There are a number of potential reasons behind a gerbil losing its hair, ranging from old age to parasitic infection. If your gerbil is losing its hair, it can be a symptom of a health problem, and so it needs to be investigated.
Are your gerbils' coats nice and shiny, or are they thinning in places?
The cause of the hair loss will vary depending on where on the body it is occurring. For example, hair loss on the belly where a bump is forming could be due to an abscess or a tumour, whereas hair loss on the face could be due to a condition known as bar rub (more information on bar rub is available on our Gerbils Chewing Bars page).
One of the more common causes is parasites - if your pet is suffering from parasites like mites, then all the itching and scratching they do to try and get rid of them could be wearing away their hair.
If you are sure that the hair loss is not due to old age, and there’s no parasitic infection, then you might want to take your pet to the vet to get an accurate diagnosis - your pet could be suffering from some form of disease or nutritional deficiency.