Comfortable Quarters for Gerbils in Research Institutions Eva Waiblinger Animal Behavior, Zoological Institute, University of Zürich Winterthurerstraße 190, CH-8057 Zürich, Switzerland E-mail: eviwai@zool.unizh.ch

• Most animal welfare laws and guidelines for the humane care of laboratory animals require:
  (a) that the environment of captive animals meets their physiological and behavioral needs;
  (b) that housing conditions facilitate the performance of natural behavior patterns;
  (c) that husbandry conditions allow for adequate social contacts.

Hence, comfortable quarters for gerbils must be based on the species' physiological and behavioral needs, behavior repertoire and regulation mechanisms as well as social structure.

• Housing-induced changes in brain and behavior
  Behavior control mechanisms evolved in a species' natural environment and require a specific set of environmental and social stimuli to function properly (Hughes and Duncan, 1988). Gerbil breeders concentrated not on altering behavior mechanisms, but rather on changing specific behavior traits [tameness, reduced aggressiveness] and physiological parameters [higher reproductive output, higher body weight] compared to wild-caught gerbils (Stuermer, 1998). Despite decades of domestication, most behavioral traits and behavioral regulation mechanisms have been retained in laboratory gerbils (Wiedenmayer, 1995) as have their behavioral needs. However, the barren laboratory cage lacks crucial environmental and social stimuli involved in eliciting and regulating behavior as well as stimulating behavior and brain development. As a result, behavioral expressions become chronically thwarted and the animal's ability to adapt to these conditions is overtaxed (Mason, 1991; Würbel, 2001). Rearing rodents in such impoverished laboratory cages can result in impaired cognitive functions, inappropriate and ill-regulated behavioral responses, stress reactions, abnormal behaviors such as stereotypic digging and bar chewing, altered brain functions and reduced stress-tolerance in adulthood, which in consequence all threaten the validity of experimental results gained from such animals (Wiedenmayer, 1995; Winterfeld et al., 1999; Würbel, 2001). Housing and husbandry should therefore be adapted to presenting the animals with an appropriate set of stimuli to regulate behavior and forestall the development of stereotypies.
    
• Standardization, variability and result validity
  Standardization of experimental procedures requires a reduction of inter- and intra-individual variation (Würbel, 2000). Notwithstanding, very large individual variation of stereotypic behavior patterns can be observed in animals under barren housing conditions, which reflect their different strategies to cope with the situation (Ödberg, 1989; Dantzer, 1991; Mason, 1991; Wechsler, 1995). Animals kept under enriched conditions should yield more robust results and greater validity, since an enriched environment better resembles the stimulus-rich, variable conditions of natural environments (Würbel, 2000, 2001).
    
• What is environmental enrichment?
  Environmental enrichment are those measures that add extra, species-appropriate environmental stimuli to standard laboratory housing conditions [e.g., 2-3 cm of bedding, pelleted food in food hopper, water ad libitum]. Environmental enrichment is often done intuitively and is based on availability and practicability of materials rather than on the species-specific needs of the confined animal. For enrichment to be most effective in eliciting natural behavior patterns and reducing the development of stereotypies, it should be designed on the basis of a thorough knowledge of the behavioral needs and behavior control mechanisms of the species (Wiedenmayer, 1996). Ethological studies under seminatural and enriched as well as under impoverished conditions can assess the influence of different environmental factors and stimuli on behavior and their relative importance for the well-being of the animals. Ideally, such studies should be accompanied by physiological measurements of welfare-indicators and stress-related parameters.
   
