The perception that dog hair grows faster than human hair is common, largely due to the frequent, visible shedding of a canine coat. However, comparing growth rates is complicated because human and dog hair are governed by fundamentally different biological cycles. While the actual speed of hair growth (measured in centimeters per month) is similar, the duration of the growth phase varies drastically. The ultimate length and shedding pattern depend entirely on the genetic programming of the hair follicle.
Understanding the Hair Growth Cycle
All hair growth in mammals operates on a cyclical process that includes four distinct phases: Anagen, Catagen, Telogen, and Exogen. This cycle, which repeats throughout the life of the follicle, determines how long a hair strand will grow and when it will be replaced. The Anagen phase is the growth stage, where cells at the base of the follicle rapidly divide to form the hair shaft, actively pushing the hair outward.
This growth stage is followed by the Catagen phase, a brief transitional period lasting only a couple of weeks. During this time, the hair follicle shrinks, and hair growth ceases as the hair detaches from the blood supply. The hair then enters the Telogen phase, a resting period where the strand is fully formed but remains anchored in the follicle for months.
Finally, the Exogen phase is the shedding or release phase, where the old, resting hair falls out to make way for a new hair shaft to begin growing in the Anagen phase. The duration a follicle spends in these phases, particularly the Anagen phase, dictates the maximum hair length and the frequency of shedding.
The Characteristics of Human Hair Growth
Human hair is characterized by an exceptionally long Anagen phase, which is the primary reason our hair can grow to great lengths. On the scalp, this active growth period can last anywhere from two to seven years, allowing hair to grow an average of 1 to 1.5 centimeters each month. At any given time, approximately 85% to 90% of the hair on the human scalp is in this active growth phase.
The human hair growth cycle is also asynchronous, meaning each hair follicle operates on its own independent timeline. This staggered process prevents mass shedding, ensuring that only a small number of hairs enter the resting and shedding phases simultaneously. This asynchronous nature is why humans typically lose only about 50 to 100 hairs daily, with the remaining hair continuously growing.
The prolonged Anagen phase ensures hair has a long lifespan before shedding, resulting in a slow but steady accumulation of length. This growth pattern requires regular cutting to maintain a desired length, as the growth window is not genetically limited to a short period.
The Characteristics of Canine Coat Growth
The canine coat cycle differs from the human cycle primarily in the duration of the Anagen phase and the synchronization of the follicles. For most dog breeds, the Anagen phase is dramatically shorter than in humans, often lasting only a few weeks to a few months. This short growth period limits the maximum length the hair can achieve before it enters the Catagen and Telogen phases, leading to coats that maintain a relatively short, uniform length.
In many breeds, particularly those with a dense undercoat like Labradors or Huskies, the hair follicles operate in a synchronized manner, often dictated by seasonal changes in light and temperature. This synchronization causes a large number of hairs to enter the Telogen and Exogen phases at the same time, resulting in the seasonal phenomenon known as “blowing the coat” or mass shedding.
However, some dog breeds, such as Poodles and Shih Tzus, have been selectively bred to possess a genetic mutation resulting in a much longer Anagen phase. Their hair grows continuously, much like human hair, and they shed far less frequently. These breeds require regular grooming and clipping because their Anagen phase is not genetically limited to a short period.
While the actual growth rate for dog hair (around 0.5 to 1.25 centimeters per month) is comparable to human hair, the difference lies in the Anagen phase duration and cycle synchronization. Most dogs appear to have faster-growing hair because their follicles quickly cycle through a short growth period and are replaced, rather than continuously growing for years.