Housing and husbandry: Hamster
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Hamsters are burrow-digging, nest-building rodents that are sensitive to seasonal changes within their environment, for example, the length of daylight within each day (photoperiod). Environmental cues associated with winter induce physiological changes in hamsters, (see torpor below). Typically, they are most active during the low-light hours of night, dawn and dusk. However, in the laboratory periods of wakefulness can also occur in daylight hours.
Laboratory species include the Syrian hamster (Mesocricetus auratus; also known as the golden hamster), the Djungarian Hamster (Phodopus sungorus; also known as the Siberian dwarf hamster) and the Chinese hamster (Cricetulus griseus). Within this guidance ‘hamster’ refers to the Syrian hamster, unless otherwise stated. Much of the information presented here can be generalised to other hamster species used in research. However, if this is not explicitly stated, it is recommended to double check the specific details for the species that you work with.
Habituation to human contact through regular, gentle handling (e.g. picking them up using cupped hands) is necessary for all species . Handling hamsters when they are most active is preferable  If this is not practical, hamsters should be gradually eased awake to minimise their distress .
Enclosures should be large enough to allow the animals to perform a range of behaviours, stand fully upright and compartmentalise their space. Hamsters are driven to make a latrine area within their enclosure and the cage should be arranged to facilitate this. Therefore, food and water should be provided at one end of the cage, leaving the remaining space free for the animals to choose where they urinate and defecate.
Minimum enclosure sizes for all hamster species in European research establishments are fully detailed in Table 1.4 of Annex III to Directive 2010/63/EU. The minimum permitted enclosure size for adult hamsters is 800 cm2, with the minimum floor area per hamster, under group housing conditions, ranging from 150 to 250 cm2 depending on body weight. The minimum permitted cage height is 14 or 15 cm according to Directive 2010/63/EU and UK Home Office Guidance, respectively. However, Syrian hamsters can reach a height of 16 cm when rearing, so a minimum cage height of 17 cm is recommended . Hamsters are good climbers and skilled at escaping, so enclosures should be covered with securely fitting lids.
Nesting material is essential to allow the animals to create appropriate microenvironments  and to facilitate burrowing and shelter building behaviour . Hamsters require enough nesting material to build large nests that provide full coverage to all the animals within. Enrichment items should be provided in order to increase cage complexity and the available floor area, and to allow spatial compartmentalisation. Floors should be solid. Grid or wire mesh floors require strong scientific justification and should be avoided as they can lead to injuries; if used, a solid area should be provided for the animals to rest on, unless specific experimental conditions prevent this and are authorised by the project licence.
Hamsters scent mark using their flank and rely heavily on olfactory cues to communicate and gain information about the environment. The removal of scent cues can cause stress and increase the risk of aggression between group housed animals. Animals should be monitored closely after a cage clean as aggression between hamsters can escalate quickly with potentially fatal consequences .
Spot cleaning (the removal of soiled bedding only) is preferable to replacing all bedding substrate with clean material. When a full cage clean does take place, or animals are moved to a new cage, some old, clean nesting material should be retained.
In the wild, Syrian, Djungarian and Chinese hamsters are solitary and social contact is limited to mating. In the laboratory, harmonious group housing is possible if special care is taken in forming social groups . Appropriate housing and husbandry will facilitate the maintenance of stable groups. Compatible animals will sleep and huddle together, and a preference for social company over solitude has been demonstrated in captive male Syrian hamsters .
Group-housed hamsters should be introduced as juveniles (e.g. by keeping littermates together) in order to minimise the risk of aggression and promote the formation of socially stable groups. Social stability can persist into adulthood, however within-group aggression becomes more likely, in both sexes, as the animals age.
Unlike some species that can find resolution after conflict, aggression in hamsters escalates quickly and immediate action should be taken if they begin to fight. If aggression occurs, both sexes are capable of inflicting severe injuries, but females tend to be the more aggressive sex. Separation of animals or division into subgroups can be necessary to prevent injury or death [7,8]
Extreme care should be taken if pairing animals for breeding purposes and the hamsters should be continually monitored by an experienced professional throughout the process. It is common for animals to vocalise immediately after pairing, however this should not be long-lasting or involve physical aggression. Fighting can occur at all stages, with the female being the main aggressor in most instances. If fighting occurs animals should be separated at once. It may be necessary to separate the pair promptly after mating to prevent the female attacking the male .
