The Growth of a Dog’s Coat

Dog coat growth, or the weakness of growth, not to mention the nuisance and problems of shedding, are constant topics of discussion for dog owners. The change from winter to summer coat is known, even to some extent how a dog’s hair grows and changes, but still, many myths are associated with the subject.

Perhaps the most common myth is that cutting or clipping the coat affects the quality and even the amount of hair. It is claimed that when the hair is cut, the new hair is either soft or coarse.

That is not true because the hair does not know if it has been cut. In fact, hair is exactly like human hair, meaning it is dead in the sense that it has no metabolism – even though hair cosmetic sellers try to claim otherwise.

The hair follicle does not know if the hair has been cut either. There is no sensory system in the hair that tells the length of the hair. The fact that a dog’s coat does not grow indefinitely, like human hair in principle, is a species difference and is due to genetics that regulates hair growth – not scissors.

I encountered an exaggerated example of this claim when the owner of a heavily trimmed breed insisted that clipping permanently – in the sense of ruining – changes the dog’s coat. The fact that he could not see the difference between a shaped coat and a clipped-down coat was somewhat comical.

TLDR;

Let’s start with a spoiler, a kind of summary for the impatient. A clipped coat feels soft or coarse for the same reason human stubble feels coarse. Short hair does not bend. Softness, on the other hand, comes from the structure of the hair itself and how many hairs from the same follicle are in the same growth phase.

There are three phases in dog coat growth for the hair itself, and two seasonal changes for the coat as a whole (summer and winter coat). Depending on the phase, it affects the growth rate of new hair in the dog’s coat and gives the illusion of softness or coarseness.

There is no difference from human hair, which also does not become softer or coarser depending on the scissors of a hairdresser or barber.

A Dog’s Hair Grows in Phases

Hair grows and is attached to a follicle. The difference in dogs, at least in some breeds, compared to humans is that while humans grow one hair per follicle, dogs can grow several.

The word follicle means three things:

• vesicle
• glandular vesicle
• hair follicle

My knowledge limits me from knowing why a hair follicle is also called a follicle – is it functionally or structurally a glandular vesicle?

There are several factors in the hair follicle that regulate hair growth or affect the quality of a dog’s coat. Most often, external factors can only weaken the hair and coat through deficiencies, hormonal problems, or diseases, but never improve them.

Side note: defining improvement

Improvement, or healing, is a tricky word. When deficiencies or diseases are cured, the dog’s hair and coat improve. Still, it only returns to a normal state. Beyond that, meaning above the natural genetic function of the dog, it cannot be improved further by adding supplements or coat/skin oils, no matter how much the owner believes and the manufacturer invents stories to get the believing owner’s money.

So, it must always be kept separate from each other, meaning:

• a deficiency or disease is corrected and returns to normal
• trying to develop or grow normal into something bigger and better

3 Phases in Hair Growth

Hair – whether we are talking about dogs, cats, humans… – has three phases related to hair growth¹:

• growth phase (anagen)
• transition phase (catagen)
• resting phase (telogen)

Growth phase grows the hair to its genetically determined length. That’s why we have bald, short-haired, and long-haired dog breeds. For the same reason, some breeds don’t have an undercoat, some always do, and some as needed.

Transition phase begins to detach the hair from the follicle. Hair growth stops, and it somehow detaches from its root.

Resting phase begins to grow a new hair start in the follicle, and it is the factor that makes the hair fall out. Either loose hairs are visible on the owner’s clothes, or the coat starts to have mats of dead hair causing gray tufts.

Each hair goes through all three phases. In humans, the different phases are more or less mixed in the scalp, and if something goes wrong, we start discussing baldness or, more often, hair thinning.

In dogs, the phases of hair follicles are grouped into different areas. Each area has hairs in different phases, but in each area, most hairs are in the same phase.

The most well-known sign of this is uneven shedding. A dog doesn’t shed its entire coat at once but in areas.

