Comfortable Quarters for Rats
in Research Institutions

Monica M. Lawlor

Psychology Department, Royal Holloway
Egham, Surrey, TW2 OEX, United Kingdom


Introduction

Where rats are used for scientific study it is of prime importance to avoid cruelty and to curtail the suffering that might be associated with experimental procedures. The humane care of animals, however, ought to go beyond that to an active attempt to promote comfort and well-being.

Although rats have lived with man for millennia, people in general know very little about them. Rats have an "image problem": very few people like them and even fewer love them. Generally rats are seen as disease carrying vermin to be exterminated, rather than creatures to be cherished. When one adds to this the consideration that rats are phenomenally adaptive, with a remarkable ability to withstand efforts to eliminate them, it is not surprising that they are capable of growing, living, and breeding in conditions that are far from ideal. They are great survivors.

Some excellent books cover the proper care of laboratory rats, but their emphasis is more often on experimental procedures and the physiological aspects of care than on the quality of life of the animals who involuntarily make such a huge contribution - in terms of millions of animals "used" and "sacrificed" - to the advancement of biomedical science. In this chapter the emphasis is reversed, and the focus shifted on behavioral considerations rather than physiological data.

The currently recommended caging parameters for rats must be re-thought and revised if they cannot be shown to satisfy basic behavioral, physiological, and exercise needs of the animals who live in such enclosures. In order to decide the best way to satisfy those needs, it is logical to start by measuring the rat rather than the cage. A description of the rat's biology will make it clear why gross body weight should not be the only measure used to determine caging standards.

Species-typical characteristics

Rats are long-tailed, conspicuously inquisitive rodents who are biologically adapted to live in groups. The most commonly used laboratory rats are mutants of the Gray Norway rat (Rattus norvegicus). Selective breeding over hundreds of years made the mutant forms "domesticated," lacking the timidness and ferocity of their wild cousins. Their natural longevity is 3-4 years. Two types of red-eyed albinos are the prevailing laboratory rats: the relatively small, fine boned, and elegant Wistar rat; and the larger, relatively coarse boned and rough coated Sprague-Dawley rat. Pigmented, black-eyed rats are less common. The most widely used types are the Wistar Hooded and Lister Hooded rats. Their vision is far better than that of albino rats and they are less distressed by bright light.

 
Figure 1. Lister hooded male rats "challenging" each other in bipedal position.

Adult rats usually weigh between 200 and 1000 grams and measure 40 to 50 cm from nose to tail. When standing on all four feet, rats require extra horizontal space for their long, normally straight held tails. To assume a natural quadrupedal position an adult rat needs 35-48 cm. The bipedal position is very often seen in rats. It serves as an orienting stance in which the weight is on the back feet and the spine extended upward. The base of the tail is used as a stabilizing tripod (Figure 1). The forepaws may be supported on a firm vertical surface allowing the animal to stretch right up until he/she is standing on tiptoe. The head is up and the ears are pricked. The head room required for an adult animal to make the bipedal orienting stance is up to 30 cm (Figure 2).

 
Figure 2. Adult rat in full bipedal posture, showing the headroom required.

Rats are social animals who live in stable groups in which each animal has a well-defined relationship with each member of the group. When kept in single-cages, rats suffer from isolation stress, which, in turn,

The presence of another rat has a protective effect in fear-provoking and stressful situations (Latané and Glass, 1968; Latané, 1969; Taylor, 1981).

Adult rats often groom each other, and by doing so spread a group-specific scent on each other with saliva. They also spend a considerable part of their time grooming themselves. They do this in the squatting or standing position with tongue, teeth, paws, and claws. Ano-genital sniffing among same-sex animals is often a prelude to a dispute. At this point a fight may be averted if one animal rolls over onto his or her back and emits a submission call. If this happens, the dominant will stand over the other and do nothing more; both may even fall asleep in this position. If, however, neither animal gives way to the other, both will rear up and start to box with their forepaws while making attempts to bite each other. Any such encounter may be ended by a submissive posture and call. If this fails to happen, the animals may injure each other badly. Overt fighting can be a serious problem when rats are kept in barren cages, because the defeated animal cannot effectively stop the attacks of the aggressor by running away and escaping from the opponent's sensory field.

