Degus fight. It is used to establish dominance, set the social hierarchy, and express dislike. This behavior is typical for all species and is present in the cage as much as it would be in the wild, possibly even more as there is less territory to control and more (sexual) frustration. On 99% of occasions you should do nothing and leave them to it, as interfering would only harm proceedings and cause more fighting. It is only when things get serious you should break them apart. Degus will tend to fight more during the breeding season, which is winter-spring time, as the breeding hormones cause a greater need for dominance.
Most fighting will be non-serious, where they are establishing the social hierarchy by displaying dominant behaviour over each other. This can include chasing each other around the degu cage, squeaking at each other, mounting each other so the one below becomes submissive and the one on top dominant, and standing on the hind legs and ‘boxing.’ This is normal behaviour and nothing to worry about, and you should not intervene as it will only set them back a stage and make them fight more.
These battles are fought in a way as to not cause injury, just a bit of friendly rough and tumble. Fighting can also occur when there are limited resources to share such as food and treats. With Degus showing such behavior as turning their backs, blocking access to the food bowls and treats, and loud squeaking to warn them away. If you see a degu for sale on his own, beware! Great care must be taken to ensure everything is equal and is placed far enough apart for the Degus to see there are 2 (or more) separate areas without coming into too much contact with each other. According to degupets.com – The food bowls should be 10cm apart.
You should only step in and separate them when there is biting and lunging at each other which causes wounds, and when they become locked in a ball rolling around. This is serious fighting and can cause injury, as both Degus are unwilling to back down and fight to the death. Any injuries sustained during these fights should be checked by a Vet, including any noticeable changes in behavior or discomfort. Remember, a Degu can not speak and tell you they need treatment, so it is up to you as a right owner to ensure their health and well-being takes priority. If you do need to separate them, it may be hard to introduce them back together and may need to be kept separate for a few weeks before being able to be reunited.