Müntener et al. (2011²) took samples from the hair follicles of dogs from the withers and thigh areas and determined the phase of the follicles.

According to the results, of the hair follicles:

• 30% were in the growth phase (12% at the beginning of the phase and 18% at the end)
• 8% were in the transition phase (2% at the beginning, 5% at the end, and 1% could not be determined)
• 27% were in the resting phase
• 35% could not be determined

The duration and activity of each of the three phases depend on several factors, whose significance is also guided by genetics:

• breed differences
• hormonal activity itself (e.g., estrogen is a strong influencer)
• amount of light in the change from summer to winter coat
• temperature to some extent

There are also external factors that affect. Most often, we are talking about problems:

• nutrition
• stress
• diseases

Two Growth Phases of a Dog’s Coat

Dogs, and actually most animals, have two larger rhythms affecting hair growth and change: summer and winter coat.

The extensive change of coat and the growth of new hair occur through the alternation of active and resting phases of hair follicles, as well as the follicle’s own function in growing hair.

In principle, it is a simple matter. Largely because it happens regardless of the owner. But if we go into the theory of what truly triggers the change from summer to winter coat and vice versa, we are on weaker ground.

We know roughly what happens. But why and what causes it… we are in the middle of a large puzzle, much of which is unassembled. Simply put: we don’t really know. But there are clear indications that the most important external factors are:

• air temperature
• amount of light

Spring Warmth, Autumn Coolness

The general understanding among dog owners is that:

• the winter coat changes to a summer coat because it is hot
• the summer coat changes to a winter coat because it becomes cold

That is not entirely wrong but not the whole truth either. In fact, in layman’s terms, one could say it is a third of the truth. Namely, even in tropical areas, dogs have a coat that changes twice a year, and it is not cold there in the same sense as here.

However, temperature affects coat change. It is due to a simple reason that the coat regulates heat, and a winter coat in summer is not good, nor is a summer coat in winter the most effective warmer. And vice versa – a summer coat does not warm sufficiently in -30 degrees Celsius, and a stronger coat is needed.

Side note: clipping a dog

If someone now takes a deep breath and starts preaching how clipping the coat in summer disrupts heat regulation, then blow your lungs empty slowly. It is about a different matter.

In winter, effective insulation is needed. The coat and hair quality aim to prevent body heat from escaping into the cold outside air. Insulation, whether in a house or a hiker’s sleeping pad, never tries to prevent cold from coming in – simply because temperatures do not move that way. The goal is always to prevent heat from moving out into the cold and to keep the heat in the insulated thing – whether it is a house, sleeping bag, or a dog’s body.

In summer, such effective insulation hinders the dog’s cooling. The dog’s muscle work and metabolism constantly increase body heat, and it must be gotten rid of. This is attempted by thinning the coat. Again, the coat does not try to prevent the sun’s heat from reaching the dog, but a thinner coat allows the dog’s internal heat, which is hotter than the outside air, to escape. So, it is done just the opposite of winter.

The owner does the same by putting their winter coat in storage and wearing shorts, a skirt, and a t-shirt. The dog changes the winter coat to a summer coat. But the summer coat is still insulation, somewhat like the owner wearing a long-sleeved cotton polo instead of a t-shirt. It is not enough in winter and is cooler than a winter coat, but it does not make summer heat cool.

The owner removes the polo and puts on a sleeveless t-shirt. The dog is clipped. Same thing, same result.

Birch Leaves and a Dog’s Summer Coat

Another major factor in the change of a dog’s winter and summer coat is the amount of light. Light affects many things, especially here at northern latitudes, where the difference in the number of daylight hours between winter and summer (and the height of the sun above the horizon) is vast.

Birch leaves do not open because of warmth but because of the amount of sunlight. That’s why birches start to green roughly at the same time every year, regardless of late winters or early springs. However, a dark cloudy weather slows down birches.

