun will heat the dog additionally, to levels which are not at all manageable by their biological processes. Additionally, if the hairs are trimmed excessively short they might even get severe sun-burns.

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Bottom line: With all the fur, the dog will feel hot. Without all his fur, the dog will feel even hotter.

Bottom line 2: learn to NOT go against the specialists in areas where you are not a specialist - not before really doing your homework and fully understanding things.

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From Wikipedia about dog odor:

Dogs only produce sweat on areas not covered with fur, such as the nose and paw pads, unlike humans who sweat almost everywhere.

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I've had groomers tell me that a dog's fur keeps them cool in hot weather as well as keeping them warm in cold.

That is the poetic* way of expressing the facts. What actually happens is:

• during cold times, the hairs prevent the body heat to dissipate - therefore keeping the dog "warm";
• during hot times, the hairs prevent additional heat to reach the body - therefore keeping the dog "cool" - actually, keeping him just "not warmer than needed"

* poetic = easy to say and easy to understand by most people, even if it is not scientifically 100% true

I've measured the dog's skin temperature using a contact thermometer and an infrared thermometer (I'm an engineer and an ex-physics geek. I have these things lying around.)

Being an engineer using a infrared gadget is one thing. Not understanding the complexity of the situation (including biological facts) and drawing the wrong conclusion is another thing. Ignoring the advice of specialists, against the well being of your dog is the ultimate thing.

So unless someone can tell me how dog hair is somehow a heat sink, instead of an insulator in these situations

The hair is not a heat sink, it is an insulator, as you stated.

it's ridiculous to assert that dog hair is helping your dog keep cool

Yes it is ridiculous, because the assumption is wrong. The hair does not cool your dog, it just prevents the dogs body to be overwhelmed with the sun's heat. In this way, the body has to deal only with the internally generated heat.

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For even more information, please search for "do dogs sweat?" in your favorite search engine.

There are some commonly cited reasons, which are mostly not true or not fully true:

1. Cutting the hair will cause it to grow back differently or not grow back. This is not true because hair is dead and has no sensory ability, so the hair follicle has no way to know if a hair has been cut or not. Thus, the follicle cannot change based on a hair being cut. (If you damage the skin/follicle, e.g. by plucking or a cut on the skin, that could change hair growth.) Mayo Clinic, Scientific American citing two specific studies

2. Another variation on this one is the idea that the coats of certain dogs grows very slowly. This has some truth but it is a little more complicated. Focusing on the breeds with double coats (because those tend to be shaved most), most of those breeds are seasonal shedders. They naturally shed most of their fur twice a year. So if you shave one of these dogs around the time they finish growing in their fall coat, you can't expect the to regrow much until the spring shedding starts. But, a further complication, the trigger for shedding is (probably) light (maybe a temperature component, too). So if your dog is kept in your house with artificial light and temperature, your dog may never receive the trigger to start shedding. No shedding, no regrowing fur. If your dog is a year-round shedder, that doesn't apply.

3. The dog needs the fur to stay cool. The evidence usually given for this is a thermal photo that has been floating around the internet. It shows a partially clipped dog, where the parts that are clipped show as hotter than the parts that aren't clipped. This is mostly a poor understanding of what thermal imaging means. (OP owns a thermal camera and probably knows this but I'm including an explanation for completeness.)

   Thermal imaging only shows the surface temperature of what it is pointed at. If there is fur, it shows the temperature of the fur. If it is pointed at skin, it shows the surface temperature of the skin. Now imagine someone under a heavy blanket. If you touch the outside of the blanket, it will feel about the same temperature as the air. But the person under the blanket may be quite warm. Because the blanket is an insulator, the surface temperature of the blanket was different than the temperature underneath. Similarly, in the photo where the fur shows as cooler than skin, that does not mean that the dog is actually cooler under that fur. To do an accurate comparison you would need to measure the skin temperature of the dog under the fur. (Or, better, the core body temperature of the dog, which is what really matters.) If your dog has very long fur, you can approximate this by putting your hands under the fur and feeling if it is warmer than the air temperature (it generally is).

4. If the fur is an insulator, shouldn't it be keeping the dog cooler in the summer (insulating it from heat)? An insulator will keep heat on one side from reaching the other side. In particular, the side that is cooler will stay cooler and the side that is warmer will stay warmer. So if the skin temperature of the dog is less than the outside temperature, then the dog will stay cooler if insulated. Skin temperature varies over the body, but the torso skin temperature of a dog is probably in the mid/high 90s (Fahrenheit) (mid-30s C) (I couldn't find a direct citation for this, but torso skin temperature for humans is only slightly lower than body temperature, so it should be similar for dogs. Also OP directly reports that the skin temperature of the specific dog is warmer than the air temperature.) So if it is more than 95 F outside, take your dog inside please. Wolves, foxes, and coyotes do this naturally by creating dens that provide cool places during the day and hunting at night.

Finally, as a simple test, if long fur was somehow beneficial in heat we would expect to see longer fur on animals in hot climates compared to animals in colder climates. We would expect the ocelot to have longer fur than the Canadian lynx, the desert fox to have longer fur than the arctic fox, etc. But somehow this doesn't happen. Even when we think of a warm climate animal that has notable fur (camel, lion) they often actually live in a mixed area (desert that gets hot during the day but very cold at night) and they usually have some areas with long fur and some with short fur so they can regulate their temperature by changing which areas are exposed. Cold climate animals which have short fur usually have either seasonal regulation via shedding (growing more fur during winter) or some other compensation such as high body fat.

One more very good study showing that light is a primary driver of shedding/fur regrowth.