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Variables are uncontrolled factors that have the potential to influence research data.
Refinement is the attempt to enhance animal welfare by reducing to an absolute minimum the amount of distress imposed (Russell and Birch, 1959) and control extraneous variables that may increase research data variability (Reinhardt and Reinhardt, 2002) and, hence, increase the number of research subjects needed to achieve statistically significant results (National Research Council, 1985; Home Office, 1989; Institute for Laboratory Animal Research, 1992).
2.1. Confinement in Barren Cages
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Being confined in a barren primary enclosure is probably the most serious stressor for animals, because it isolates them from their surroundings (Figure 1). For rodents and rabbits, empty space is biologically extremely frightening as it exposes them to the eyes of potential predators. But there is an absolute minimum empty space that the caged animals need, otherwise there is a risk that their physiological equilibrium will become disturbed.
Kuhnen (1998,1999) compared the fever response a non-specific immune response to the administration of Salmonella typhosa lipopolysaccharide in golden hamsters, who were individually housed either in small unfurnished cages (200 cm2) or in large unfurnished cages (1815 cm2). Subjects in the small cages showed a significantly reduced febrile response relative to those in the large cages. This was taken as an indicator that housing in too small cages induced chronic stress. Fullwood et al. (1998) examined trios of male C57BL/6 mice housed in barren cages that were very small (32 cm2/mouse), small (64 cm2/mouse), meeting official space recommendations (97 cm2/mouse) or relatively large (129 cm2/mouse). Mice in the very small cage had a significantly greater lymphocyte blastogenesis than all the other mice, who showed no significant differences in this particular stress parameter. Both adrenal weights and plasma corticosterone concentrations, however, increased significantly with reduced cage size.
Beyond a minimum required for freely engaging in species-typical postures and postural adjustments (e.g., outstretched lying posture, turning) and species-typical locomotion patterns (e.g., hopping, leaping, climbing) space has little value unless:
Being confined in an unstructured primary enclosure is often associated with:
Numerous attempts have been made to ameliorate these problems by furnishing the animals' impoverished living quarters with species-appropriate structures, offering the animals objects to interact with in non-injurious ways, and providing them with compatible companionship.
2.1.2.1. Reduction of Stress, Distress, Fear and Anxiety
Stress is an effect produced by acute events, referred to as stressors, that induces an alteration in the subject's biologic equilibrium.
Distress is a subjective state that results from the inability to adapt to a chronic stressor or to a repeatedly occurring stressor. The inability to adapt is reflected in aversive gestures and behavioral pathologies and in significant physiological deviations from the undisturbed state.
Anxiety is a subjective state triggered by an unknown potential danger. For example, an animal who has repeatedly been exposed to a painful, life-threatening situation by various investigators might retreat to the back of his/her homecage whenever a person enters the room.
Fear is a subjective state triggered by a known danger. For example, an animal who has repeatedly been exposed to a life-threatening procedure by a certain investigator squeals and crouches in the back of his/her homecage when this investigator enters the room and approaches the cage.
Gardiner and Bennett (1978) measured the blood pressure of Wistar rats who were kept either alone in a barren cage or as pairs in two interconnected barren cages. While all five single rats developed hypertension after five days, none of the five paired rats became hypertensive during this time period. This suggests that the presence of a companion mitigated the stress response (social buffer) to a rat-inadequate living environment.
Cambardella et al. (1994) noticed that individually caged Sprague-Dawley rats experienced more stress, as reflected in significantly higher concentrations of plasma corticosterone and prolactin, than subjects housed six per cage (Figure 2). Sharp et al. (2002, 2003) recorded the behavior and the telemetrically measured heart rate of Sprague-Dawley rats housed either alone or in groups of four same-sex partners in 930-cm2 standard cages. In both sexes resting heart rates were lower in group-housed rats. Moreover, group-housed rats had significantly lower heart rates in response to disturbing husbandry procedures (e.g., cage change). Husbandry-induced arousal behaviors were less frequent and of shorter duration in group- than in singe-housed rats.
