For instance, gerbils and rats are very social animals that do best with cagemates of the same gender. (petbusiness.com)
Any hamster beyond their immediate group of cagemates is perceived as a threat. (smallanimalchanel.com)
Environmental causes include: low temperature, low humidity, excessive handling of shy species, overcrowding (may occur with only two dragons in a cage), aggression from cagemates, lack of hide box for visual security, lack of UV light and lack of proper diet. (safarivet.com)