Parturient females have a strong need for nesting material and will readily work for the acquisition of it (Oley and Slotnick, 1970). Apart from females approaching parturition, rats do not show much purposive nest building activity. Preferred sleeping sites are "nest-like" but do not usually have much structure.

Mating is largely opportunistic. When in full oestrus the female becomes unusually active and accepts copulation from any sexually competent male. During coitus, ultrasonic calls are emitted by females, and males purr during mating. These vocalizations seem to serve to deter other group members from interfering with the mating couple.

The newborn not only need the attentive care of their mothers but also a distinctive nest to keep them warm and safe during the short periods when the mother leaves them. The mother carefully covers the pups when she leaves the nest. Sometimes, however, pups may explore the environment outside of the nest. Females - and to a lesser degree also males - have a strong drive to retrieve such pups even if the young are already too old to relish the attention.

When rats are comfortable and relaxed they normally nurse in the "half moon position." In less benign circumstances they nurse in a "cover position," standing over the pups in a protective manner (Figure 3). When there is sufficient nest material, and the ambient temperature is adequate, the mother will leave the nest between feeds. In more demanding conditions, when the temperature is too low, and there is insufficient nest material, she will remain crouched over the pups to keep them warm. If the caging system allows her to do so, the mother will "escape" from the increasingly mobile and demanding offspring once they are two or three weeks old; however, she will allow some sporadic nursing until the pups are four or five weeks old.

 
Figure 3. Lister hooded rat nursing her ten-day-old pups in "cover position."

Young rats play a lot. Play is a form of vigorous exercise that is essential for the well-being and normal social and sexual development of young rats. Adult animals play only very rarely with each other.

Rats are proficient diggers and build their own tunnels in which they usually sleep during the light phase of the day. They will sleep in a heap or separately depending on the ambient temperature. They are easily aroused and do not normally sleep for long periods without waking intervals. Mature animals settle down to sleep by tucking their heads between the forepaws while in the quadrupedal position. As they move into a deeper sleep phase, they suddenly keel over onto a side, typically at full length with tail extended. To assume this natural sleeping posture an adult rat requires a floor space of 15 x 35-45 cm. Rats sleep in a curled position only when chilled. When waking up, they stretch and yawn with fully opened mouth while the head is thrown back and the forelegs extended. When a rat stretches, one forepaw goes forward of the head while a back foot is stretched out beyond the tail base, the tail itself being arched; then the feet are reversed. Finally, the animal shakes itself.

Rats will progress by creeping when they are nervous, insecure, or alarmed. In this mode of locomotion the animal flattens his/her belly against the floor and shoves the trunk forward by "paddling" with laterally extended limbs. When a rat is walking normally, the tail is carried off the ground straight out behind the trunk. Any other pattern is distorted and indicative that the animal is kept in a too small enclosure. The running pace is probably three or four times longer than the walking pace. When a rat is running, the tail is carried straight out behind the body with the tip of the tail upturned. Juveniles and females in estrus spend conspicuous amounts of time running around. Rats can achieve considerable speed, especially in a panic-stricken dash. When merely exploring a new place they lope along at a much more modest speed.

The motivation to forage is very strong in rats, and they will readily work for the retrieval of food in the presence of freely accessible identical food (Neuringer, 1969; Carder and Berkowitz, 1970; Hothersall et al., 1973). They will eat powdered or mushy food from a dish or from the floor, but their species-specific habit is to secure a piece of food in their teeth and carry it to a suitable spot where they adopt a squatting posture and transfer the food to the forepaws. Holding the food in their paws, they nibble gently at it; if they do not like the taste they drop it immediately. The opportunity to gnaw is an essential physiological and behavioral need for rats. If they are not given the chance to regularly gnaw, their front teeth overgrow and make it more or less impossible for them to eat at all or to engage in grooming.

