A hairball is a small collection of hair or fur formed in the stomach of animals that is occasionally vomited up when it becomes too big. Hairballs are primarily a tight elongated cylinder of packed fur, but may include bits of other elements such as swallowed food. Cats are especially prone to hairball formation since they groom themselves by licking their fur, and thereby ingest it. Rabbits are also prone to hairballs because they groom themselves in the same fashion as cats, but hairballs are especially dangerous for rabbits because they cannot regurgitate them. Because the digestive system of a rabbit is very fragile, rabbit hairballs must be treated immediately or they may cause the animal to stop feeding and ultimately die due to dehydration. Cattle are also known to accumulate hairballs, but as they do not vomit, these are found usually after death and can be quite large.