Introduction
The term 'vice' is often used to describe abnormal equine
behaviour which is usually stereotypic in nature. This term
can be misleading since it attributes to the horse the ability
to make moral decisions as to whether or not to act in an
anti-social manner. This affects our perception of these behaviours,
normally in a negative context, and has implications about
how we subsequently treat horses that perform them. It also
results in behaviours that are not stereotypic by definition,
being classified as such; common examples being wood-chewing
and door kicking. In accordance with the current definition
of stereotypies (a behaviour pattern that is repetitive, consistent
and has no obvious goal or function), the major equine stereotypic
behaviours are crib-biting, weaving and box-walking. It is
very important to make this distinction if we are to understand
why these behaviours develop and to try and prevent or reduce
their performance.
General Causes of Equine Stereotypic Behaviour
Many people have talked in very general terms about the
causes of equine stereotypy normally in terms of frustration,
boredom or stress of the animal. Only recently have studies
started to support this speculation. A study carried out in
1995 indicated that the risk of horses developing weaving
decreases when levels of forage are increased (above 6.8kg/day)
and that a similar pattern is observed if forages other than
hay are given. With regard to the latter, the number of forages
given to the horse appears to be important because choice
is something that the horse would naturally have in the wild.
A more recent study has shown that increasing the number of
forages available to the horse (2-3 presented at the same
time) reduces the amount of time that it will stand in an
inactive state; inactivity being associated with boredom,
a considered causal factor of stereotypies, it is apparent
why a choice of forages would reduce stereotypy incidence.
In actual fact, a crib-biting horse was used in the latter
experimental trial no stereotypy was performed when more than
one forage was offered.
The risk of abnormal behaviour generally (stereotypies and
wood-chewing) has been reported to increase when stable design
prevents visual contact with other horses. This observation
has been recently supported by another study which tested
the effect of different levels of visual stimulation within
a stable (number of windows) on weaving behaviour in horses.
It was observed that the more windows within the stable design,
the less stereotypy the horse performed. A fourth study carried
out in Sweden demonstrated that the occurrence of stereotypies
was directly related to the amount of cereal-based concentrate
and forage given to the horse, as well as the number of horses
per trainer. The authors concluded that horses that spent
more time eating or were in greater contact with stable staff,
were less likely to perform a stereotypy. These studies support
the idea that stereotypies arise from frustrated motivation
and in fact may be replacement behaviours for those that the
animal cannot perform due the environment that it is kept
in. For example, oral stereotypies (crib-biting) may be the
result of frustrated eating motivation due to restricted or
monotonous food supply (insufficient ad-libitum forage or
number of forages). Locomotory stereotypies (weaving) may
result from frustrated motivation to perform locomotory action
to gain food or access other horses. These causal factors
are strong indicators of the types of management strategies
that could prevent or reduce these types of behaviour.
Neurochemical mechanisms underlying the cause of equine
stereotypic behaviour.
Not all individual horses develop stereotypies when placed
in the same environment. For example, when the 280 horses
held at the Household Cavalry Mounted Regiment all kept in
the same husbandry conditions were examined in 1995, only
four were reported to crib-bite and three to animals weave.
This suggests individual predisposition to performing these
behaviours. Inheritance-based studies using stereotypy horses
have, however, been unable to come to any meaningful conclusions
about genotypic predisposition of equine stereotypy due to
difficulties in obtaining data sets of sufficient size. Horses
are not, however, the only species to perform stereotypies
and much research has been carried out using rodents to try
and understand why this predisposition exists. Research to
date suggests that the activity level of certain neurotransmitters
within the brain (e.g. dopamine) are important in this respect
and this is affected by the way the individual animal responds
to a stressful environment. The individual animal's genes
dictate this latter response.
Recent work has suggested that this same mechanism exists
for equine stereotypy where the same differences in neurochemistry
as seen in mice have now been reported for the horse. This
is preliminary data, however, and further research* needs
to be carried out to establish fully the neurochemical profile
of stereotypy horses. Such information will help determine
whether individual horses are predisposed from the outset
to perform stereotypic behaviour.Neurochemical mechanisms
underlying the function of equine stereotypic behaviour.
The function of stereotypic behaviour may differ depending
on its stage of development in the animal. In the initial
stages the behaviour may simply be replacing other behaviours
that the animal cannot perform because of the restrictive
nature of its environment. However, when the behaviour becomes
more established, it is performed during periods which are
not normally associated with the replacement of other oral
or locomotory behaviours (Figure 1).
Here it is thought that the animal may be using these behaviours
to allow it to cope with a stressful or unstimulating environment.