Rats have a spontaneous fear of people and avoid being handled. Handling can be a powerful stressor for them (Brown and Martin, 1974; Kvetnansky, 1978; Berkey et al., 1990; Briese and Cabanac, 1991) introducing uncontrolled variability into research data (Shyu et al., 1987; Brockway et al., 1993; Claassen, 1994). Rats are also very sensitive to environmental disturbances. Even ordinary animal husbandry procedures such as moving a cage to a different area or moving animals to a clean cage can induce transient, but significant, physiological, and behavioral changes that may confound experiments conducted shortly thereafter (Gärtner et al., 1980; York and Regan, 1982; Saibaba et al., 1996; Duke et al., 2001).

The natural defense of rats who experience threat is not to hang about but to run and hide, and if possible huddle with other conspecifics in a safe place. Being placed on an open surface is an especially threatening situation (cf., Latané, 1969).
 

Minimum recommendations for rat-adequate housing and handling conditions

The ethogram provides a base from which the behavioral needs of a species can be derived and that allows one to make recommendations regarding the minimum space and caging conditions required by the animals to satisfy those needs and experience a state of behavioral and physical well-being.

The cage in which a mature rat can adopt species-typical postures and stances and can carry out essential activities has to measure between 35 x 25 x 18 [height] cm for the smallest females and 50 x 30 x 30 [height] cm for the largest males. Table 1 lists the minimum space requirements by sex and body weight. It must be emphasized that young animals require more space, relatively, for play activities. Therefore, they should not be allocated less space than is appropriate for the smallest females [35 x 25 x 18 cm].

 BODY WEIGHT OF RAT
[1g = 0.035 Oz.]

MINIMUM FLOOR AREA cm2 (in2)

 MINIMUM HEIGHT
cm (in)

 for 1-3 rats

 for an additional rat

male up to 150 g
female up to 140 g

  900 (140)

  300 (47)

 18 (7)

 male 150-250 g
female 140-170 g

 1200 (186)

  450 (70)

 20 (8)

 male 250-450 g
female 170-310 g

 1500 (233)

  600 (93)

 22 (9)

 male 450-900 g
female 310-615 g

 1800 (279)

  800 (124)

 26 (10)

 male over 900 g
female over 615 g

 1800 (279)

 1000 (155)

 30 (12)

Table 1. Minimum space recommendations for laboratory rats

Rats of any age should not be caged singly or in large groups. For adults the group should not be more than six animals, for juveniles not more than ten animals. Rats kept in larger groups tend to be too aggressive and are more prone to disease. Pair-housing is probably the optimal alternative both to single-housing and to group-housing (Heath, 1999). Separation from conspecifics is a distressing situation for rats leading to significant physiological alterations (Ehlers et al., 1993; Young et al., 1996; Lawson and Churchill, 2000). Individually caged animals are susceptible to stress (Hurst et al., 1997), which again jeopardizes the validity of research data collected from such animals (Pérez et al., 1997). Rats show more pronounced stress-like changes in behavior and cardiovascular function during common husbandry and experimental procedures when they are housed alone than when housed with another rat (Zammit et al., 2001). If an animal has to be single-caged for veterinary reasons provision must be made that he/she can keep visual and auditory contact with other rats to buffer the stress associated with isolation (cf., Latané and Glass, 1968).

