What triggers cats to grow a winter coat?

Most pet owners will notice that their cat’s coat grows thicker during the cooler months and once spring arrives, the coat sheds, leaving behind a much thinner. Scientists have found that growth of the winter coat is triggered by shortening daylight hours, and not a drop in temperature.

Once daylight hours begin to shorten, information is relayed via the retinohypothalamic tract to a master clock known as suprachiasmatic nucleus (SCN) in the hypothalamus which encodes these signals and disseminates them throughout the cat to drive the circadian and circannual changes in physiology. At the pineal gland, the SCN signal affects melatonin secretion (a hormone that modulates hair growth) and the length of daylight hours is proportional to the duration of melatonin production. Melatonin suppresses prolactin secretion, which regulates the hair follicle cycle.  Increased production of melatonin and declining prolactin trigger winter coat growth in cats. As days become longer in spring, melatonin production declines and prolactin increases, leading to shedding.

Animals who live in the warmer southern hemisphere experience longer winter hours, than their northern counterparts, and produce less melatonin, resulting in coats more suited to their climate. For comparison, below are the daylight hours from Sydney and London on the shortest days of the year.

London-21.12.2022

• Sunrise/sunset: 8.03 AM – 3.53 PM=Day length 7.49 hours

Sydney-21.06.2022

• Sunrise/sunset: 6.59 AM – 4.53 PM=Day length 9.53 hours

This makes sense, as temperatures are warmer in regions close to the equator and animals don’t need a heavy winter coat as an animal living in say Norway or Denmark.

Function of fur

The cat’s coat acts as a barrier between the cat and the outside world and consists of three types of hair, guard hair, awn hair and down hair.

• Guard hairs: The longest and thickest, and make up the cat’s protective outer coat to retard water, guard hairs contain melanin which gives the coat its colour.
• Awn hairs: The intermediate hairs that are shorter than the guard hairs but longer than down. Awn hairs help with insulation and protect the down hairs underneath. Most of the visible coat is awn hairs.
• Down hairs: The soft and silky hair which forms a thick undercoat to insulate the body and reduce heat loss.

Together these three types of hair keep cats warm and help to keep the cat dry. You may notice on cold days the coat looks thicker, as each hair springs up to trap air pockets and provide extra insulation by retaining body heat.

Why do cats shed even when it’s not spring?

A small amount of shedding is normal in all cats, year-round. This is no different to humans, who lose between 50-150 strands of hair a day. Hair (and fur) growth occurs in four phases:

• Anagen: The active growing phase of the hair
• Catagen: This phase signals the end of the active growing phase, hair growth slows down
• Telogen: The resting phase of the hair cycle.
• Exogen: Shedding phase.

Stress, nutrition and overall health can also influence the amount of hair we and our cats lose. Feeding a healthy diet, and maintaining a stress-free environment may influence the amount of fur a cat sheds, but as a small amount of shedding is normal, it is not possible to completely eliminate it. Daily brushing removes loose hair before it gets into the environment or onto your clothes and the addition of omega 3 fatty acids can also help reduce shedding.

Coat colour changes in pointed cats

Interestingly, while the photoperiod is responsible for triggering the winter coat, cooler temperatures can influence coat colour in pointed cats (Siamese, Birman, Ragdoll etc). The gene responsible for the seal point colouration is known as the Himalayan gene and is a form of albinism. The medical term for this unique coat colour is Type I oculocutaneous albinism (OCA 1b TS).

The Himalayan pattern is the result of a missense mutation (a change in a single base pair that causes the substitution of a different amino acid in the resulting protein) in the TYR tyrosinase gene (c^s = c.940G >) which produces tyrosinase, an enzyme for controlling the production of melanin in the hair, skin and eyes. Tyrosinase normally works at body temperature, but in the pointed cat, tyrosinase can only function below certain temperatures, leaving the warmer parts of the body with hair that lacks melanin and the cooler parts pigmented. As temperatures drop, the coat of pointed cats becomes darker. This is less obvious for indoor cats who live in a constant temperature.

Frequently asked questions

How long does shedding last?

The heavy shedding that occurs in spring typically lasts 2-4 weeks.

Do inside cats get a winter coat?

If the cat is exposed to daylight, he or she will develop a winter coat, just like their outdoor counterparts.

How often do cats shed?

Fur is continually shed in small quantities year-round, however, cats go through large scale sheds in spring and autumn. In spring they lose their heavy winter coat, and a second shed occurs in autumn to replace the summer coat with a warmer winter coat.

Do longhaired cats get winter coats?

Longhaired cats also grow winter coats as the days grow shorter.

Feature image: Alkhym, Shutterstock