  

The Mongolian gerbil (Meriones unguiculatus) is a non-murine rodent of intermediate size between a rat and a mouse [body length 10-12.5 cm, adult weight 70-130 g]. The wild type has a golden brown, agouti-speckled coat on back and face and a cream-colored belly with a clear demarcation line between the two (Figure 1). Males tend to have a reddish tinge in the fur and are generally more heavily built than females. The gerbil has a fur-covered tail almost as long as the body [9.5-11 cm] with a small tuft of black hair at the tip. Since the first coat color mutations, "black" and "spotted," a wide variety of coat-colors has emerged. Differences in behavior and physiology between the different color varieties are not pronounced (Dizinno and Clancy, 1978). The hind legs of gerbils are relatively long compared to the forelegs due to longer hind paws. The animals often sit on their haunches to look around, feed or groom themselves, and the long hind paws are used extensively during digging behavior to vigorously kick out and remove loose substrate from under the belly. Gerbils perform rapid foot-thumping with the hind feet during alarming situations, and the males foot-thump too after copulation (Holman and Seale 1991). In erect, attentive posture, gerbils stand up on the toes, and the tail acts as extra support. An erect adult gerbil is about 12 cm high. Gerbils are not well equipped to climb, at least not upside down at the cage top as mice do (Roper and Polioudakis, 1977). This is due to the fact that their hind paws are not equipped with friction pads and partially opposable toes, and that the soles are covered with fur. Gerbil cages, however, have to be covered because the animals might crawl or jump out. If startled, gerbils can readily jump over a 30 cm high barrier. Adult animals can comfortably crawl through tubes of a diameter of 5 cm. Tubes narrower than 4 cm, however, may pose a problem for the more massively built adult males who might get stuck

Figure 1. Wild-type [agouti] male Mongolian gerbil (Meriones unguiculatus).

Gerbil claws grow too long if not worn down by regular digging, or walking on rough surfaces. Gerbil teeth also overgrow quickly if not worn down by regular gnawing on hard food or chewing material. The animals are highly motivated to shred gnawable material such as cardboard, hay, straw, woodsticks (Glickman et al., 1967), and they should always have access to it. They shred such material into small pieces and afterwards collect it and carry it to the nest site to build or pad the nest. Wood chip bedding is too finely grained to trigger gnawing behavior.

Gerbil tails are well covered with fur. Gerbils should not be picked up by the tail, because they can shed the outer layer of skin and hair of the tail if in danger and escape with the bloody tail remains of bones and muscle. Dangling a gerbil from his/her tail base is an inhumane practice that can induce an epileptic-like seizure (Tumblebrook Farm, 1980; Fenske, 1996). The marking of same-colored individuals can be done by cutting away the brown upper, light fur layers in defined patterns. The dark under-fur will then show and allow for an easy recognition of individuals (Figure 2). This simple marking method is quite durable in adult gerbils (4-8 weeks). Transpondering might offer a more sophisticated identification method, which has been used with success in house mice and in much smaller species such as insectivorous bats (Neuhäusser-Wespy and König 2000).

Figure 2. Fur-clipping in an agouti gerbil - a method to mark animals individually.

To allow for normal locomotion, the flooring of the laboratory cage must be covered with substrate or at least be somewhat rough, not just wire or slippery polycarbonate. Most small rodents, including gerbils, prefer bedding to wire or bare plastic as cage floor (Pettijohn and Barkes, 1978; Arnold and Estep, 1994; Patterson-Kane et al., 2001). The Guide for the Care and Use of Laboraory Animals, therefore, recommends “solid-bottom caging, with bedding ... for rodents” (National Research Council, 1996, p. 24). We tested several bedding types and found that the gerbils' fur became greasy and lost its shine when the animals were kept on sawdust or dry sand. Gerbils kept on rough-grained sawdust, woodchips, hay, straw, or moist sand had shiny, healthy-looking fur. This was particularly noticeable in animals who had access to moist sand in which they could dig tunnels (own unpublished observation). As an alternative to the heavy moist sand, rough-grained, well-absorbing wood chips provide a suitable bedding for gerbils. The bedding should have a depth of 3-5 cm to allow for digging, and it should always be supplemented with hay or straw to provide more structure and opportunities for gnawing. The addition of a rough-surfaced object [e.g., a stone or the backside of a bathroom-tile] helps wear down the claws and provides a suitable object for marking behavior (Roper and Polioudakis, 1977; Thiessen and Yahr, 1977; Dizinno and Clancy, 1978). Gerbils should be able to stand up in fully erect posture. Therefore, the top of the cage should be slightly higher than an erect gerbil [12 cm] plus the depth of the bedding layer [3-5 cm]. This means that gerbil cages must have a height of at least 17 cm.

