and Cathy Bickel
The scent gland is a waxy-looking, hairless spot in the middle of a gerbil's underside. It is more prominent in males than females and gets larger as the gerbil matures.
Gerbils rub their scent gland across surfaces, or other gerbils, to mark them with their scent. If you see a gerbil dragging his belly along the ground you may be seeing scent-marking behavior. Mounting another gerbil from behind can be an aggressive action, or part of mating, but it could also be just scent-marking behavior. The scent left behind is imperceptible to humans.
Scent glands are susceptible to tumors, both benign and malignant. These can often be removed by the vet. Deal with them early before they have a chance to metastasize.