Baldwin et al. (1995) kept Sprague-Dawley rats in 1200-cm2 unfurnished cages alone or as a group of five. Basal corticosterone levels and lymphocyte percentages were significantly higher in single-housed rats than in group-housed rats, regardless of the fact that single rats had access to a relatively much larger, albeit unstructured, living space. Unlike space, social companionship seemingly buffered the stress attendant with confinement. The lower stress level in group-housed rats may account for the observation that the incidence of pituitary tumors is conspicuously lower in animals housed in small groups than in those housed alone (Nyska et al., 1998).
Brown and Grunberg (1995) showed in female Wistar rats that the high corticosterone level associated with small living quarters is significantly reduced by the presence of other females, even if this implies crowding (four animals in a 405-cm2 cage). This effect could not be verified in males who, unlike females, are more stressed (higher corticosterone concentration) under crowded than under single-housing conditions. Social deprivation, therefore, is a particularly potent stressor for female rats.
Shaw and Gallagher (1984) noticed in Sprague-Dawley rats that animals housed in same-sex groups of five in 1820-cm2 cages had a significantly longer survival rate under undisturbed conditions than individuals kept alone in 600-cm2 cages. Male rats were prone to develop foot lesions resulting from the wire mesh floors. The incidence of such lesions was markedly lower in group-housed than in singly housed rats, presumably because of the potential for more exercise in the large group-cages.
Conger (1957) placed Hooded Norway rats in a distressing conflict situation and noted that pair-housed animals were protected from stress, showing a greater resistance to the development of gastric ulcers than singly housed rats. Patterson-Kane et al. (1999) recorded significantly higher latencies to emerge from a box and explore a novel environment in Hooded Norway rats who were housed alone compared to those housed in pairs. From this it was inferred that the pair-housed rats were less fearful.
Heath (1999) observed rats of unspecified strain who were caged either alone or with another rat. While singly caged individuals spent most of their time in the back of the cage, pairs spent most of their time in the front of the cage, suggesting that they experienced less fear in the presence of the observer.
Davitz and Mason (1955) subjected Wistar rats to an open-field situation and noted that the behavioral fear response of the subjects was significantly less intense when they were tested together with another rat versus alone. Latané (1968) and Taylor (1981) extended these studies and demonstrated that:
Green et al. (2002) implanted male Sprague-Dawley rats with jugular catheters that allowed the animals to self-administer the psychoactive drug amphetamine. Animals living alone in barren 410-cm2 standard cages self-administered significantly more of the drug than animals who shared a 7200-cm2 cage with nine other males. The combined effect of companionship and more space may have decreased the urge for the presumably hedonic effect of the drug in an environment that was more species-appropriate than the boring single-cage.
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| Figure 2 Compatible social companions and species-appropriate environmental enrichment can buffer the stress resulting from permanent confinement in artificial living quarters. (Photo by Maureen Hargaden, Hoffmann-La Roche Inc., Nutley, NJ, USA. ©Roche Laboratories Inc., 2005) |
Sharp et al. (2003) monitored telemetrically heart rate and blood pressure in undisturbed male Sprague-Dawley rats who were housed with three other rats in small (920 cm2) unfurnished cages or in large (1250 cm2) unfurnished cages. The parameters measured did not differ between the two test groups, showing that additional unstructured space is unlikely to reduce potential stress resulting from confinement. Hirsjärvi (1994), however, found in pairs of male Wistar rats that the subjects were less fearful as measured by shortened latency to rear when the cage top is removed when the floor area of their standard cages (800 cm2) was enlarged by 16 percent (to 930 cm2) and the height of their cages (15 cm) increased by 33 percent (to 20 cm).
Belz et al. (2003) examined the effects of environmental enrichment on stress-sensitive hormones in vein-cannulated individually caged Sprague-Dawley rats. The cages of control animals were barren while those of the test animals were enriched with rubber toys and squares of compressed cotton fiber (nestlets), two items that were used for gnawing and shredding. Rats with enrichment had significantly lower plasma adrenocorticotropin and corticosterone concentrations than those with no enrichment. Foulkes (2004) kept Wistar rats in single-cages that were barren or enriched with a polyvinyl chloride (PVC) tube for hiding. Chromodacryorrhea, a rat-specific stress indicator (Mason et al., 2004), was significantly less pronounced in the enriched as compared with the non-enriched rats, suggesting that the animals were less stressed when they had access to a tube.
