Chewing on leather
From www.leather-dictionary.com - The Leather Dictionary
Some animals, such as horses or alpacas, are occasionally seen to be chewing leather, for example leather straps. This habit is interpreted by veterinarians in such a way that chewing is relieves stress and is soothing. Furthermore, the chewing process - similar to humans chewing gum in - increases the salivary flow which in turn regulates the acid-base ratio in the mouth and prevents dental caries
Video about a leather chewing alpaca.
Additional information
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