Ball fighting is where two rodents curl into a ball and this is where a fight has escalated and does need to be separated. Gerbils in particular must be separated if this happens.
Sometimes rodents that have lived together for a long time can have serious fights and decide they can no longer live together. This is known as declanning. Female gerbils are known for this in later life, if kept in numbers greater than a pair. If declanning occurs in a pair then you may be able to reunite using the split-cage method, but you cannot introduce a single gerbil to an existing pair or clan.
If blood hasn’t been drawn then one of the most common problems may be odour, say if you handled one rodent without handling the other or if there is as strong smell in the room such as if you have recently painted. A dust bath is a good start to try and neutralise this for some species.
Sometimes this isn't declanning, but where a species has been sold to an owner without the proviso that they may not have a good chance at staying together for life. Dwarf hamsters from pet shops for instance, can be difficult as they will be hybrids - and so may have inherited different percentages of traits and not be compatible with each other.
Rodents moult their fur from pup to adult where a baby coat is replaced with a smoother adult version. Some changes also occur as adults, Nutmeg gerbils for example, have quite dramatic colour changes and the lines of this can sometimes be seen quite clearly along their fur.
Winter White Hamsters will change coat colour as the light changes in winter, as a natural camouflage against the snow.
Some rodents will use their scent glands to mark their territory. It is often a high-ranking or alpha that will do this most, and they may also secrete small droplets of urine as a form of marking. This does not indicate incontinence and not all rodents will do this.
Cleaning out rodents too often may lead to excessive scent marking. Not only will this actually lead to a greater overall smell, but more importantly this puts a greater strain on their kidneys.
Seizures are a very upsetting thing to watch and it is best to adapt your cage layout to ensure your rodents safety if this is something they are prone to. They can have a variety of causes, including epilepsy, and a vet should be consulted.
Young gerbils may experience seizures that they grow out of.