Mares rise to normal heat cycles with the amount of light. The same applies to female dogs, although breeding has shortened their heat cycle. But the fact that so many females start their heat in spring as the days lengthen is only due to the sun.

In heat problems, especially in mares, more attention should be paid to light and circadian rhythm, and less to raspberry leaves – but that’s a topic of its own.

What Temperature and Light Affect in a Dog

I have always said that a dog, its body, food, and nutrition, are divided into precise boxes only to make it easier to learn things in large entities. In addition, understanding logical groups is easier.

The body is not built from logical blocks. It is like a tangled fishing net, where everything affects everything. It looks like chaos. It is chaos, but of course, necessary and precisely regulated. There are just an unmanageable number of regulations and effects, and some regulation is also a self-consuming snake.

Briefly: we know a few guidelines that likely react to both heat and light and thus guide coat change in a certain way in the normal cycle as well as in seasonal variation when a dog changes its summer and winter coat.

The change in the amount of light and a longer-term temperature change affect at least:

• pineal gland (melatonin; internal clock)
• hypothalamus (several hormones; e.g., body temperature, internal clock)
• pituitary gland (several hormones; e.g., growth, body temperature)
• thyroid gland (thyroxine; e.g., metabolism rate)

The assumption, and partly also demonstrated effect, is that the hormones of that quartet, which regulate all cycles in general, also regulate the cyclicity related to dog hair growth.

How exactly, is largely an open question. Melatonin is used a lot in treating hair-related problems, such as alopecia, but it is not known for sure what happens.

Melatonin does not directly affect the hair follicle. But it regulates the prolactin hormone secreted by the pituitary gland.

Prolactin is perceived as a hormone that increases milk production after childbirth, but it is much more and regulates, among other things, immunity, metabolism, etc. Prolactin is secreted in bursts, for example, after eating, and is one factor in regulating and causing the cyclical functioning of the body.

In addition, prolactin is involved in regulating growth and cell specialization at the cellular level.

Fun fact: a hair follicle always needs stem cells to grow hair, not already specialized “hair cells.”

Which, then, is a stronger factor, cold or darkness, heat or midnight sun. That is not known. Depending on what has been studied and how, the results are remarkably contradictory. That probably means that both affect, and the emphasis depends on where you live and what breed the dog is.

Breed Differences Affect

In Brazil, the hair change of three different breeds was studied in a tropical region for a year (Favarato & Conceição 2008³). There was some variation in the amount of sunlight, but not much. But there wasn’t much variation in temperature either. The fact that winter is characterized as colder is probably due to the rainy seasons.

The dogs were divided into three groups based on their breed:

• boxer representing short coat,
• Labrador retriever representing medium-length, and
• schnauzer representing longer coat

The size and variety of each breed and dog were not mentioned.

Schnauzers differed quite a bit from the others. Schnauzers consistently had more hair follicles in the growth phase and fewer in the resting phase than boxers or Labradors.

For some strange reason, the study completely overlooked the fact that the schnauzer is a breed that needs to be plucked. Due to the different functioning of the follicles, it must be plucked properly at least a couple of times a year and tidied in between.

Boxers and Labrador retrievers did not differ in hair growth cycles and coat change from each other. In the hottest summer, a larger portion of the hair follicles were in the resting phase. On the other hand, the coolness of winter increased the number of active hair follicles in the growth phase.

Both breeds reacted more strongly to climate than light with their coats, boxers slightly more than Labradors. Schnauzers’ coat grew and changed at its own pace all the time, regardless of heat and light.

Winter Coat in July and Summer Coat in February

Changing the coat twice a year is a given, at least for most breeds. But why the dog doesn’t read the calendar can be puzzling. It is well into summer before the winter coat starts to change. Or winter coat begins to be pushed only during ski holidays.

No one knows the reason for that, but guesses can be made.

