Ch 21: Gerbils

Lesson 1: Introduction

Gerbils, also known as “jirds” or “desert rats,” are rodents native to the desert regions of Mongolia, northeastern China, and other semi-arid parts of the world. Gerbils are well-known for their ease in handling, mild temperament, adaptability to environmental changes, and low occurrence of natural diseases. Gerbils are often used in research to study stroke and epilepsy. Since they are prone to dental caries (cavities) and periodontal disease as they age, gerbils are also used in dental research. ^1,2

Lesson 2: Anatomy and Physiology

Gerbils are longer and more slender than hamsters. They have strong hind legs and a tendency to jump. They also have a very long furred tail with a tufted end. Gerbils of both sexes have a large sebaceous gland on their abdomen called the ventral marking gland. This gland is controlled by reproductive hormones. It is covered by a distinct mid-ventral abdominal pad, and is larger in males. Secretions from this gland are used in territorial marking, and pup identification, and may also serve as a sexual attractant.

Species-Specific Traits

Spontaneous seizures are hereditary in gerbils. The incidence of seizures is reported in 20% to 50% or more of the gerbil population, depending on the strain. The seizures begin at about 2 months of age and may be induced by sudden noises or handling. As with rats and mice, gerbils do not have canine or premolar teeth. They have one pair of upper and lower incisors and three upper and lower molars on each side. Their incisors grow continuously, but their molars do not. Malocclusion of the incisors can occur, although not as often as in mice and rats.

Lesson 3: Taxonomy

Species Used in Biomedical Research

The most common species of gerbil studied in biomedical research is the Mongolian gerbil (Figure 21.1), Meriones unguiculatus. Other species are also studied, but less commonly (for example, Meriones libycus, Meriones crassus).

Lesson 4: Sexing and Reproduction

Sexing and Reproduction 1

As in most other rodents, the anogenital distance is greater in the male gerbil than in the female. The female may become pregnant while she is still nursing a litter due to the postpartum estrus. In this case, implantation of the embryos may be delayed and gestation may be extended. During her reproductive life, the female may bear seven or more litters. Table 21.1 provides information about litter size and number of young.

Although group housing is common, monogamous pairs show the best breeding results. Significant fighting and even deaths can occur during the establishment of breeding pairs. Therefore, pairs should be formed at 9 to 10 weeks of age, before the gerbils are sexually mature. If one of the breeding pair of gerbils should die or be removed, the remaining gerbil may not form a new pairing with another animal.

Sexing and Reproduction 2

Males “thump” with their hind feet around the time of mating. After mating, a vaginal plug forms in the female, but is usually too small and deep within the vagina to be detectable. Females have a high incidence of ovarian cysts, and affected gerbils may cease reproducing prematurely.³ The nests of lactating females should be disturbed as little as possible to prevent cannibalism.

Lesson 5: Handling and Restraint

A gerbil may be grasped by the base of the tail and the scruff of the neck; however, extra care must be taken not to pull too hard on the tail. If the tail is grasped tightly or if the animal is held along the length of its tail, the skin will slough off if the animal moves against the restraint, resulting in permanent damage known as degloving or tail-slip. Surgical amputation may be necessary if the tail is injured. Since tail degloving injuries can occur easily in gerbils, it is recommended to use handling methods that are gentle and only hold the base of the tail.

Most gerbils are nonaggressive, easy to work with, and rarely bite. They may be picked up by simply scooping them up with both hands. A secure hold may also be obtained with the V-hold. This method involves grasping the animal with the index and middle fingers around its neck and the thumb and ring fingers around the body. Gerbils can also be restrained by wrapping them in a towel or using a restraint box.⁴

Lesson 6: Identification

As for other rodents, cage cards are typically used to identify gerbils. Subcutaneous microchips, ear tags, ear notching, and colored markers may also be used for individual identification.

Lesson 7: Husbandry and Diet

Caging

Gerbils are a USDA-regulated species, so their housing, husbandry, and record-keeping requirements are governed by the Animal Welfare Act and Regulations. Gerbils often stand erect on their hind legs (Figure 21.2). Therefore, it is important to use tall cages with a solid bottom. The use of wire-bottom cages should be avoided. Gerbils need escape-proof cages with secure lids because of their tendency to jump. Being desert rodents, gerbils excrete little urine and very dry feces. Even in nonventilated caging, cage changes may be needed as seldom as every other week. Depth of bedding should be about 3-5 cm to permit burrowing behavior. Environmental temperature recommendations are similar to that for mice and rats, with the exception of humidity. Humidity levels greater than 50% may be associated with rough hair coats and occasional skin problems (nasal dermatitis). Noise levels from human activity in housing areas should be kept to a minimum, and handling during cage changes should be performed gently to avoid inducing seizures.

Water and Feed

Gerbils thrive on any of the commercially available rodent diets. Clean, fresh water should be provided ad libitum (Table 21.1). The height of the food and water should always be checked to be sure even the smallest gerbil can reach it, especially when using tall cages designed for rats. Feed supplements should be placed on the floor of the cages of nursing animals. The young are unable to eat feed from a raised hopper due to the height and may also not be able to reach watering devices. Without adequate access to food and water from moistened feed pellets placed on the floor of the cage, weaning-aged animals may die from malnutrition or dehydration.⁵

Lesson 8: Environmental Enrichment

Enrichment items for gerbils are generally similar to those for rats and mice. Gerbils are avid chewers and anything placed in their cage needs to be either durable or safe to be chewed and ingested. Some enrichment items are made of paper materials which can be safely chewed, whereas others are made of hard materials, like PVC, which need to be regularly inspected for damage or sharp edges and replaced regularly to avoid safety concerns. Fruit tree branches and wood blocks for gnawing are excellent enrichment items for gerbils. Gerbils may also benefit from the addition of shelters to the home cage as these can provide a hiding place, and assist with thermoregulation. Because of their tendency to burrow, gerbils benefit from deep bedding.⁶ Both deep bedding and nesting materials should be provided to allow for expression of normal behaviors.

Lesson 9: Signs of Pain, Distress, or Illness

Sick or injured gerbils often display similar clinical signs to other laboratory rodent species, regardless of what is causing their illness. Common clinical signs include ruffled fur, hunched posture, decreased activity, and weight loss.

Lesson 10: Euthanasia

Euthanasia methods for gerbils include cervical dislocation and exposure to high levels of inhalant gases, such as carbon dioxide or isoflurane. An overdose of barbiturates administered intraperitoneally is also a commonly used method. As with other laboratory animal species, death must be verified. Your institution’s SOPs may require an additional procedure to be performed for this verification. Carcass disposal should be according to the facility’s guidelines.