Sharp et al. (2005) measured heart rate and blood pressure of SH (spontaneously hypertensive) and Sprague-Dawley rats housed individually in 930-cm2 cages that were barren or enriched with a simulated burrow, a feeding enrichment gadget, and a shredding-and-nesting item. Blood pressure was not affected by enrichment. Heart rate, however, was significantly lower in SH rats who lived in enriched vs. barren cages. This effect could not be confirmed in Sprague-Dawley rats.
Eskola and Kaliste-Korhonen (1998) furnished the home cages of trio-housed Wistar rats each with three blocks of aspen wood. The animals gnawed these blocks into small pieces and did not lose interest in them over time. They ate less, as reflected in significantly lower weight gains, were more active, and exhibited the alert-posture less frequently when tested in an unfamiliar open-field area than control animals kept in barren cages of the same size. It was concluded that access to the gnawing blocks made the rats less timid.
Klein et al. (1994) exposed isosexually group-housed Sprague-Dawley rats six animals per group to cat urine. Animals living in cages that were enriched with eight daily changed, unspecified toys showed significantly fewer freeze-and-concealment responses to this natural predator stressor than animals in barren cages of the same size. This was evident in both sexes and taken as an indicator that toys mitigated the stress response and lowered the level of anxiety in the subjects.
Mlynarik et al. (2004) compared the corticosterone response induced by repeated injections of Escherichia coli lipopolysaccharide in Wistar rats, who were kept either in groups of three in small, barren cages (1200 cm2) or in groups of ten in large cages (5000 cm2) enriched with several platforms, swings, tunnels, glass jars, branches, running wheels, cardboard boxes and a water pool. Lipopolysaccharide injections resulted in plasma and adrenal corticosterone levels that were significantly increased in the first but not in the second group of animals. Living in a larger and more complex environment may have made the second group of animals more resistant to the immune challenge.
Ader et al. (1991) assessed emotionality in NOD (non-obese diabetic) mice who were housed either alone or in groups of five or eight same-sex animals in barren standard cages. Individually caged mice of both sexes exhibited significantly more fear and anxiety as determined by resistance to being picked up, vocalizing, struggling, spontaneous urination and defecation than mice caged in the company of other mice.
Einstein et al. (2000) used a telemetry system to monitor the heart rate of male BALB/c mice over an 18-day period. The animals were housed alone or as trios in unfurnished cages. Throughout the experimental period, individually caged mice had significantly higher heart rates than the group-housed mice, suggesting that they were more distressed. Späni et al. (2003) confirmed these findings in male outbred mice who were kept alone or as pairs with ovarectomized females in same-sized cages furnished with hay and paper towels. Single mice had significantly higher heart rates than their nine pair-housed counterparts. This is in line with Herreid and Schlenker's (1980) observation of male RR mice who had significantly higher metabolic and heart rates when they were kept alone than when they shared a cage with another mouse. Späni et al. (2003) also noticed striking differences in the subjects' sleep patterns, with singly housed mice showing more frequent, short phases of sleeping than pair-housed mice.
That the welfare of singly housed mice is impaired relative to those living with companions has also been emphasized by Andervont (1944), because C3H mice housed in groups of eight were less susceptible to developing mammary tumors than mice housed alone.
Chamove (1989) kept groups of six CLFP mice in same-sized cages that were either unstructured or furnished with several vertical partitions with passage holes, structuring the cage floor into a complex burrow-like system. Animals from the burrow-cage were less inclined to escape when the cage top was removed, and they deposited significantly fewer fecal boluses when tested in an open field, than control subjects from the unstructured cage. This was taken as a sign that the provision of vertical cage dividers reduced the stress associated with confinement and made the animals less fearful. Van Loo et al. (2004) measured significantly decreased urinary corticosterone concentrations in BALB/c mice kept in trios, when their cages were enriched with paper tissues that allowed individuals to break visual contact with each other and hide from disturbing environmental influences.
