From:
Church, Robert Ray (UMC-Student)
Date: 2001-08-20 01:31:00 UTC
Subject: Bob C: Ferret Dentition, part IV: Premolars
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 teethâespecially the
carnassialâto 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 sizeâand to
some degree the shapeâof 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 dentitionâlike the permanent
dentitionâis 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 âself-sharpeningâ; 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 foodâespecially
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 tartarâa selling point).
While food remains basically safe and fresh in the dish, it is not without
cost to the ferretâs 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 sidesâthe
lingual-vestibular surfacesâof 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 âbladeâ 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 domesticationâas
with the dogâis 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).
Bob C