In order to understand headshaking, it is first necessary to understand the normal
The horse has successfully evolved over millions of years, longer
than human evolution. Horse headshaking, or head tossing, is a common condition,
increasing in prevalence throughout the world. It seems very unlikely or impossible
that headshaking could be caused by some unexplained recently developed neurological
or eye defect. Instead let us look at the horse and examine what mechanisms the horse,
with all its power and speed, has evolved to survive so successfully.
The horse may
be large, but it has a very small and lethal enemy, the fly. The horse's principal
defence against flies is movement. A horse at the gallop is not likely to be bothered
by flies. Moving less quickly, or at rest, the horse has a number of movement methods
for keeping flies away. Tail swishing is obvious, as is flicking the ears. The body
of the horse is covered with a layer of panniculus muscles just under the skin, and
these muscles react to flick away flies whenever a fly disturbs the end of the coat
hairs. This reaction has to be very fast, and so messages are conveyed locally through
nerve junctions in the spine. The lower legs have no panniculus muscles. A fly detected
in these four areas will be removed by foot stamping.
Now we come to the interesting
There are no panniculus muscles on the neck and head. When a fly disturbs the
hairs of the neck or head, the fly is removed by headshaking. Again this has to be
rapid, before the fly can strike, and so the message is passed very quickly via the
nerve junctions of the spine. If this anti-fly headshaking is observed carefully,
it will be seen that it is an automatic action over which, the horse (that is the
brain of the horse) has no control.
All of these automatic actions have an energy
cost for the horse, and so the horse has evolved to switch the whole fly defence
system off when there are no flies around. How does the horse know that there are
flies around ? The answer is daylight and warmth. As the ambient temperature rises
in the spring, the horse switches on the anti-fly reflex during the day, and off
again at night.
This can be tested on a warm day by lightly brushing the hairs of
the flank of the horse with the finger tips. The horse will automatically flick the
panniculus muscles, seen as a ripple on the skin. If the horse is now taken into
a dark barn out of the light, the whole anti-fly reflex will shut down in the absence
Note that even a small shaft of daylight into the barn will tell the
horse that it is day, not night, and the normal anti-fly reflex may be switched on.
now we can see that headshaking to remove flies is automatic and will occur during
the longer warmer days of the spring, summer and early autumn, but will not occur
at the gallop, in the dark, or in winter.