There has been a lot of speculation about the mechanism by
which this is taking place, much of which has centred on the
theory of opioid release (naturally occurring substances equivalent
to morphine being released into the brain). Two studies have
provided information that may help us understand if this is
the case for equine stereotypies. The first study assessed
pain thresholds in horses before and after crib-biting. Here
it was considered that if opioids are being released during
stereotypy, then there should be an increase in pain-threshold
after crib-biting compared to before. Results in fact suggested
the opposite, that the pain-threshold decreases, suggesting
that these behaviours do not seem to function in a coping
capacity. Other studies have assessed the behavioural effect
of drugs that block the effect of opioids (opioid antagonists).
These are pharmaceuticals routinely used to help reduce addiction
in humans by blocking neurochemical receptors for opioids.
For horses with stereotypies, they cause an 80-90% reduction
in stereotypic behaviour. What is important here is that these
drugs also reduce other behaviours known to have reward characteristics
(i.e. behaviours that produce a sensation of pleasure), such
as eating.
It is argued that because these drugs reduce these behaviours
and also reduce stereotypy, then stereotypies must also be
reward behaviours that could then be allowing the animal to
cope with its environment.Interestingly, the majority of crib-biting
is performed after eating, and especially after eating foods
that are highly palatable (e.g. cereal-based concentrates).
In fact the most reliable way to test for a crib-biting horse
is to give a small handful of concentrate and observe the
immediate behavioural response. Palatable foods also cause
opioid release in the brain and there has been some speculation
that the crib-biting response is greatest after eating because
eating has a 'neurochemical priming effect'. In other words,
the neurochemical effect of eating palatable food (through
opioid release) primes the brain reward centre such that there
is more activity than normal when stereotypy is performed.
In this way, equine stereotypies may be similar to other reward-seeking
behaviours performed by humans; cigarette smoking for example
is rewarding to the participant, however, it is most rewarding
when performed after other reward behaviours, such as eating,
alcohol and caffeine consumption. All of this evidence points
to the fact that stereotypy may be a reward behaviour which
allows the animal to cope with an environment that it finds
stressful.
Figure 1 Horse with an established
stereotypy crib-biting on a field -post. The behaviour has
become emancipating from the original causal stimuli.
Equine stereotypy and animal welfare
Crib-biting, weaving and box walking are all equine behavioural
conditions that owners have actively sought to avoid in their
horses since this issue was first discussed over one hundred
and sixty years ago. This is mainly attributable to the clinical
effects of these behaviours (e.g. reduced performance, incisor
wear, strained ligaments), but also the apparent social stigma
associated with the ownership of these supposedly 'vice-ridden'
animals.
In general, horse-owners are also under the belief that these
behaviours are learned. The result of these two perceptions
is that stereotypic animals are currently being subjected
to management practices that may be seriously reducing their
welfare. For example, in a recent study it was observed that
around 32% of riding schools prohibit stereotypic animals
from entering onto their premises and on average 39% of racing,
riding school and competition yards socially isolate horses
which perform stereotypies. This is a practice known to induce
a stress response in the horse. In addition, equine stereotypies
are physically prevented using devices such as the crib-strap
(Figure 2) and anti-weave bar (Figure 3) (77, 67 and 79% of
racing, riding school and competition yards respectively),
the implications of which are twofold. Firstly, if stereotypies
are acting to allow that animal to cope with a stressful environment,
then the restriction of such behaviours may prevent activation
of the coping response, thereby placing the animal at risk
from a welfare perspective. Secondly, even if equine stereotypies
are not acting in this capacity, other results have shown
that the use of the crib-strap and anti-weave bar are themselves
stressful to the animal. Overall, therefore, the current management
of animals may be open to criticism if the welfare of the
animal is considered important.
Figure 2 Horse wearing a
crib-strap preventing contraction of the neck muscles.
Figure 3 Anti-weave bar placed
on the stable door to prevent lateral movement of head (weaving).
Practical means of reducing stereotypic behaviour without
affecting the horse's welfareOnly some horses are predisposed
to performing stereotypic behaviour. Until methods have been
devised to identify these individuals, it is difficult to
test for husbandry conditions that would prevent the onset
of equine stereotypies. However, we can make recommendations
to reduce the performance of stereotypies once established,
and more importantly, in a way that is not compromising the
animal's welfare.· Move away from the discrete meal
feeding system (i.e two meals of cereal-based concentrates
per day) towards a more ad libitum feeding system. Although
this may not be possible for concentrates, it is possible
for forage-based substitutes.
· Increase the level of visual contact time with other
animals.
· Increase the amount of visual stimulus generally
within the stable environment.
· Increase the level of exercise or turn-out
*If you are interested in helping current research into
the neurochemistry of equine stereotypy, veterinarians or
horse-owners in the unfortunate position of having to put
down either stereotypy or non-stereotypy animals can donate
brain material by contacting The University of Wales Aberystwyth
on 01970 621690 or by e-mailing sdm@aber.ac.uk