Dogs no longer live much outdoors. At least most don’t. They don’t then get the same kind of encouragement from the cold air to change their coat. When living in a centrally heated house, when a heat-retaining fireplace warms the back nicely, it doesn’t matter how cold it is outside.

The same indoor living causes light pollution, but it also applies to the environment – at least in cities. The body’s internal clock regulating circadian rhythm is very strongly light-dependent. In fact, everything on Earth is, except perhaps the deep-sea life forms.

The disappearance of the difference between night and day due to artificial lighting messes up everything. House plants suffer, the owner’s mental health and stamina waver and fluctuate, and dogs can’t calm down – nor change their seasonal coat.

Computer blue light is blamed for much. But if we stayed up less during dark times and were less dependent on artificial light, maybe the owner’s psyche and the dog’s coat would be better off.

Implementing that in practice is just very difficult.

Nutritional Support for Hair and Coat

Nutritional support for a dog’s coat growth is very much the same as caring for a dog’s skin. The reason is very clear. Hair is not actually a living metabolic organism, but as part of the skin, the hair follicle is.

When a dog is unwell, the skin is unwell. When the skin is unwell, the hair follicle is unwell. When the hair follicle is unwell, you have a dog whose:

• skin flakes
• hair is of poor quality
• coat is matte and dead

The coat is thus a measure of problems, not a problem itself. This is something that should be realized. The health of the stomach and intestines is measured by the amount and quality of the dog’s stools, but the stool itself is not a problem. The health of the kidneys and urinary tract is measured by the amount and quality of urine, but urine itself is not a problem.

A dog’s skin, and thus also the coat in terms of hair follicle function, is the largest organ of the body. It consumes an enormous amount of everything, including energy. And because skin function is so strongly dependent on the functioning of the body’s major hormones, internal organ problems quickly show in the coat.

A poor coat is not always due to dietary problems. The thinning of the coat caused by hypothyroidism is a well-known and recognized consequence. The effect of spaying on the coat through estrogen is known.

Side note. Owners of female dogs almost always worry about coat softening during spaying. Maybe such a risk is the easiest to internalize, but it doesn’t affect the quality of life of the female dog. It affects the owner’s aesthetic eye and show activities. The real concern should be urinary incontinence problems, which is the most common reason for premature euthanasia after spaying. So, prioritize things, please.

The question is that if coat problems are due to disease or other hormonal issues, nutritional support treatments rarely help.

The Most Important Growth Factors for Coat

Because the coat is cared for by caring for the skin, the same things are needed:

• water
• fat (specifically omega-6 linoleic acid)
• proteins (especially sulfur-containing methionine)
• zinc
• vitamin E
• plus generally a balanced diet

What exactly you should do for your dog’s matte or poor-quality coat is a much more difficult question and does not fit the scope of this article.

But three things need to be clarified:

• are the problems due to possible disease or the body itself
• are the problems perhaps due to external factors
• are the problems due to nutritional deficiencies

Most often, the cause is at least two factors, but every now and then, it is enough to correct one influencer. What the target to be corrected is often requires knowing the dog’s history – and still, from time to time, an educated guess must be made according to which to start trying to fix the situation.

That task can be done alone. As long as you understand the basics, what things affect, and how each detail is adjusted. The task might be easier by booking a phone consultation.

1. Welle MM, Wiener DJ. The Hair Follicle: A Comparative Review of Canine Hair Follicle Anatomy and Physiology. Toxicol Pathol. 2016 Jun;44(4):564-74.[↩]
2. Müntener T, Doherr MG, Guscetti F, Suter MM, Welle MM. The canine hair cycle – a guide for the assessment of morphological and immunohistochemical criteria. Vet Dermatol. 2011 Oct;22(5):383-95.[↩]
3. Favarato ES, Conceição LG. Hair cycle in dogs with different hair types in a tropical region of Brazil. Vet Dermatol. 2008 Feb;19(1):15-20.[↩]