Although pocket pets such as mice, rats, hamsters, and gerbils may like running in a hamster ball, guinea pigs should never be put inside exercise balls. Since a guinea pig has a different body structure, putting it in an exercise ball will badly injure its back. Hamster balls can also be dangerous for hedgehogs, as being confined in an enclosed space may stress them out. Moreover, their nails can get caught in the ventilation slits and break off, causing severe injury.