Gerbils who have curly coats like Rex gerbils may have temporary bald patches while molting. The hair falls out just before the new hair grows in leaving your Rex gerbil bald for a few days.
Young pups that have not molted (grown a new coat) may lose hair more easily than adult gerbils. Some pups may fail to grow in a coat due to poor growth. Often they also have fever coat.
Gerbils may also have cancers or other skin irrations that cause they to scratch escessively at particular spots. A veternarian may need to be consulted.
Gerbils that are fed poor diets or do not recieve adequate amounts of food can fail to grow in hair, or to grow hair that is washed out. Some gerbils that are undernourished may have Fever Coat. Diets that are high in fillers like Timorthy Hay, Alfalfa, Milo, Sorghum or Corn often do not provide enough nutrition, fat and protein for the gerbils to grow healthy coats.
Gerbils with parasites like mites, scabies, and even internal parasites often lose fur from scratching. A veternarians should be consulted for parasite treatment.
Gerbils use grooming as a way of enforcing social order. It is not uncommon that gerbils may overgroom their subordinate tankmates. Common areas of overgrooming include the whiskers, tails, and shoulders. Providing your gerbils with toys and other distractions can help alleviate over grooming.
Gerbils that are declanning may attack each other. Losing hair on the rump is fairly common as the gerbils bite eachother and rip hair out. Seperate the gerbils immediatly. Seek medical attention for the bite wounds.
Gerbils who bite cage bars can often rub the hair off their cheeks or off the bridge of their nose. Obtaining a cage with no wire bars will stop the behavior.
Some gerbils dig a lot. If they are diffing under wooden objects they may rub their fur off in their digging. Moving the objects around so the gerbil can easily dig without scraping themselves will stop further damage to the hair.