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Mice, Rats, Hamsters
Introduction
4.1. Laboratory rodents are highly adaptable animals, selected for important traits
such as ability to breed in laboratory conditions and docility. However, they do retain
many of the traits of their wild counterparts, for example, grooming, exploratory
activity, searching for food, burrowing and gnawing, and housing systems should aim
to encompass these behavioural needs.
Mouse
The laboratory mouse is derived from a largely nocturnal burrowing and climbing
ancestor which favoured building nests for temperature regulation and reproduction.
Mice do not readily cross open spaces, as confirmed by utilisation of cage space
studies. Mice are capable of assuming a wide range of social organisations and
intense territoriality may be seen in reproductively active males. Pregnant and
lactating females may prove aggressive in nest defence. As mice, particularly albino
strains, have poor sight they rely heavily on the sense of smell and create patterns
of urine markings in their environment.
Rat
As the rat is a very much more social animal than the mouse disruption to social
groups should be minimised. Young animals are very exploratory and interact to an
enormous degree (37). Rats are excellent climbers, avoid open spaces, and use urine
spotting as a territorial marker. The senses of smell and hearing are highly
developed, and these animals are particularly sensitive to ultrasound. Daylight vision
is poor, but dim-light vision is effective in some pigmented strains. Activity is higher
during hours of darkness.
Hamster
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This species is very different from the rat and the mouse. The female is larger and
more aggressive than the male. During pregnancy and lactation the female can be
intensely aggressive, and can inflict serious injury on her mate. Group housing may
reduce aggressiveness in this species (38). Female hamsters often provide a latrine
area within the cage, mark areas with secretions from a flank gland, and frequently
selectively reduce the size of their own litter by cannibalism. Careful control of
environmental features, and prevention of disruption during routine husbandry
practices are of particular importance in this species.
THE ENVIRONMENT
4.2. Laboratory rodents are species which choose to manipulate their own
microenvironments via activities such as huddling, nest building and tunnelling. In
general, the rodent's ability to control temperature', humidity and lighting is more
important to its welfare than specifying ambient conditions within the room. The
microclimate within the cage is of most importance to the animal, and welfare seems
facilitated when rodents arc able to control this, for example by provision of bedding
material.
Temperature
4.3. The optimal temperature band for mice, rats and hamsters is 19-230C.
Temperatures within the cages will often be higher than room temperatures. Even
with grid floors and adequate ventilation the cage temperatures may be 3-6OC
above room temperature. The difference is likely to be greater in the solid-floored
cages used for breeding.
Provision of bedding or nesting material allows the animal an opportunity to
manipulate its own immediate environments, and provide a warm nest for its young.
This may also promote greater utilisation of the available space.
Relative Humidity
4.4. Humidity control is an important consideration for laboratory rodents.
For rodents relative humidities in the range of 55% + or -15 are acceptable.
As low relative humidities may contribute to the development of ring-tail in rats,
levels of less than 40% should be avoided.
Ventilation
4.5. Specific air change rates in the room are less important than ensuring that there
is an efficient flow within the rooms to keep the level of ammonia within the animals'
immediate environment at an acceptable level. Stocking densities, husbandry
practices and cage types (for example, solid or grid bottom) will influence air flow
patterns and the consequent ventilation rate required.
Lighting
4.6. Light levels within cages are more important to the welfare of breeding rats,
mice and hamsters than the light level in the room. Lighting intensity should be that
only which is required by husbandry practices or safety reasons.
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More use should be made of subdued lighting (e.g. red lighting which rodents cannot
detect). All racks (especially those that are relatively high) should have shaded tops
to prevent animals in the top row being exposed to excessive light (which can cause
retinal degeneration). The importance of light to dark cycles in regulating circadian
rhythms and stimulating and synchronising breeding cycles is well documented. A
daily cycle of 12:12 is usual.
Animals, especially when breeding, should be given the opportunity to withdraw to
shaded areas within the cage, for example by provision of adequate nesting
materials.
Noise
4.7. Sudden irregular noises create more disturbance in breeding rodents than
continuous or predictable sounds.
As rodent neonates use ultrasound production to communicate distress, it is
important that extraneous noise is minimised. Ultrasound from cleaning devices,
pressure hoses, trolley wheels, vacuum cleaners, computer VDU's may result in
abnormal behaviour and disturbed breeding cycles.
Noise cannot be eliminated from an animal unit but care should be taken to minimise
the generation of sudden extraneous audible and ultrasound noise in the vicinity of
animals.
ANIMAL CARE AND HEALTH
4.8. Unless there is good husbandry/veterinary justification for individual housing,
animals should be maintained in sociable groups. These groups should remain stable.
Frequent mixing of groups of breeding mice and hamsters is strongly discouraged as
this can be a source of intense stressful conflict (39).
Bedding and Nesting Material
4.9. Nesting materials are crucial to breeding rats, mice and hamsters to enable
them to engineer appropriate microenvironments that facilitate the successful
rearing of young. The bedding is also an important material on which all three
species lay down patterns of odour cues. These cues are important to the animal's
sense of security.
Food and Water
4.10. Where large numbers of breeding or stock animals are maintained in a single
cage or pen, it is important to ensure that there are sufficient feeding and watering
stations to avoid undue competition.
Cleaning
4.11. Routine cleaning and maintenance, and a high standard of hygiene are
essential for good husbandry.
There is, however, a real danger of over cleaning cages used by pregnant animals
and females with litters. Such disturbances can result in mismothering or
cannibalism.
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Odour marking is an important activity in these rodent species, and cleaning
disturbances will cause a degree of social disruption.
Decisions on frequency of cleaning should therefore be based on cage system, type
of animal, stocking densities, and the ability of ventilation systems to maintain
suitable air quality.
Partial cleaning, for example removal and replacement of soiled bedding permits
some odour cues to remain in the cage and reduces the disturbance to the animals.
4.13. Rats, mice and hamsters must be bred on solid floors, and provided with
suitable bedding material (e.g. shredded paper or wood chippings) from which a nest
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can be constructed. This is important in thermoregulation of the microenvironment,
and keeps the young together for efficient lactation.
Disturbance to the animals should be minimised during late pregnancy and early
lactation to reduce the risk of mismothering or cannibalism.
Environmental Enrichment
4.14. Many rodent species attempt to divide up their own cages into areas for
feeding, resting, urination and food storage. These divisions may be based on odour
marks rather than physical division but partial barriers may be beneficial. To increase
environmental complexity the addition of some form of cage enrichment is strongly
recommended. Corrugated devices or tubes are examples of devices which have
been used successfully for rodents and these have the added benefit of increasing
utilisable floor areas for the animals.
As these rodent species are generally social animals, disruption of established groups
should be minimised as this can be very stressful.

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