Kingston and Hoffman-Goetz (1996) noticed in C57BL/6 mice living alone or in groups of eight in barren cages that enrichment in the form of a running wheel, nesting material, small bottles, and tubes significantly buffered the animals' immunosuppression at times of stress. Benaroya-Milshtein et al. (2004) demonstrated in single- and group-caged CrH/eB mice that the provision of ladders, tunnels and a running wheel mitigated immune responses to acute stress and significantly reduced anxiety- and fear-related behavioral responses in an unfamiliar environment.
Manosevitz (1970) tested random-bred mice in an empty arena. Animals raised with their littermates in large cages (5350 cm2), enriched with a variety of structures for climbing and hiding, defecated less often than animals raised in small, barren standard cages (500 cm2). Similar findings were reported for outbred mice as well for BALB/c and C57BL/6 mice (Scharmann, 1994; Chapillon et al., 1999), suggesting that anxiety and fear can be buffered by inanimate environmental enrichment.
Sherwin and Olsson (2004) offered C57BL/6 mice free access to a solution of a psychoactive anxiolytic (midazolam) and compared trios housed in unfurnished 540-cm2 cages with trios kept in more spacious 1090-cm2 cages that were enriched with a nest box, a running wheel, cardboard tubes and nesting material. The enriched mice drank significantly less of the midazolam solution than the non-enriched mice, suggesting that more space along with enrichment made the animals less anxious. Van de Weerd et al. (2002) came to a similar conclusion when assessing fear-related behaviors of RIVM mice kept in groups of eight in small barren cages (375 cm2) or in large cages (840 m2) furnished with gnawing blocks, climbing structures, a nest box and wood-wool.
Roy et al. (2001) assessed plasma corticosterone concentrations of BALB/c mice who had been exposed to cat feces in their familiar home environment. The subjects were raised with their littermates either in large (1300 cm2) cages each containing a running wheel and unspecified enrichment objects or in small (260 cm2) unfurnished standard cages. Mice from large enriched cages showed a significantly lower corticosterone stress response to the natural stressor than mice from small, unenriched cages.
Manosevitz and Pryor (1975) demonstrated in C57BL/6 mice that space alone can have a stress-buffering effect. The test animals were all kept individually in barren cages that were either small (440 cm2) or large (5350 cm2). When exposed to an unprotected open field, mice from large cages demonstrated significantly fewer signs of anxiety and fear (i.e., less defecation and more exploration) than the mice from small cages.
McMahon et al. (2005) determined that breeding trios of C57BL/6 mice had a much higher birth rate (9.6 pups/female) in presumably unfurnished, relatively large 860 cm2 cages than in small 375 cm2 standard cages (7.2 pups/female). Keeping the mice in larger cages also provided a better microenvironment as measured in lower ammonia levels (17 ppm vs. 24 ppm).
2.1.2.2. Alleviation of Maladaptive Behaviors
Maladaptive behaviors reflect the inability of a subject to adapt to species-inappropriate living conditions. Maladaptive behaviors are generally referred to as abnormal behaviors. This misleading term is avoided here, since it is the species-inappropriate conditions under which the subject is forced to exist that are really abnormal, not the attempts to adapt to them.
Baenninger (1967) compared the behavior of singly caged rats of unspecified strain with that of rats kept in groups of six. Both categories of animals were housed in 1740-cm2 cages and their behavior was monitored from the day of weaning (21 days old) until the age of 92 days. Stereotypic pawing and tail-manipulating developed under both housing conditions, but their occurrence was significantly lower in the group-housed rats.
Callard et al. (2000) videotaped isosexually pair-housed roof rats in 1900-cm2 cages that were either empty or furnished with 13 x 25 x 20-cm wooden shelters. The rats engaged in stereotypic backflipping under both conditions, but the incidence and frequency of this behavior were significantly lower when they had a shelter.