Light and sound
All hamster species are physiologically and behaviourally sensitive to photoperiod (the duration of daylight). For example, the length of daylight hours can trigger reversible changes in weight, coat colour, reproductive behaviours, gut microbiota, food preferences, levels of aggression, immune function and expression of torpor [10, 11].
Lighting regime should be informed by the research being conducted and the natural history of the species, but for general research purposes hamsters can be maintained and bred on a standard 12:12 light:dark cycle . Breeding must be carried out when hamsters are in their long-day (i.e. summer) physiological state. Hamsters tend to be more aggressive under short-day lighting regimes than they are under a long-day conditions .
A reversed light cycle, where the dark period overlaps with usual working hours, allows hamsters to be monitored when they are most active. However, care should be taken not to accidentally expose the animals to light during their dark phase. Areas of low light should be provided within the cage, for example, opaque nest boxes and tunnels.
Hamsters have poor eyesight but can still easily detect the movement of objects, they can therefore be startled if they are not approached carefully. Avoiding sudden movements and allowing hamsters to smell and hear you before making gentle contact can reduce handling-associated stress and the risk of biting. It is advisable to avoid sudden loud noises.
Hamsters are able to perceive both infrasound and ultrasound (0.1 – 46.5 kHz)  and use ultrasonic vocalisations for communication (20 – 55 kHz) . Different species use different frequencies . Anthropogenic (human-made) sources of infrasound include air conditioners, heavy machinery and wind turbines . Sources of ultrasound in laboratories include dripping taps, trolley wheels and computer monitors. As some sources of noise disturbance cannot be eliminated (e.g. ultrasound from laboratory equipment), playing background music may help mask stressful sounds, as has been suggested for mice , although this has not been validated for hamsters and should therefore be carefully evaluated.
Temperature and humidity
UK Home Office guidance states that hamster rooms should be maintained at a constant temperature between 20 – 24 °C. However, the thermoneutral zone of hamsters is between 28°C and 30°C (82.4 – 86°F; Mulder 2012); therefore, nesting material is essential for them to regulate their body temperature and the microclimate within the cage. Torpor can be triggered by low temperatures. It is advisable that the relative humidity in rodent facilities should be kept at 45 to 65%.
Torpor is a state of increased lethargy and reduced metabolic activity and body temperature which can be subdivided into daily torpor, lasting just a few hours, or hibernation, lasting for weeks or months at a time. Some species of hamster are true hibernators (e.g. the Syrian hamster), while others exhibit daily torpor (e.g. the Djungarian hamster). For simplicity, torpor is used here to describe both daily torpor and true hibernation.
Torpor is triggered by winter-like conditions, such as food scarcity, low temperatures (e.g. 3 – 5 °C for the Syrian hamster)  and shorter daylight hours. The expression of daily torpor may be lost over generations if hamsters are not allowed to experience a short-day ‘winter’ photoperiod prior to being used for breeding.
During torpor hamsters undergo pronounced behavioural and physiological changes that can be mistaken for illness, or even death. Hamsters can appear highly lethargic and hunched with an unkept coat. Their body may also be limp and cool to the touch, particularly at the extremities. It is very important to understand the signs of torpor and how to differentiate these from signs of ill health.
Torpor can be reversed by rousing the animal awake, for example by opening the cage or through gentle handling using cupped hands. Gently rubbing the animal can increase its body temperature. An ill animal will show no change in state, whereas a torpid animal will begin to rewarm, becoming more active within approximately five to ten minutes after rousing.
As research animals are maintained under tightly controlled conditions, hamsters becoming unexpectedly torpid is indicative of issues with housing and husbandry and it is essential to consult your named veterinary surgeon if this occurs. Environmental conditions and the quantity and nutritional content of the food provided should be reviewed. It should be noted that the same conditions that trigger torpor also increase the risk of infanticide.
