Crib biting is when a horse bites onto something, leans back and contracts the muscles. It has been found to develop from an early age and has been suggested to be caused by the animal's behavioural need to suckle at weaning.
It has been seen for over a hundred years but it is still not clearly understood. It used to be considered to be due to idleness, lack of social contact and the restrictive nature of the stable environment. These factors are still considered today but in terms of frustration, boredom or stress seen in the animal. There are many theories into the causes of crib biting.
Feeding concentrates theory
Once the behaviour has been established, it is often seen after the horse has been fed. This is particularly when they have highly palatable feeds such as cereal-based concentrates which are high in starch. These reduce the pH in the caecum due to the mircrobes that break down the food working rapidly to produce volatile fatty acids. It has been speculated that this lower acidity in the gut leads to crib biting. When the horse crib bites it produced more saliva which will change the pH in the gut and prevent it from being as acidic.
This theory was developed after giving horses virginiamycin supplements which prevented the normal decrease in pH after feeding concentrates and significantly reduced crib biting behaviours. Other studies have also looked at giving horses ant-acid tablets and have found similar effects.
Research has suggested that stereotypies may be caused by a dysregulation of the basal ganglia in the brain. Dysfunctions in this area also show when a horse is demonstrating differences carrying out a learning task. Therefore several studies have showed a link between inappropriate repeat responding and stereotypy behaviours. An example of this has been seen in a trial where horses were rewarded for pressing a button. They then stopped rewarding them and counted how many times they pressed the button before they gave up. Crib biting horses demonstrated increased perseverance suggesting they had alterations in basal ganglia physiology.
More work is still needed to be carried out to better understand crib biting. When this knowledge has been gained this will lead to an improved management of horses with regards to preventing crib biting behaviours.