Wrightson and Dickson (1999) and Van Berkum (2000) controlled the risk of excessive eating resulting from boredom in group-housed Sprague-Dawley rats by inserting metal plates over the food hoppers, so that only a small section of the original food access area remained available. The animals worked harder for their food, which made them burn more calories and allowed them to eat throughout the day. After an eight-month test period they were slim, healthy and significantly lighter than control rats with unrestricted food hoppers (Wrightson and Dickson, 1999).
DeLuca (1997) assessed alopecia resulting from partner-directed hair-pulling ("barbering") in groups of ten mice of unspecified strain. The animals were kept in cages of unspecified size that were either barren or furnished with nest boxes and various commercial, regularly replaced toys. The incidence of hair loss due to barbering was about 60 percent in unenriched mice, but only 23 percent in enriched mice.
Leach et al. (2000) tested a custom-made cage insert consisting of two raised platforms and a shelter. Videorecordings of pair-housed BALB/c mice revealed that the animals showed significantly less bar-gnawing with the insert. Würbel et al. (1998) made a similar observation in pairs of male ICR mice. The provision of cover, in the form of a cardboard tube, in an otherwise barren, hence potentially fear-inducing cage significantly reduced, but did not eradicate stereotypic wire-gnawing.
Powell et al. (2000) studied the behavior of deer mice (Peromyscus maniculatus bairdii) reared (a) in same-sex pairs or trios in barren standard cages or (b) in a same-sex group of 16 animals in a cage that was more than 500 times larger and furnished with wire mesh cylinders, PVC tubes, and dividers. Stereotypic jumping, backward somersaulting, and patterned running dominated the behavioral repertoire of the animals in the small barren cages (Powell et al. 1999). Given the constraint of confinement, these stereotypies developed even in the very large and complex enclosure, but their occurrence was significantly lower than in the small, unfurnished cages (Powell et al., 1999, 2000).
Wiedenmayer (1997) developed an artificial burrow system for gerbils, who are notorious for their stereotypic digging. The burrow consists of an opaque nestbox with an angled access tube. Providing the cages of gerbil families with this burrow system almost entirely prevented the development of stereotypic digging in young animals (Wiedenmayer, 1997) and reduced its occurrence significantly in adults (Waiblinger and König, 2004).
McClure and Thomson (1992) noticed that golden hamsters, kept individually in suspended wire cages, developed bizarre aggressive behavior. They spent much time growling, hissing, posturing aggressively toward humans, destroying the rubber stoppers of their water bottles, and attacking any objects introduced into the cage. Many animals developed inappetence which progressed to anorexia, depression, and unresponsiveness. When the hamsters were given cotton nestlets their appetite and responsiveness improved, but their aggressive behavior remained unchanged. When the nestlets were replaced by a PVC pipe section (13 cm long, 5.5-cm diameter) functioning as a burrow, the aggressive behavior diminished within three days and was no longer discernible after 14 days.
Krohn et al. (1999) adjusted the routine feeding time of singly caged Ssc:CPH rabbits from four hours after daybreak to two hours before nightfall, which is about the natural time of day when wild rabbits would forage and eat. This simple change in feeding time resulted in a significant reduction of stereotypic activities from approximately 4.5 to 0.5 percent of the time during the night (dark phase of the artificial light cycle). The rabbits displayed stereotypies during the day (light phase of the artificial light cycle) only 0.5 percent of the time. The change in the feeding schedule did not diminish the occurrence of stereotypic behavior even further.
Brummer (1975) noticed hair-pulling and -eating as a common behavioral problem in a colony of New Zealand White (NZW) rabbits housed in barren cages. When 14 does were provided with straw, they stopped pulling their hair, and the development of this behavioral disorder was prevented in all their offspring. Unfortunately, this clinical study is not supported by data.
Potter and Borkowski (1998) diagnosed three NZW rabbits, housed singly in barren cages, with psychogenic polydipsia (over-drinking). Placing hay and unspecified toys into the cages resulted in a conspicuous decrease in the manifestation of this maladaptive behavior in all three cases.