As with all animals, the provision of adequate nutrition is essential and the nutritional requirements of hamsters will vary depending on the species, age, sex and reproductive state (see Nutrient requirements of the hamster) .
Food supplementation should be provided for breeding hamsters (e.g. wheat germ for Chinese hamsters) . Both the availability and the nutritional content of food influence the risk of infanticide. An additional food source (e.g. oats) should be provided to females with pups.
In some cases, the wider muzzle of the Syrian hamster can make accessing food from traditional food hoppers challenging . If this is evident, pellets should be provided in a bowl within the cage. Scattering food within the cage is also recommended for enrichment purposes.
Environmental enrichment should include nesting material, a refuge area (e.g. tube, plastic shelter), roughage (e.g. hay) and gnawing objects (e.g. hard nylon chews and wooden blocks) [7, 21]. Gnawing objects are essential to prevent the overgrowth of teeth. Hamsters are good climbers and providing grid space or other climbing devices, such as ladders, lofts and mezzanines, will allow them to make use of available space.
In addition to substrate at the bottom of the cage (e.g. dust-free woodchip), hamsters require nest-building materials. This is particularly important as hamsters are unlikely to have bedding of an optimum depth (> 40 cm) within a laboratory setting [21, 22]. Suitable nesting material includes large handfuls of shredded paper, hay, straw and shreddable materials like cardboard and paper towels . A sufficient amount of nesting material should be provided for the hamsters to ‘bulldoze through’ . Hamsters should be able to build nests that fully cover all group members.
As hamsters are burrowing animals, providing areas of low light within the enclosure is important and this can be achieved using enrichment items such as shelters, nest boxes and tunnels. Hamsters will huddle inside a shelter or nest box in a group, so there should be enough space to fit all cage-mates at once. Hamsters will nest inside pipes (7.6 cm diameter), preferring pipes that are closed at one end .
Hamsters are hoarders and food storing behaviour should be enabled (e.g. by providing food pellets within the cage) [4, 7]. Scatter feeding facilitates hamsters’ natural foraging and hoarding behaviours. When providing treats or scatter feeding it is important to ensure that enough is provided to prevent competition between individuals. Additional sources of food, such fruits and vegetables (e.g. apple and cucumber) can provide good enrichment for hamsters, however your named veterinarian should be consulted before new dietary items are introduced.
Summary of key features of housing and husbandry
Cages should be large enough to provide enough space for exercise and normal social behaviour, with solid floors and a minimum height of 17 cm. The living area should be arranged in a way that allows hamsters to compartmentalise and customise their space.
The light-dark cycle should be appropriate for the species of hamster and the research requirements. Sensitivity to photoperiod should be well considered.
Hamster rooms and cages should be well ventilated and kept at a temperature range of 20 – 24 oC with relative humidity between 45 – 65 %. Extraneous noise, and in particular ultrasound and sounds that may startle animals, should be kept to the minimum.
Socially-housed hamsters should be introduced as juveniles and care should be taken in forming socially stable groups. Hamsters should be monitored closely for aggression and if it is observed intervention should be immediate.
Sufficient nesting material for the hamsters to fully cover themselves (e.g. a depth of 3 – 5 cm, or enough for the animals to ‘bulldoze’ through) should be provided.
Appropriate enrichment should be provided. Gnawing materials are essential to avoid overgrown teeth.
Providing an additional food source and scatter feeding food items facilitates natural behaviours and improves welfare. An additional food source is essential for breeding colonies and females with pups.
Cage cleaning frequency should be kept to the minimum and nesting material should be transferred to clean cages to maintain familiar odours and minimise stress.
Hamsters should be approached carefully, taking into account their poor eyesight. They should be handled during their active period or roused awake gently. Hamsters should be handled using cupped hands and habituation to handling should start when they are juveniles.
Hamsters in ill health should be differentiated from those experiencing torpor. Unexpected torpor should be investigated, and conditions reviewed with the named veterinary surgeon.
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