I'm not sure but I think it started after the last update. Previously when animals matured it would usually result in same sex fights, but lately it is my adult breeding pair that start fighting and not the newly matured animals and it happens with all species. If the fighting adults are getting old anyway I just release them to the wild. But if they are still young and I want to keep them all I can do is release the young ones and just let the breeders fight and send the looser to the vet afterwards.