We have a long hair cat. The cat recently had an accident and a part of its body was shaved. The hair is growing back nicely but at some point it will stop growing when it reaches its normal length. For a short hair cat this would be different than for a long hair cat.
This applies to humans as well. Arm hair stops growing after a time but head hair will grow continually.
What is the mechanism that controls hair length?
Hair alternates between periods of growth and dormancy. Body hair has a much shorter growth period than hair on the head does and so hair on the head can reach a longer length than body hair before it sheds. I imagine that in animal breeds with shorter fur they have shorter growth periods and longer periods of dormancy, while animals with longer fur have longer growth periods and shorter periods of dormancy.
How does body hair know it’s been cut and grow back?
Why does head hair grow indefinitely but other body hair doesn’t?
Each follicle turns on for some amount of time, producing a hair of a certain length, and then turns off, at which point that hair falls out (as the owner of a long-haired cat, you’re probably quite familiar with this latter process), and the process repeats. So if a cat or human head is allowed to grow out to max length, at any given time, some hairs will be at maximum length, some will be just starting off, and some will be in between. When you shave the cat, the cat’s follicles don’t know the difference, and just keep following this pattern. Some of the hairs that got shaved off (the longest ones) were about to fall out anyway, so they’ll only be very short when they fall out. Some of them (the shorter ones) had a lot of growth ahead of them, so they’ll keep on growing until they reach almost full-length before falling out.
A concrete example: Suppose that a given cat has hair follicles that grow for four months, then turn off for one month, then grow again, and suppose that the follicles produce hair at a rate of an inch per month. At any given time, some follicles will be in their first month, some will be in their second, and so on. So you’ll have something like this:
5
3—
4----
1-
2–
So you’ll have a variety of hair lengths, but the longest will be 4 inches. The next month, the 4-inch hair will have fallen out, and each of the others will have gotten an inch longer:
1-
4----
5
2–
3—
You still have a mixture of different hair lengths, and the longest is still four inches, it’s just a different follicle that has the longest hair now. And in another month,
2–
5
1-
3—
4----
Same story. Now, someone shaves the cat. The follicles are unchanged, but the hair is gone:
2
5
1
3
4
The month after that, like in every month, a 4-month follicle turns into a 5-month follicle and loses its hair, and all the others add one inch:
3-
1-
2-
4-
5
The next month:
4–
2–
3–
5
1-
Next:
5
3—
4—
1-
2–
(notice how the longest hair is now 3 inches long, but there are two 3-inch follicles at this moment, not just 1)
And in one more month,
1-
4----
5
2–
3—
and we’re back to a full-fluff, 4-inch-maximum coat.