Gerbils have a relatively broad snout (cf., Figure 7). They have problems feeding on pellets presented in the food hopper with standardized 0.7 cm bar spacing, which is suitable for rats and mice. Competition between cage-mates increases when food is not so easily obtainable due to narrow bar spacing. Subordinates who manage to retrieve a food item from the food hopper are often chased and pestered by dominant cage mates (Waiblinger, unpublished data). Cage-tops with 0.9 - 1.2 cm bar spacing are preferable for gerbils. Unfortunately such cage-tops are not commercially available. However, standardized cage-tops can easily be altered for gerbils by removing every second bar of the food hopper. Scattering the food on the bedding of the cage circumvents the problems associated with the food hopper and also provides behavioral enrichment: the gerbils can now dig and search for food items in the substrate (Cheal, 1987; Forkman, 1996) as well as carry food around and hoard it (Agren et al. 1989b). We found that adult gerbils dedicate up to 30% of their active time to foraging for food that is scattered on the bedding of their cage. Thanks to the dry feces and concentrated urine of gerbils, the bedding doesn't become wet so quickly, and the scattered food will remain relatively clean.
 

The Mongolian gerbil can be found in all types of semi-desert in Mongolia, Inner Mongolia [China], Manchuria [China] and South Russia. Gerbils are found in habitats as diverse as dry sandy steppes, moist river banks and grassy semi-desert salt pans, but also man-made artificial earth structures, as long as the ground is sandy and easy to dig out (Milne-Edwards, 1867; Bannikov, 1954). The climate in these regions is continental. A range of physiological and behavioral adaptations enable gerbils to survive under often extreme climatic conditions.

Gerbils excrete highly concentrated urine and almost dry fecal pellets. Although wild gerbils do not need to take up large amounts of water, they might lick dew or otherwise get their fluid through plant matter (Bannikov, 1954). In the wild, gerbils feed on the green parts of wormwood (Artemisia spp.), the seeds of numerous grasses, bulbs, buckwheat (Fagopyrum) and a wide range of other annuals and perennials (Bannikov, 1954; Naumov and Lobachev, 1975; Agren et al., 1989a). In the laboratory, gerbils cannot survive without water when they are fed only dry food (Marston and Chang, 1965; Tumblebrook Farm, 1975). The older a gerbil gets, the more water he/she needs (Spangler et al., 1997). Gerbils will drink water even if they are regularly provided with fresh fruit and vegetables. It is therefore advisable to always provide them with fresh water. Fruit [pears, melons with seeds, apples, oranges], vegetables [cucumber, carrot, pumpkin, fennel] and salads are readily accepted and enrich the gerbils' menu. Experience shows that gerbils prefer sweet and/or juicy produce. Since gerbils tend to hoard everything that is edible, cages need to be checked daily to remove any rotten fruit or vegetable material.

Although gerbils do not have cheek pouches like hamsters, they exhibit a very pronounced hoarding behavior. In the wild, 2-3 oval food stores per burrow have been found of 30-75 cm length and 15-20 cm height, containing green plant matter and seeds (Bannikov, 1954). Gerbils may hoard up to 1.5 kg of grains in one store (Bannikov, 1954; Naumov and Lobachev, 1975). In the laboratory, even under controlled temperature, humidity and day-length conditions, gerbils engage in extensive hoarding behavior (Tumblebrook Farm, 1977; Agren et al., 1989b; Forkman, 1996).