The main food staple for laboratory rats is ordinarily a commercial high quality pelleted diet fed ad libitum. Hard pellets usually provide for sufficient gnawing. Natural food items, however, such as carrots, grain/seeds, and/or pieces of soft wood, are more species-appropriate items for gnawing. Rats should always have free access to them. Wooden gnawing blocks are attractive enrichment objects (Chmiel and Noonan, 1996) that not only reduce the incidence of stereotypic chewing of metal cage bars (Orok-Edem and Key, 1994) and make the animals less timid (Eskola and Kaliste-Korhonen, 1998) but are available with certificates of analysis, a particularly important aspect for toxicological studies (Robertson, 1999). Rats "want" to forage (cf., Neuringer, 1969), and they can easily be induced to "work" for their food by soldering metal plates over their food hoppers, so that only a small segment of the original area remains available. This method of "food restriction" is preferable to giving less food to avoid obesity. Rather than rapidly eating a reduced ration and feeling hungry for long periods, the animals work harder for their food, which enables them to burn more calories and eat throughout the day. This reduces the incidence of obesity and its associated disorders and also encourages more "natural" behavior patterns, both of which improve welfare (Wrightson and Dickson, 1999).

In order to provide rats a sense of security and options of breaking visual contact with each other during agonistic conflicts, it is recommended to add vertical barriers (cf., Anzaldo et al., 1994) and/or tubes - made of PVC or aspen wood (Mering, 2000) - in their cages. This offers the animals additional wall contact, tactile comfort, escape routes, and areas for exploration, thereby increasing cage complexity and the usable floor space of the cage. Evidence suggests that a more complex housing environment - in sharp contrast to the barren cage - buffers anxiety responses to potential stressors (Levine, 1985). A well-designed cage provides a distinctive sheltered nest area away from the feeding location. Rats with access to an appropriate shelter are more explorative and less timid than those in barren cages (Townsend, 1997). Nest-boxes of opaque or semi-opaque materials are particularly suitable shelters (Manser et al., 1998). Ideally, rats should always have access to one cage section that is covered with a black perspex screen serving as dark-and-sheltered sleeping and hiding area and another section serving as living area (Figures 4; cf., Dickson and Wrightson, 1999). The living area section should be covered with a wire lid for gymnastics.

 
Figure 4. The ideal double-cage arrangement: The right cage section is covered with a black perspex screen and serves as dark-and-sheltered sleeping/nesting area. The left cage section serves as living area, which is covered with a wire lid for gymnastics.

It should go without saying that solid floors are much more appropriate for the feet of rats than wire floors, which impact the feet in a biologically abnormal manner (Grover-Johnson and Spencer, 1981) and may cause discomfort, pressure sores, and pain. They may also cause chilling even in a warm room. While rats housed on grid or mesh floors tend to pile up in heaps when resting, rats with access to solid flooring spread out on the bedding (Dickson and Wrightson, 1999). Under experimental conditions, rats are prepared to make considerable efforts to reach a solid floor when they wish to rest. Preference testing revealed that the animals chose to dwell on solid floors rather than grids, regardless of previous housing experience. Thus there is ground for suggesting that laboratory rats be housed on solid rather then grid floors, because solid floor housing improves their welfare (Manser et al., 1995; Manser et al., 1996; Stauffacher, 1996). The Guide for the Care and Use of Laboratory Animals aptly recommends “solid-bottom caging, with [emphasis added] bedding ... for rodents” (National Research Council, 1996, p. 24). As a warning however, it must be pointed out that on solid PVC (polyvinyl chloride) floors the claws of rats may "over-grow" because the surface is not abrasive. If this happens, the claws must be clipped or the animal will experience considerable discomfort. The widely used concept of housing rats on one type of cage flooring should be abandoned and replaced by a cage concept with different types of flooring - and bedding - to enable the animals to express a more complete behavioral repertoire (van de Weerd et al., 1996; Dickson and Wrightson, 1999).

It is impossible to fully satisfy the instinctive need for digging in caged rats. Dust-free woodchip (preferably irradiated) bedding is a good compromise solution that allows the animals not only to engage in quasi-digging maneuvers but also to forage, i.e., search for food particles. Woodchips - unlike sawdust - and corn-cob are the preferred bedding for rats (Blom et al., 1995; Patterson-Kane et al., 2001) and should be regarded as a basic, inexpensive means of environmental enrichment. The bedding also absorbs urine and moisture from feces. Regularly changed bedding is the best guarantee of a hygienic cage environment.

