20 Aug 2018
Horses habitually gnawing on objects around their stables may be relieving anxiety linked to housing conditions, according to new research.
Experts from the Royal Agricultural University (RAU) and Aberystwyth University found differences in brain function pointed to a stress-relieving effect for this biting behaviour known as ‘cribbing’.
The grasping motion appears to mimic grazing or trying to find food suggesting a connection with housing conditions that restrict natural feeding patterns, according to the study.
The findings also hint that horse behaviour of this kind may hold clues to future understanding of human mental health, because crib-biting and conditions such as schizophrenia show similar brain differences.
Up to 10% of domestic horses can be seen performing this distinctive biting of nearby items of stable fittings and equipment with their front teeth while grunting audibly.
Lead researcher Dr Andrew Hemmings, Principal Lecturer in Equine Science at the RAU explained: “Many owners consider cribbing to be unsightly and contagious, and as a result often physically prevent the behaviour with harsh devices (crib-straps) which fit around the neck and cause discomfort when the animal starts to crib.
“This research casts doubt on physical prevention as the findings suggest that crib-biting may have a stress relieving effect.
“The presence of this behaviour raises a range of welfare related questions because these activities normally develop under housing conditions which deprive the animal of food, social interaction and the opportunity to move freely.
“To date very little has been known about the brain basis of cribbing. We discovered some widespread neurological differences in horses who performed crib-biting behaviour.
“The clear evidence of ‘neural sensitisation’ detailed in research paper suggests that the performance of the food-motivated grasping behaviour might have self-stimulatory or rewarding consequences for the horse.
“On that basis behaviours such as crib-biting could allow horses to ‘cope’ with stressful situations, in the same manner that humans use rewarding activities or commodities to counter the effects of stress.
“From a horse management perspective, physical prevention of crib-biting is not advisable as we could be interfering with an important coping strategy, leaving the horse vulnerable to the damaging and performance reducing effects of stress.
“Rodents acting in this way are sometimes used to model human conditions such as Schizophrenia, which are associated with altered function of the brain’s striatum. However, the anatomy of the rodent striatum is very different to that of the human, whereas the horse provides a much closer approximation. Therefore, there is potential for equine work such as this to inform future studies into human mental health.”
A sub-region of the striatum (the nucleus accumbens), where the new work concentrated, is a key component of the brain’s pleasure and motivational circuitry, which receives the neurotransmitter dopamine from nerve cells in the midbrain.
Midbrain dopamine systems are under the influence of other neurotransmitters known as opioids which include the endorphins such as Beta-Endorphin. Some of the well-known functions of opioids include pleasure and analgesia.
Dr Hemmings added: “We found large scale increases in the number of docking molecules on nerve cells to which the opioids bind (opioid receptors), a change which has been linked to hyperactivity and sensitisation of dopamine systems in other species. One effect of sensitisation is a lowering of the brain’s reward threshold whereby activities which previously led to modest reward become more pleasurable in the sensitised animal.”
The research was a collaboration between Dr Hemmings of the RAU and Dr Sebastian McBride of Aberystwyth University, supported by Dr Matthew Parker of University of Portsmouth and independent equine nutritionist, Catherine Hale.
The full paper Causal and functional interpretation of mu- and delta-opioid receptor profiles in mesoaccumbens and nigrostriatal pathways of an oral stereotypy phenotype has been published by Behavioural Brain Research and made freely available to download through ScienceDirect, ahead of its appearance in the printed edition of the journal on 1 November.