Very few studies of how sound negatively impact ferrets have been done,
but this failure follows the general trend of neglecting the problems
of noise impacting ANY animal.  Ferret auditory ranges have been studied
and documented, but nothing of real significance on HOW sound impacts
ferrets, especially those living in captivity, has been attempted.  It
is probably true sounds loud enough to harm human hearing would likewise
damage a ferret's hearing, but such assumptions lack demonstrable
evidence; it may be a ferret's hearing can be damaged by noise lower or
higher in decibels than those required to injure humans.  Hypothetically,
it is a simple matter to assume ferrets, like most mammals, become
habituated to sound over time, but while such an assumption is probably
valid, it is yet to be proven.  Sounds have an immediate and sometimes
drastic impact on ferrets that ARE NOT habituated; when I was building my
basement bone lab, I was using a cartridge-powered nailer to fasten
cabinets to the concrete floor.  My floor is VERY hard, so I had to use
27 caliber #7 cartridges, which let out a bit of noise.  At first, even
in another room, the ferrets freaked, but after a half-dozen-or-so loud
bangs, the sudden, loud sound was completely ignored.
 
Sudden, periodic loud sounds over a limited period of time probably do
not have a long-term effect on ferrets (assuming the sound isn't
painful); the 27 caliber cartridges from my nailer are loud, but so is
the thunder from nearby lightning strikes, and as far as the decibel
load goes, ferret screams during fights can get pretty nasty.  A far
worse problem is exposure to constant, long-term noise, such as that
experienced in some shelters filled with radios, TV, street noise,
telephones, people and animals.  Private homes can be worse; my hearing
was damaged when I was serving in the Navy, and I can only decipher the
loudest sound in a room.  Imagine my problems trying to hear conversation
during a party, or when the TV is playing loud.  Can you imagine the
onslaught of noise some ferrets face on a daily basis?  It would be bad
enough if the ferret had a hearing sensitivity similar to humans, but
they don't; they are FAR more sensitive to sound.
 
There are two lines of evidence suggesting ferrets have an extremely
sensitive sense of hearing.  The first is the structure of the ferret's
skull, and the second are the published reports of the auditory range of
ferret hearing.  The structure of the ferret skull leaves no doubt that
ferrets have sensitive hearing.  There is a bony structure in mammals
called the auditory (or tympanic) bulla that houses the middle ear (see
the accompanying photograph).  It is more-or-less shaped like a cooked
shell pasta glued rim-down to the bottom of the skull, and--among other
things--it houses and protects the bones of the middle ear.  Mammals
possessing a proportionately large auditory bulla (the size of the bulla
in proportion to the size of the skull) tend to have more sensitive
hearing compared to those than have proportionately smaller bulla.
 
For example, dogs have about 4 times the hearing sensitivity of humans
in the 500 to 16000 Hz range, presumably due to the size of the auditory
bulla.  There are several reasons for this, but three in particular are
the most important.  First, the shape of the auditory bulla acts like a
parabolic microphone; sound passes down the external ear, through the
tympanic membrane (the eardrum), then reflects off the bulla back to
the eardrum, intensifying the sound vibration.  Second, like the hollow
body of an acoustic guitar, the bulla acts like a resonating chamber,
increasing the strength of barely audible sounds and making them easier
to hear.  Third, the malleus (or hammer) is attached to the auditory
bulla by a ligament; as the auditory bulla resonates like the soundboard
of an acoustic guitar, vibrations are transferred from the bone through
the malleus to the tympanic membrane.  All three events (and more) occur
at the same time, increasing the sensitivity of the ear to detect sound.
The auditory bullae in ferrets are extremely large in comparison to the
rest of the skull.  They extend from the external ear canal to the back
of the skull, and from the outer edge (lateral edge) of the skull to the
middle (medial line).
 
Bob C
[Posted in FML issue 4213]