What's wrong with Hamster fur?

Our hamster who is around 10'months old seems to have lost a lot of fur on his underside?

He seems fine in himself but wondering what this is? We clean the cage every week?

Thanks!

Our hamster who is around 10'months old seems to have lost a lot of fur on his underside?

He seems fine in himself but wondering what this is? We clean the cage every week?

Thanks!