Males can become more aggressive around 4 – 5 months old and can become hyper-aggressive if he has a hormone imbalance. Some indicators of this include biting but also hissing, puffing up his fur, scratching at things, peeing everywhere and rubbing his sides on objects to leave his scent.
If you find that he threatens to bite you or hisses at you then you need to take some action. This can include castration which will normalize his hormone levels and see his behavior return to normal. This will usually see the buck become a lot more happy and calm as a result. Being in a perpetual state of aggression is not the best state for a rat to be in. See a vet who has performed many castrations on rats before as it is a very different procedure from other animals.
Female rats on the other hand can also become a bit ‘bitey’ at times and this is usually during pregnancy. Once the babies are weaned however, her behavior should return to normal again.
Rats that bite should not be bred as they can pass along the trait to their offspring. This can cause further complications whereby if the mother is biting it’s breeders whenever it’s offspring are being handled, then it’s very difficult to properly socialize the babies and they may end up aggressive towards humans later in life. This is to be avoided as much as possible.
Never try and stop a rat fight by handling them physically. This will only get you bitten by one or both of the rats. The best thing to do is to squirt them with a plant sprayer filled with cool water. This will stop them from fighting immediately and distract them long enough for you to physically separate them with your hands.
If you feed your rats through their bars with treats then expect to get nipped on a rare occasion. They don’t mean to harm you, they just missed and thought your hands were food. You can easily teach them the difference by using voice commands when you are giving them food, and a different command for when it’s your bare hand going into the cage. This will teach them to detect the difference slowly but surely, and the biting should decrease over time.
Genetic abnormalities can occur in rat breeding which causes a biting rat to be born, no matter how many precautions are taken. Even if the cutest, most healthy, non-biting rats are bred from, occasionally there will be a nasty, biting rat that will be born. This is just part of the genetic lottery.
Of course it’s much more common if the rats are bred from indiscriminate stock with healthy and unhealthy put together. In these cases, it’s best to not let rats like these breed. In fact, the most humane option is to have them put down. A rat with this type of temperament will be left in a cage with no contact from other rats and left to stew in their aggression and frustration. That is not a quality life for a rat, so it’s best that they are not left to endure it.