Hair grows in cycles. When it reaches a certain length that is determined by the individual's genetic profile, growing stops. Each hair’s growth cycle consists of four phases: growth, regressing, rest and shedding.
In the growth phase (1) the new hair comes in and starts growing. In the regressing phase (2) the hair stops growing. During the rest phase (3) the hair dies. Hairs that are in the resting phase are held in the follicle by friction and are not anchored in. They fall out very easily, especially during grooming. And in the final shedding phase (4) the hair falls out and the follicle moves back into the growth phase (1).
That means the hair’s growth cycle directly influences the shedding.
Dogs, cats and rabbits go through these phases continuously. Several factors affect the amount of hair the animal will shed and when shedding is more pronounced.
Most animals shed during the whole year and undergo a complete change of coat in spring and again in autumn. The exact time this occurs depends on the amount of daylight and the temperature, but it is also affected by the animal’s hormones, which is one reason why female dogs often tend to shed hair at the start of their heat period, whilst neutered dogs shed fur all year round. However, the fur of some breeds, such as the poodle, is constantly growing. Since these animals do not go through a seasonal coat change, they shed significantly less than other breeds and therefore need to be clipped or trimmed on a regular basis. For hunting terriers (e.g. Airedale, Fox Terrier), the seasonal change of coat has been bred out and these breeds thus need to be trimmed regularly.