[Sasami]

How large is the cage? Sometimes this happens when males become sexually mature and it usually calms down. Did he draw any blood?

 

[Sasami]

Are they neutured?

Degus aren't normally neutered unless someone wants multiple sexes together . There's no evidence that neutering actually calms them down (it might, though, who knows), just like cavies.

 

[Sasami]

If there's no blood it sounds like typical bossy male behavior. Hopefully they'll settle it themselves. Is the fighting usually related to cage cleaning? A fix for that is to clean half the cage at a time instead of washing out the whole cage at once. Degus are scent-based animals when it comes to territoriality so a newly cleaned cage can set off fights. It doesn't smell like either degu now and lacks any territorial markings so they fight for territory all over again.