Dentes premolares:
 
While ferret premolars have a name that suggests their primary use are as
grinders, they neither grind, nor are they molars.  Ferret premolars are
slicing blades, flattened into knives having thin, cutting edges and at
least a single sharp point; the mammalian equivalent of shark teeth.  The
numbers of premolars have been reduced from the ancestral mammalian
pattern; the 1st premolar has been lost, leaving the 2nd, 3rd and 4th
premolars to do the job.  In many cases, lost teeth are due to a size
reduction in the jaw, but in the case of the ferret, it allows the
remaining teeth-especially the carnassial-to become larger, increasing
the efficiency of flesh rendering.  In the case of the ferret, the 2nd and
3rd premolars are very small, and do not touch when the jaw is closed.
These teeth are initially used to help hold prey, reducing stress on the
canines.  They are later used as saw blades to cut the prey apart; when
the ferret braces their feet on the carcass and uses powerful neck muscles
to pull tissue, the premolars act like serrated blades to cut chunks of
meat and bone free.  In the ferret, the 4th maxillary premolar is modified
into a scissor-like cutting blade.  A single glance at the ferret premolar
dentition is positive evidence of the obligate, primary carnivorous habits
of the ferret; these are the teeth of a highly specialized carnivore, not
an herbivore.
 
In the human dentition, the difference between the deciduous and permanent
premolars is readily apparent; the molar-like (= molariform) premolars of
the child are replaced by bicuspid premolars in the adult, signaling a
shift from grinding to cutting teeth.  While differences exist in the
size-and to some degree the shape-of ferret deciduous and permanent
premolars, the two dentitions are predominately equivalent.  There is
little doubt the deciduous dentition is designed to perform the same tasks
as the adult teeth.  In other words, the ferret deciduous dentition-like
the permanent dentition-is designed to process animal flesh to a point
where it can be safely swallowed.  To do this, the deciduous and permanent
premolars are compressed laterally (= buccolingually, lingual-vestibular)
into backward pointing, thin, blade-like teeth designed to puncture and
grip food, secure it, and cut it into small chunks that are swallowed by
bolting; that is, without extended mastication (= chewing, grinding).  One
special type of tooth, the carnassial (= dentes sectorius, dentes sectoria,
sectorial tooth, cutting cheek tooth, shearing teeth), is superbly designed
to shear flesh.  The maxillary carnassial (= maxillary 4th premolar, PM4/,
pm4/) is designed to move across the mandibular carnassial (= mandibular
1st molar, M/1, m/1) like the blades of a pair of scissors, and can cut
muscle, tendon, ligaments, tough connective tissue, and bone.
 
Technically, the ferret does not masticate food.  That is, they do not chew
in it the sense that they use blunt grinding surfaces to crush food to a
pulp and mix it with salivary juices, predigesting it.  Instead, the ferret
cuts food into chunks small enough to swallow, and then bolts them whole.
While ferrets use their reduced molars to break or crush small bones, and
insect and crustacean carapaces (and other hard components), they are not
used to masticate food.  This lack of ability to chew is related to two
important anatomical features of the ferret jaw.  First, because the ferret
has to open their mouth wide to apply the killing bite (which approximates
the width exhibited during yawns), the tongue can get in the way, bitten
or even severed.  To reduce risk of injury, the tongue lacks some of the
mobility seen in other mammals and is held low in the lower jaw.  Because
of this lack of mobility, the ferret has a difficult time keeping food on
the tooth biting surfaces; most ferret owners have observed their ferrets
tilting their heads to the side in an effort to keep small pieces of dry
food over the cutting surfaces of the teeth.  Animals that masticate their
food have very mobile tongues, which are used to keep food over the chewing
surfaces (they also generally have small mouths and tight cheeks).
 
The second important adaptation is the structure of the temporomandibular
joint (= TMJ, jaw joint), which locks the mandible in position on the
skull (I have described this joint in detail in the section describing the
skull).  Because of the TMJ, the jaw can only move in an up-and-down motion
and cannot make the side-to-side or back-and-forth motions required for
grinding food.  Because a ferret cannot grind materials and mix them with
significant portions of salivary juices, they are at a disadvantage when
consuming plants, or dry, processed foods.  This type of jaw adaptation
is primarily found in animals highly specialized to eat other animals.
 