Parturient females must have access to shredded paper or soft wood to build appropriate nests for the successful rearing of their offspring (Figure 5). The use of shredded paper, which incidentally makes a nest similar to that of the wild rat, allows the female with her young to burrow and insulate themselves from disturbing environmental factors, thus enhancing the mother's feeling of security. Access to shredded paper can drastically reduce infant mortality (Nolen and Alexander, 1966).

 
Figure 5. A three-day-old litter of Lister hooded rat pups. The well-structured nest is built with shredded dye-free paper.

All rats of a given experiment/test have to be caged at the same distance from the light source to assure uniform illumination. Traditionally, however, rat cages are arranged in racks with several rows being stacked on top of each other. This allows for maximal usage of room space but bears serious implications for scientifically valid research. Since the cages are arranged at different levels and most of them are located in the shade area cast by upper rows, it is impossible to guarantee uniform illumination in all cages. Light intensities in stacked cages vary substantially (Bellhorn, 1980; Clough, 1982). This introduces an uncontrolled variable into research data and unnecessarily increases the number of animals needed to obtain statistically significant results. The different degrees of illumination may be one of the explanations for variations in experimental results (Lockard, 1962; Weihe et al., 1969) and the need for unduly high numbers of animals to obtain statistically significant research findings. Obviously, the multi-tier caging systems are not compatible with scientifically sound research methodology.

Rats can lose their shyness of people if a little time is spent handling them as juveniles. An effective technique is to put half a dozen three to four weeks old cage mates into a bucket - using cupped hands to make the transfer from the cage - and then putting your hand into the bucket and allowing the young to explore it thoroughly. By putting your hand over and under the animals, they get used to the contact and can, in a day or two, be easily lifted a few inches in one hand. The bucket prevents the juveniles from avoiding the hand, thereby conditioning them to accept the human hand as a neutral, non-threatening environment. It is only necessary to do this exercise a few times over five days to "create" a rat who will readily accept proper handling throughout his or her lifetime (Figure 6). This simple technique - which is equally effective for mice and hamsters - is relatively labor-intensive, but it is reliable and makes life a great deal easier for both the rat and the handler. Gentle handling during infancy makes rats less fearful, quasi-tame in situations in which control rats remain timidly crouched at the back of the cage (Wells, 1985).

 
Figure 6. Once a rat is conditioned as a juvenile to accept the human hand as a neutral, non-threatening environment, he or she will readily accept proper handling throughout his or her lifetime.

In an ideal world all rodents are best handled by being picked up with a firm-and-gentle hold over the shoulders and quickly supported by allowing their feet to rest on your other hand or sleeve. To a considerable extent proper handling depends on the handler rather than on the animal subject. Nervous people make animals [and other people] nervous and consequently unpredictable in their reactions to handling. Inexperienced personnel often grip too hard, thereby stressing not only the handled animal but also other rats who witness this disturbing situation (Fuchs et al., 1987). It is unusual for laboratory rats to object to being picked up in shoulder hold. Small rats fit comfortably into the hand when lifted and may be held in one hand if the tail is anchored between the third and fourth finger, and the thumb kept under the jaw. Bigger rats need the support of the second hand or a sleeve to make them feel secure when lifted.

Conclusion

Even though U. S. federal regulations currently do not regard rats as "animals" (United States Department of Agriculture, 1995), rats require and deserve the same professional care as other, perhaps more charismatic laboratory animals, because their well-being determines no less the quality and reliability of scientific research data collected from them. In order to design a species-appropriate and scientifically sound housing protocol it is essential to view the world with the eyes of a rat. It is through patient observation and a grain of humbleness that such a view can be cultivated.


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Monica Lawlor is a psychologist specialized in ethology. She maintained a rat colony for thirty years and conducted ground-breaking research in species-adequate housing arrangements and humane handling practices of rats kept in research laboratories. Monica Lawlor lives in London, United Kingdom, where she retired from the Psychology Department at Royal Holloway & Bedford New College.


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