Kim, I'll comment here only on the techniques for ageing a horse by the
teeth. Galvayne's groove is as accurate as any of the other signs in the
incisive dentition that are used for ageing, vis., the "stars" (occlusal
plane of the pulp cavity) and the shape, size, and presence of the
infundibulae. How accurate any of these are is, after the age of six when
the canine teeth are fully erupted, about plus or minus two years for the
population as a whole. In other words, within the whole domestic horse
population, you will find some horses that are chronologically eight years
old but whose dentition, by whatever book you may be using, "says" he is
nine or seven. The books are not absolutely uniform either, and this is
because different authors looked at different horse populations to create
their charts or illustrations showing the ageing of the teeth.
Other factors also matter. One thing I would love to see y'all do is
document the presence and magnitude of malocclusions in the teeth, and any
evidence of tooth-rasping or any attempt at equine dentistry, i.e.
extractions, bite equilibration, reduction of hooks, etc. When domestic
horses are kept off of pasturage and thus do not graze very much, their
incisors do not receive the same amount of wear as the cheek dentition and
indeed may receive no wear at all if the animal is kept in a stall or a
drylot and fed hay and/or grain, i.e. any type of processed feed. Horse
incisors only receive wear if they are actually used in grazing, but
nonetheless by an internal, genetically-mandated process, the incisors,
like the cheek teeth, extrude continuously through the gums at the rate of
about 1/8th of an inch per year.
This means that, if the animal is kept in a pen and not permitted to graze
for four years, his incisor teeth will have extruded 1/8th of an inch X 4
on the top and 1/8th of an inch X 4 on the bottom, for a total of one inch
of tooth in excess of the cheek teeth, which will have received enough
wear in the same time period to keep their tables at the same height over
the gums.
What horses do in response to this is a kind of "reverse self-orthodonty":
since the buildup of excess length in the bluntly-occluding incisors can
do nothing but act to pry the jaws apart -- gradually, from front to back,
making it more and more difficult for the animal to bring the cheek tables
into occlusion -- the animal will strain to close its jaws. Immediately
this results in visible hypertrophy of the masseter and temporalis
musculature, while little by little over time the pressure induces the
incisor teeth to rotate into a more horizontal angle than the eccentric
curvature native to the incisors would have dictated, so that the animal
becomes markedly "buck-toothed" as he ages. In a horse over 15 years who
has been kept in a stall or pen most of his life, the incisor teeth can
become so long and procumbent that the animal cannot comfortably bring his
lips closed over them.
Now, how this relates to ageing the horse is this: that any horse with
excess accumulated incisor length is going to present a younger occlusal
table than he should for his actual chronological age. One must therefore
section or "reduce" the tooth down to the length it should have had, had
the animal been able to graze all its life. What length this is I can
teach you if you would like to know; there is a technique for determining
it used by those equine dentists who properly understand equine bite
biomechanics.
You might be desirous of serially sectioning some of the teeth under study
anyway -- or you might not; we can do it "graphically" also -- which is an
estimate, but it won't be far out -- if you don't want to damage any
teeth.
-- Deb Bennett
> I am helping a beginning MA student who is working on a collection of
> Bronze-Age sacrificial horse's heads from northern Mongolia. We are
> starting by his learning the measuring points and developing his
> measurement consistency, including adjusting for incomplete skulls. Then
> we will go to
>
> -ageing
> -sexing
> -seasonality
> -use (bit abrasion at least) / behavior / treatment
>
> -size
> -injury and disease
> -correlations with which additional bones are present
> -diet.
>
> I would appreciate any tips. While Bishop's paper is extensive, later
> works that I've seen (e.g., U. Colorado) generally disagree with it on the
> third incisor's groove's value for ageing older horses. We are going to
> start with maco-studies, given a lack of experience and resources.
>
>
> For measurements, do we need all of the thirty or so standard
> measurements, given his interests?
>
> Any-thing about the development of the horse (breeds etc.) in this
> part of the world would be greatly appreciated.
>
> Also, if any-one knows of early Chinese references (especially if
> translated) to horseson the steppes back before the time of Christ, they
> might be of use.
>
> The student does not read Russian but can work his way through English
> material.
>
> What cheer,
> Kim
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