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)
- Gender (spayed/neutered dogs can have more pronounced undercoats, so shedding can be more noticeable)
- Hormonal status (determine hair growth phases)
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.