In their natural habitat, gerbils dig out vast subterranean burrows up to 170 cm deep into the ground. These labyrinths may extend horizontally over 6 to 8 meters. They consist of a complicated net of tunnels and nest chambers, food stores and litter nests (Figure 3; Bannikov, 1954; Naumov and Lobachev, 1975; Roper and Polioudakis, 1977; Agren et al., 1989a). The burrow serves as retreat and refuge from predators, food-storage and protection from extreme temperature and desiccation. If a gerbil perceives a threat, he or she will thump the hind feet in a fast staccato, and this will prompt the whole group to rush to the various entrances of the borrow and disappear. Gerbils show surface activity day and night throughout the summer. During the winter they hide in the burrow and emerge only on the most sunny days (Bannikov, 1954; Naumov and Lobachev, 1975; Agren et al., 1989a,b). In summer, the burrows probably provide more even temperatures and air humidity, whereas they isolate against the cold in winter (Bannikov, 1954; Naumov and Lobachev, 1975).

Figure 3. Gerbils in a seminatural burrow.

Gerbils use a wide range of shredded, dry plant material to isolate and pad their nest chambers (Bannikov, 1954). They exhibit shredding behavior when presented with hay, straw, paper, cardboard, branches or wood sticks, collecting the shredded pieces and using them to build a nest (Glickman et al., 1967; Waiblinger and König, 1999). Most of these materials can be autoclaved, are cheap and provide the animals with excellent enrichment material. Gerbils are proficient shredders who reduce a cardboard tube to dime-sized pieces within 5 minutes! It is therefore important to regularly replenish the gnawing material with new supply to keep the animals busy. Paper and cardboard should not contain ink. Empty toilet paper rolls, egg containers or plain cardboard boxes are ideal shredding materials. Material from willow (Salix spp.), hazel (Corylus avellana), beech (Fagus sylvatica), birch (Betula pendula), maple (Acer spp.), elder (Sambucus spp.), fruit trees, as well as pine trees can provide natural gnawing sticks that are very attractive for gerbils. In the case of pine tree material, an animal will skillfully gnaw off all the needles before tackling and shredding the branch itself. It goes without saying that all poisonous plants such as ivy (Hedera helix), yew (Taxus baccata) and holly (Ilex spp.) have no place in the gerbil cage.

Under laboratory housing conditions, gerbils develop stereotypic digging behavior (Figure 4). Neither additional space allowance [7600 cm² instead of 1900 cm² for a family of gerbils] (Wiedenmayer, 1996) nor the performance of digging in natural substrate [dry, heavy sand] (Wiedenmayer, 1997a) is effective in preventing the development of stereotypic digging behavior in juveniles. However, the presence of a burrow during ontogeny results in significantly reduced stereotypic digging (Wiedenmayer, 1995,1997b).

Figure 4. Gerbil engaged in stereotypic digging.

The ordinary cage set-up does not allow for digging a stable, dark burrow system. Nevertheless, the gerbils try to dig a burrow in the lab cage, but this behavior strategy is not successful. Yet, the digging continues with great perseverance and gradually develops into a persisting stereotypy, indicating that the animals' behavior control mechanism is chronically overtaxed (Wiedenmayer, 1997a,b). Ideally gerbils should be able to construct their own burrow. In standard laboratory cages, this is impossible because they are not high enough to contain a sufficiently thick layer of bedding in which a stable burrow can be constructed by the animals. Also, most of the bedding will be kicked and shoved out of the cage, since the walls are not high enough.

Spacious, 50 cm high terrariums with wire-mesh tops are the alternative. Wood-chip bedding, mixed with hay, straw, cardboard boxes/tubes and branches provides a stable substrate. It needs to be filled in 20-30 cm high to allow the gerbils to dig their own complex burrow. Such terrariums are very suitable for breeding pairs or same-sex groups. Even with the opportunity to retreat into their burrow, most gerbils will nevertheless inquisitively emerge from it in the presence of familiar humans. Individuals can be swiftly caught and transported in a cardboard tube, as they have an innate urge to dive into the nearest tube when they feel threatened by personnel.

