Bar biting is probably a response to boredom, frustration or some other stress, – rather like nail biting in humans (Hewson 2008a). Bar biting means your hamster is unhappy. Hamsters start biting the bars of their cage if they are kept in small cages, without enough bedding, and with no proper nest to hide in. Bar biting then becomes a compulsive habit, also known as a stereotypy.
Chewing opportunities are helpful but not enough on their own. Hamsters have teeth that continuously grow and need to be worn down by chewing. The easiest way to give your hamster a chew toy is to attach a wooden clothes peg to the inside of the cage bars. Other chew items are branches of fruit trees, hazel (not poisonous conifers or yew), a whole walnut or cob nut, or a coconut shell after the birds have eaten the inside. Bar biting is more likely if there is nothing to chew but just putting in a chew toy is often not enough to stop it (Fischer et al., 2007).
You have to do more. Read Keeping Your Hamster Busy not Bored and Cage and Equipment for Your Hamster. A depth of 40 cm of litter, if your cage design allows it, helps reduce or even end bar biting (Hewson 2008b). Giving your hamster more to do in other ways is also essential to reduce his stress.
Stressed hamsters are more aggressive than happy hamsters so if your hamster is aggressive, see if making his living conditions better will help (Reinhardt 2004). Here are some of the common causes of stress in hamsters:
Fischer, K., Gebhardt-Henrich, S. G. & Steiger, A., (2007), ‘Behaviour of golden hamsters (Mesocricetus auratus) kept in four different cage sizes,’ Animal Welfare, 16, 85-93
Hewson, C., (2008a), ‘How to recognise stress in your patients,’ Proceedings of the 33rd World Small Animal Veterinary Congress, 82-83
Hewson, C., (2008b ), ‘Stress in small animal patients: why it matters and what to do about it,’ Irish Veterinary Journal, 61, 249-254
Reinhardt, V., (2004), ‘Common husbandry-related variables in biomedical research with animals,’ Laboratory Animals, 38, 213–235
Russell, W. M.S., (2002), ‘The Ill-Effects of Uncomfortable Quarters,’ in eds. Reinhardt, V., & Reinhardt, A., Comfortable Quarters for Laboratory Animals, Animal Welfare Institute, 1-5s. Available at http://www.awionline.org/pubs/cq02/cqindex.html Accessed 14 July 2013.