Lidfors (1997) offered NZW rabbits housed individually in barren cages (a) a plastic bottle filled with 20 g hay twice a day or (b) a regularly replaced aspen gnawing stick. It was a relatively lengthy task for the rabbits to pull hay out of the bottle, but this opportunity to spend extra time foraging correlated with a significant decrease in the occurrence of excessive fur-licking, sham chewing and bar-gnawing. The gnawing stick was chosen rarely and did not alter the rabbits' engagement in these behavior patterns. No changes in maladaptive behaviors were registered when the animals were given access to a wooden or plastic box. The rabbits hardly ever made use of such a potential shelter (Lidfors, 1997; Berthelsen and Hansen, 1999, Hansen and Berthelsen, 2000). Berthelsen and Hansen (1999) daily replenished hay on top of the barren cages of individually housed NZW x French Lop rabbits. This decreased, albeit insignificantly, the percentage of time that the animals spent bar-gnawing from approximately nine to four percent.
Moore and Beeston (1999) developed an unconventional cage for singly housed rabbits. Its front protrudes, giving the subjects improved vision out of the cage; a shelf in the back of the cage offers a comfortable resting site and a covered retreat area underneath; and a hay rack serves as a feeding enrichment device. The benefit of this cage design for the animals has yet to be demonstrated.
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Edgar (2004) furnished the cages of individually housed NZW rabbits with an interior mirror (Figure 3). Seven-day videorecordings revealed that this reduced the time spent engaged in bar-gnawing from 1.2 to 0.2 percent in males and from 0.3 to 0.2 percent in females.
Gunn-Dore (1999) provisioned singly caged NZW rabbits with brass wire balls (5-cm diameter) that the animals could chew and push around. During a three-week test period female and male rabbits progressively lost interest in these toys, but spent significantly less time engaged in stereotypic hair-chewing and bar-gnawing.
Stauffacher (2000) compared the behavior of female Belgian Hare rabbits who were kept alone or with another female. Stereotypic wire-gnawing and wire-licking occurred under both housing conditions, but the incidence of these behaviors was significantly lower in the pair- than in the single-housed animals. Chu et al. (2004) raised female NZW rabbits in barren standard cages or as pairs in double-size cages. The animals showed bar-gnawing in both housing conditions, but while the incidence of this behavioral pathology remained relatively low in paired rabbits, it showed a significant and progressive increase in single rabbits. Kalagassy et al. (1999) confirmed in Flemish Giant x Giant Chinchilla rabbits that stereotypic behaviors occur only in animals who are housed alone, but not in those housed with a companion.
Gunn and Morton (1994) observed female NZW rabbits who were caged individually in small barren quarters or as a group of nine in a large, enriched enclosure. While each of the singly caged does showed wire-gnawing about 13 percent of the time, stereotypic behaviors were not observed in the socially housed does, who had access to straw and cardboard boxes. It is not clear which of these environmental differences accounted for the different manifestation of maladaptive behavior.
Held et al. (2001) studied groups of four adult female NZW rabbits in pens furnished with cardboard boxes for hiding and gnawing and raised platforms for rearing up, jumping up, and resting in an elevated position. In addition, the animals had access to straw and hay for burrowing and foraging. The subjects were watched for a total of 189 hours over a period of 2.5 years, but no stereotypic behaviors were ever observed. The absence of stereotypic behavior in group-housed does has been emphasized by Loeffler et al. (1991). Krohn et al. (1999), however, did observe some stereotypies at night, but not during the day. This suggests that group-housed rabbits may well engage in stereotypic behaviors during the night when they are active, but nobody is observing them.
2.1.2.3. Reduction of Aggression
Gwinn et al. (1999) provisioned groups of five male Swiss nude mice with nestlets and noticed a 31 percent decrease in fight injuries compared to controls. Armstrong et al. (1998) compared aggression among groups of eight male BALB/c mice in same-sized cages that either had standard bedding or were enriched with natural cornhusk covering the cage floor to a depth of 2.5 cm. Mice maintained on the cornhusk had significantly fewer wounds than those on standard bedding, probably because the husks encouraged burrowing and nesting and hence the option of moving out of the immediate vicinity of cagemates.