If it is necessary to have regular access to the animals, artificial burrows can be used to house gerbils. Wiedenmayer (1995) developed a simple artificial burrow system consisting of an opaque nestbox placed outside the cage. The animals have access to it through a 20 cm long tube inserted through the back wall of the cage. This quasi external burrow - unlike a dark nestbox attached to the cage without interconnecting tube - reduces stereotypic digging significantly (Wiedenmayer, 1995,1997a,b). For practical reasons, an artificial burrow system should be integrated into a standard laboratory cage rather than attached to it. For a laboratory cage type IV [38 x 58 cm x 20 cm height] the system consists of three modules (Figures 5 and 6a,b):

Figures 5. Modules of the artificial burrow system: opaque access tunnel (a), transparent separation wall (b) with transparent box (c) and supports (d) for access tunnel, opaque nestbox (f) [with front wall removed, and with lid (e)].

• An 18.5 cm high transparent separation wall, which divides the cage into the burrow area and an activity area with food hopper and water bottle. Attached to the separation wall is a transparent chamber [12.5 x 12.5 cm]. It helps to prevent conflicts of use between nest, food store and latrine (Müller, 1998).  
• An opaque access tunnel [5 cm in diameter; total length approximately 34 cm] with a right angle to minimize light leakage into the nestbox and to save space. The tunnel can be constructed from plastic, but metal is preferable, since this element is the only one susceptible to gerbil gnawing.  
• An opaque nestbox [12.5 x 19 cm] with removable lid, which is held in place by the cage-top pressing down on it.

Figure 6a,b. Gerbil-inhabited laboratory cages type IV with integrated artificial burrow system; top removed (Figure 6a) and top in place (Figure 6b).

The three modules can be separately cleaned or replaced. Their integration in a cage type IV does not affect stacking density.

Gerbils who grow up in this opaque, artificial burrow system develop significantly less stereotypic digging than gerbils who grow up in a transparent burrow (Waiblinger and König 2001). A drawback of letting the animals grow up in an opaque burrow is their pronounced shyness resulting from the dark, secluded living environment. On the other hand, the burrow makes it possible to remove the animals from the cage and easily transport them within the nestbox without disturbing them unduly. The animals should not be taken out of the nestbox, since this experience is likely to make them avoid the burrow system altogether thereafter! Lifting the lid of the nestbox to check on the animals without handling them has proved to be not that problematic. Patient training with positive reinforcement helps taming and training the animals for routine handling. The following daily procedure assures that gerbils can readily be picked up:

• By sliding the top 5-10 cm backwards, the cage opens just enough for a gerbil to stand up and nose about.  
• An animal emerges from the burrow, comes forward and inquisitively peeks out and sniffs (Figure 7).  
• The animal is tamed by offering preferred food.
• Once tame, the animal is gently lifted out of the cage, returned back to the cage and rewarded with sunflower seeds. This exercise is repeated on different occasions until the animal promptly comes forward and accepts being picked up and held in the hand for a short while.

Figure 7. Gerbils are inquisitive and will emerge from a burrow [artificial or self-dug] in the presence of a familiar person. Using positive reinforcement, they can easily be tamed and trained to allow being picked up and held in the cupped hand.

 

Gerbils are social animals. In the wild, group sizes range from 2 to 15 animals of all ages and sexes. Groups are founded by a breeding pair and extended by their offspring and other relatives (Bannikov, 1954; Naumov and Lobachev, 1975; Agren et al., 1989a). There is usually only one reproductively active female per group. Other adult, subordinate females are sexually suppressed (Norris and Adams, 1974; Payman and Swanson, 1980; Salo and French, 1989). The reproductive, dominant male is as involved in territorial defense and pup-care as the female, with the exception of retrieving pups (Waring and Perper, 1980; Weinandy and Gattermann, 1999). Older, already weaned offspring take the role of "alloparents" or "helpers" who participate in the care of younger siblings. By doing so they learn parental skills that will make them proficient parents. Breeding pairs with at least one alloparenting-experienced partner successfully raise more offspring than inexperienced pairs (Salo and French, 1989). Group members join forces in defending and marking the territory, constantly digging and extending the burrow, and hoarding food and nesting material (Agren et al. 1989a,b).