The key to efficient prey consumption is the cutting cheek tooth.  However,
like all teeth, the carnassials are subject to wear and abuse.  Carnivores,
including the ferret, have evolved a brilliant solution.  The carnassials
are "self-sharpening"; that is, normal use creates a wear facet on the
facing parts of the upper and lower cutting teeth, maintaining the sharp
edge.  This occurs because rather than the top surfaces impacting one
another, like with human premolars, in the ferret they are offset and slide
past each other.  In this way, even in older ferrets, the carnassials
remain sharp enough to cut tissue.
 
This is not the case in modern ferrets fed manufactured food-especially
those eating dry kibbled or extruded foods.  These foods are designed
to be fed to ferrets without danger of putrefaction (decay), and have
antibacterial qualities because their water content is so low (a
side-benefit is that the hardness helps reduce tartar-a selling point).
While food remains basically safe and fresh in the dish, it is not without
cost to the ferret's teeth.  These dry foods are considerably harder than
animal tissue and significantly increase the wear rates of the tooth.
Moreover, ferrets do not have sufficient molar surfaces to pulverize hard
foods, nor do they have the mechanical ability to grind them (more on the
function of ferret molars in the succeeding section).  Instead, they turn
their carnassials into makeshift molars, breaking the dry foods into
fragments small enough to swallow.  While this may not seem to be of
much importance, what is does is shift the wear facet from the sides-the
lingual-vestibular surfaces-of the carnassials, to the top of the teeth;
the occlusal surface.  The carnassial is blade-like and thin, and normally
the wear is minor when the two teeth slide past each other.  By shifting
the wear facet to the top, the "blade" of the carnassial is dulled and
ultimately flattened.  The carnassial is simply not designed to grind food
anymore than the sharp edge of a pocketknife is designed to mash potatoes.
 
The premolars are subject to a variety of problems, most due to gingival
disease or periodontitis.  Tartar is commonly found, despite the
proclamations of dry food manufacturers that their product helps prevent
the buildup of the calcified material.  Often tartar will form a
tight-fitting glove-like mold around the carnassial, especially on the
lingual surface, discoloring the tooth.  If the ferret has gum disease,
often the 2nd or even the 3rd premolar will be missing.  Caries is
uncommon, except for the carnassial, which will often have a cavity at
the gum line, or sometimes in the recesses of the grooves on the cutting
surfaces.  As already described, eating hard, dry foods causes extensive
wear to the carnassial, which is easily identified by the shift of the
wear facet from the cutting edge to the top of the tooth.  In older
ferrets, the blade of the carnassial can be completely blunted, and the
other premolars can be worn down to the gum line.  The premolars are
rarely broken; fractures are generally associated with caries or serious
accidents.  Sometimes the 1st premolar will show up, or a supernumerary
premolar can be found, but these are rare events.  On very rare occasions,
the carnassial premolar will be malformed.
 
One of the oft-quoted signs of domestication is a reduction in face length
resulting from juvenilization (= developmental neotony).  In dogs, as well
as in some breeds of cats, this reduction can cause considerable crowding
of the cheek teeth.  However, in ferrets, such reductions are minor, and
the teeth are not usually crowded.  It should not be inferred that changes
have not occurred; ferret jaws are crowded compared to polecats, but not
to such a degree that teeth are commonly rotated to fit within the mouth.
The reason ferret jaws are not as significantly impacted by domestication-
as with the dog-is related to ferret skull allometry, or how one structure
changes over time in relationship to others (I will be explaining this
better in the section discussing osteological changes caused by
domestication).  In short, the face/cranium ratio in ferrets shows very
little change when comparing juveniles to adults, while in dogs, the
changes are more significant.  Thus, developmental neotony, which
maintains a juvenile-like head in the adult, impacts dogs significantly,
but since there is a smaller difference between the face-to-cranium ratios
between juvenile and adult ferrets, there is also very little apparent
juvenilization.
 
Adult ferrets have a total of twelve premolars; three each in four
quadrants.  Dentes premolares (= p, P, pm, PM, dp, dpm, dentes praemolares,
premolar, cheek tooth, bicuspid, deciduous molar, posterior tooth,
postcanine tooth, temporary molars, deciduous molars, sectorial teeth).
[Posted in FML issue 3519]