What causes a dog or a cat to shed?

Question

My Long haired cat sheds a lot. It's insane how much fur he loses, and yet there always seems to be more of it. It doesn't seem like it's tied to the season because he sheds during the winter too, but he's also inside all the time, so I can kind of understand that he wouldn't need more fur since he's not out in the cold.

On the other hand, I had a labrador as a kid who would shed tons in the spring. It seemed pretty clear that there was a winter coat of fur, and a summer coat. Which makes sense since she spent most of her time outside.

So what makes dogs and cats shed their fur? Is it defined by seasons, or are there other reasons?

Answer 1

Most of this is what I remember from my old biology classes, so the usual caveats apply.

Animals shed their fur for the same reason we shed hair: older, less healthy hair falls away and gets replaced by new growth. The more hair there is, the more of it gets shed - which is the same for us as it is for our pets (I have long hair, and I "shed" a LOT more than my husband).

Animals that evolved in colder climates and don't hibernate typically get a summer and winter coat - they'll have a thick undercoat that sheds and thins out in spring, and a coarser overcoat that is often more or less waterproof (so that cold rain and snow doesn't get to their skin, allowing them to stay warm). They shed heavily in spring for two reasons - to clear out the older fur and to reduce the amount of fur they're carrying for the warmer weather. The pattern isn't as obvious when these animals are in climate-controlled areas all the time but it does still happen.

(As a side note - the old breeds of sheep shed their wool in clumps in spring. Breeding to get more wool had a side effect of losing this adaptation, which is why we need to shear modern breeds. My mind is a swamp of odd trivia).

Answer 2

You are right to suggest that shedding in cats, dogs and other animals (both domestic and wild) are closely linked to seasonal variations:

Shedding in animals is intimately related to seasonal cycles. In most cases, the cycle of shedding is cued by changes in the amount of daylight. Hair growth and shedding are regulated by fluctuations in the amount of melatonin, the "hormone of darkness", secreted by the pineal gland in response to seasonal sunlight variations.

Increasing day length stimulates hair growth in the Spring. Spring shedding is typically heavier because the winter coat is progressively replaced for a lighter, Summer coat.

Although seasonal variations seem to be the main cause, the article continues to discuss other factors influencing shedding:

• Temperature (seasonal, governed by day length)
• Lifestyle (indoor vs. outdoor animals)
• Nutrition (hair loss due to poor nutrition)
• Breed
• Gender (spayed/neutered dogs can have more pronounced undercoats, so shedding can be more noticeable)
• Age
• Hormonal status (determine hair growth phases)
• Behavior
• Health
• Owner

As already stated, it greatly depends on the breed of the cat or dog. Different breeds have different characteristics (for example, longer hair or a double coat) that will make their shedding amounts differ.

Factors such as skin composition including layers (epidermis, ceramides, dermis, basal cell, hypodermis) and function; the hair itself including composition, structure, layers, follicle pattern, and different hair growth phases, and hair types (including guard hairs, under-hairs and tactile hairs) all influence the hair type and therefore the shedding patterns.

Shedding patterns for dogs and cats are also discussed:

Double-coated dogs generally drop their soft undercoats twice a year and lose their topcoat once a year. If they shed all at once, the fur will come out in tufts and is often called "blowing a coat". Other dogs might shed continuously throughout the year or only every 10-12 months. Shedding can take anywhere from three weeks to two months. Dogs that live outside usually shed heavily as days lengthen in the Spring, but those that live mostly indoors, often seem to shed at least a little all year.

In cats, there is no single period of hair shedding. Neighboring follicles are in different phases of the hair cycle at any one time. Domestic cats tend to shed continuously throughout the year, with peaks of activity occurring during Spring and Fall.

Answer 3

These animals tend to have two distinct coats... if they are in an environment that doesn't vary much in temperature it is possible they could have one more continuous coat. However, all the cats I've had have had the two distinct coats (despite being indoors and thus not having the variation in temperature they would outside). My one longhaired cat (a Maine Coon) seems to shed more than all my other cats combined (even back when I had three others).

I don't notice the two coats as much but I do know that my one cat who gets constipated from time to time has more problems with this in the spring and fall, precisely the times when one coat is being jettisoned for another.