Arnold and Westbrook (1997/1998) observed same-sex groups of four golden hamsters in cages that were either barren or furnished with a T-shaped PVC pipe or with a pint-sized clear glass jar. Hamsters with access to these objects displayed only 34 and 22 percent, respectively, of the aggression displayed by hamsters kept in barren cages.
Arnold and Gillaspy (1994) kept female and male golden hamsters alone or as a group of four same-sex siblings in barren cages. Socially housed subjects were relatively docile. Their number of biting attempts against handling personnel was about a third of that shown by individually housed hamsters.
2.1.2.4. Enhanced Development of Species-Typical Brain Functions
Tagney (1973) studied the sleep patterns of rats in relation to their housing conditions. Animals kept in groups of six in cages equipped with a variety of enrichment objects and structures spent significantly more total time asleep as seen in both SWS (slow wave sleep) and REM (rapid eye movement) sleeping times than their littermates kept alone in barren cages. Evidence suggests that sleep is the time during which the brain synthesizes macromolecules required for its restoration after mental activity. The extra sleep of the rats living in a mentally stimulating environment presumably served this function. This assumption is supported by the findings of Diamond et al. (1964, 1972) and Diamond (1988, 2001), who demonstrated that the thickness of the cerebral cortex was lowest in rats kept alone. The provision of more space and access to running wheels, ladders and small mazes did not promote cortical thickness as long as the rat was kept alone. Cortical thickness increased significantly when a rat was allowed to share a cage with other rats. When enrichment was then provided in the context of social housing, cortical thickness increased even further. The positive effect of social partners and the combination of companionship plus enrichment was significant in subjects of all age classes. These findings indicate that living with other rats was the key factor for enhanced brain function and that companionship facilitated the brain-stimulating effect of environmental enrichment.
Valzelli (1973) reported that mice housed in groups of eight quickly learned to press lightly with their tongues on a lever in order to receive water from a drinking device. By contrast, 32 percent of an unspecified number of mice housed singly seemed unable to learn how to operate the automatic water distributor and died due to dehydration within one week. It is difficult to understand why the author did not intervene in time to spare the animals the agony of death resulting from dehydration.
Henderson (1970) raised groups of four mice in small, barren standard cages, and five-times larger cages that either were also barren, or enriched with a variety of objects for climbing, exploring, hiding and gnawing. The brain weight of subjects raised in a barren environment was not affected by the size of the cage, but it was significantly increased in subjects raised in enriched cages. Apparently, environmental enrichment was more important to brain development than additional unstructured space.
2.1.2.5. Recovery from Brain Injuries and Neurodegenerative Processes
Passineau et al. (2001) compared the recovery from traumatic brain injury in rats who were housed (a) alone in small, barren standard cages or (b) in groups of 14 animals in a large cage furnished with a running wheel, tunnels, a hammock, branches and a variety of toys. The combined effect of companionship, enrichment, and more space was reflected in significantly higher preservation of brain tissue integrity, along with significant attenuation of cognitive deficits. The enhanced recovery of cognitive function after brain injury in a social, more spacious, and more complex environment relative to the barren single-cage environment confirms the findings by Hamm et al. (1996). Farrell et al. (2001) made similar observations in gerbils kept in groups of five in large cages containing tubes, shelves, a running wheel and a wooden log versus alone in small barren cages.
It can be hypothesized that a more species-appropriate, complex environment stimulates mechanisms restoring function after brain damage (Ohlsson and Johansson, 1995; Mattsson et al., 1997; Belayev et al., 2003). Risedal et al. (2002) noticed in rats with experimental brain infarction that subjects recovered from surgery as measured in motor function significantly better when they shared a barren cage with other rats than when they were kept alone but had access to a running wheel. Johansson (1996) concluded from similar findings that companionship is more important to the recovery process than inanimate enrichment.
Ickes et al. (2000), Van Dellen et al. (2000), Hockly et al. (2002) and Spires et al. (2004) were able to slow neurodegenerative processes in group-housed transgenic mice by enriching the animals' cages with cardboard tubes for hiding and gnawing.
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