Immobility is caused by pain or inability to walk.  If the break is severe
enough, since the blood vessels and nerves follow the bone, there is a good
chance that they will be injured by or during the break.  This is
especially true if the fractured bone ends are sticking through the skin.
An animal with nerve damage rarely survives in the wild (they do just fine
as pets).  I have seen dozens of skeletons of predators where a fracture
was apparent, but it only healed for a couple of weeks, just long enough
for the animal to starve to death (I have never seen a similar case in prey
species because they are generally culled by predators soon after injury).
Predators, such as the ferret, have evolved the ability to go considerable
time without eating (the idea that a ferret HAS to eat every few hours is
a myth; excepting sick ferrets, most can easily go a day or two without
food with NO ill effect).  So even if a break causes short term immobility,
there is a fair chance that as the callus on the bone begins to harden,
the animal can start to hunt and get enough to eat to survive (which must
have happened in each and every case where a wild animal has survived a
fracture).
 
Infection is the real problem, and why *I* think every ferret with a
broken bone needs to see a vet.  Bone infections are deadly, debilitating,
painful, and even with modern drugs and knowledge, may not be totally
curable.  Because bone is the center for fat storage and blood cell
production, it is very vascular; that is, there are a LOT of blood and
lymph vessels which are perfect highways to carry infection to the rest
of the body.  Infected bone dies little by little, weakening and eroding
away, leaving behind horrific gaps and holes and large sections where the
bone is bubbly and weak and just plain nasty.  The absolute best cure is
prevention.  Do not assume that just because the bone has not poked through
the skin, it will not become infected.  I have seen dozens of examples
where a simple, minor fracture has become infected, leading the the death
of the animal.  Ferrets with fractured bones need to see a vet.
 
Wild animals never get to see vets and they do survive.  It takes about
a week for the blood clot around the bone to begin to harden enough to
prevent the bone ends from grating against each other.  It still cannot
bear weight, BUT, it allows mobility and a search for food.  After two
weeks, the blood clot around the bone has been turned into a spongy, soft
bone-like substance called a callus.  In three weeks, it will easily
support the animal's weight, although it is weaker than the surrounding
bone and may still be painful.  After about a month, while the bone is
not fully healed and the callus is still rough and spongy, the bone is
well enough to be considered functional.  This process occurs regardless
if the bone was splinted or not.
 
The difference is, splinted bones do not have ends which move against each
other, breaking up the callus or causing additional bleeding (or tissue
damage) which makes the callus larger.  Splinted bones are not very
painful; if you have ever suffered from a broken bone, you know that after
the initial pain goes away (2-5 days or so), it doesn't hurt much.  That
is NOT true with unsplinted bones, which can hurt for weeks.  Obviously, a
broken bone, if not splinted straight, could heal crooked, but it can also
heal without knitting, resulting in a false joint where the two ends of the
bone did not heal together.
[Posted in FML issue 3436]