The highly social gerbils should never be kept alone! Group-housing is a simple way of effective environmental enhancement and welfare improvement. Breeding pairs can be kept together uninterruptedly without risk for their whole reproductive lifetime (Agren and Meyerson, 1977; Gerling and Yahr, 1978). The offspring should remain with the parents as long as there is enough space. They will not reproduce or show aggression under this housing condition (Ostermeyer and Elwood, 1984; Salo and French, 1989). A family group with two generations of offspring should never be kept in a [small] cage but in a large terrarium. Besides keeping gerbils in breeding pairs or family groups, same-sex groups are also possible. It doesn't matter whether males or females are kept together. Both social arrangements work well. It should be emphasized that adult gerbil males are not aggressive with each other, provided they know each other since they were juveniles.

The formation of a new group can be problematic. Our own experience shows that adult animals, especially females, are not likely to accept any strange partner(s) of either sex, except juveniles less than five weeks of age. We had best results with the formation of isosexual groups of eight-week-old animals who were introduced in cages with fresh bedding [neutral territory] and tubes for escape. We also noticed that powdering of the animals with a non-toxic, dry animal shampoo can help camouflage previous group odors thereby buffering territorial antagonism among the members of a newly established group. Careful observation of the animals during the first day after group formation helps to remove individuals who are persistent targets of overt aggression. Affiliative behaviors such as nose-to-nose contact, mutual grooming, sniffing each others' corners of the mouth, and nudging the head beneath each others' rump are signs that the new group is compatible.

Separating juvenile gerbils from their parents can also pose a problem. Despite our initial hypothesis that stereotypical bar-chewing may result from a lack of chewable nesting material or from reinforced bar-manipulation, caused by the close proximity of food pellets and cage bars in the food hopper, our experiments showed that neither of these factors has an influence on the development of this behavior pattern. Bar-chewing, however, significantly increases when juveniles are separated from their families before the next litter is born and housed in separate cages with fresh bedding material (Waiblinger and König, 1999; Waiblinger and König 2001). This suggests that juvenile gerbils should not be separated from their parents before a new litter is born and that at least part of the bedding in the new cage should originate from the parental cage.
 

Gerbils of both sexes have a strong urge to scent mark their territory (Roper and Polioudakis, 1977; Yahr, 1977; Agren et al., 1989b). They deposit sebum from the ventral scent gland on stable objects and surfaces by pressing their bellies against them and walking over them. Prominent objects and territory borders may also be marked by using fecal pellets and urine (Agren, 1976; Thiessen and Yahr, 1977). Gerbils recognize group members by their scent (Halpin, 1975). Females and males show strong xenophobic reactions towards unfamiliar animals or their scent marks by grinding their teeth and lashing their tails. If not separated in time, two strange gerbils will attack and possibly kill each other during the fight.

Cage cleaning is usually a very disturbing experience for gerbils, probably because the fresh bedding is void of any familiar scent marks. After cage cleaning, gerbils typically show acute stress reactions (Weinandy, 1995) and an enhanced susceptibility to diseases (Peckham et al., 1974; Bucklar, personal communication). They need a much longer heart-rate recovery time after cage cleaning than after handling or confrontation with an unfamiliar adult conspecific (Weinandy and Gattermann, 1995). Our own experience shows that these effects can be slightly buffered by leaving some soiled bedding [i.e., familiar odor] during the cage cleaning process.
 

To create adequate quarters for gerbils in research laboratories the following provisions must be made to address the animals' basic conditions for well-being:

• Species-adequate artificial burrow system;
• Social housing;
• Offsprings stay long enough in the family group to get experienced in the care of juveniles.

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Eva Waiblinger is a zoologist with a strong interest in ethology and animal welfare. She wrote her dissertation on laboratory gerbil welfare at the University of Zürich, Switzerland. She is currently working for the Swiss Animal Protection (Schweizer Tierschutz) in the area of companion animal information and services.

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