When its only divisors are 1 and itself? No, wait, wrong prime.
Fur is prime when the coyote is the leader of a band of robots from outer space. Dang it! That's not right either. Maybe it's when they got a really good rate on their bank loans? Or is it a coyote that you shoot while watching a terrible movie with Uma Thurman and Meryl Streep?
Sorry, couldn't help myself this morning.
In general, we start thickening a bit in sept, but most aren't in good shape until late October or early November. It varies a bit from year to year. With our climate and habitat, they can get pretty tasseled by late January so really we're looking our best November and December.
But that's not to say dogs aren't blowing out their summer coats and pushing up their undercoats yet, because they are, they just don't reach their best fur yet for a month or so.