[grifter (OP)]

My dog recently had an operation in her side[]. The vet shaved a patch of her hair for the op. The hair from the (formerly) bald patch grew back over the next few weeks, but stopped growing when it reached the same length as the surrounding hair. How does the hair 'know' when to change its speed of growth (it must still be growing [?] as hair keeps falling out in moults etc.) What is the mechanism by which the change in speed happens?

[daveshorts]

As you have probably noticed your dog's hair is falling out all the time (more so when she is molting). Hair will grow for a bit, stop for a bit, then fall out of it's own accord. Depending on how long the growing period is the hair will have a maximum length that can be short or long. - this is why some people can't grow their hair very long even if they never cut it.

If you shave your dog the hair will just grow as normal up to it's maximum length as normal and then stop. The hair that was halfway through this process will end up shorter than it would otherwise be, but eventually this will fall out and more hair will grow up to the maximum length.