Hibernation in Syrian Hamsters

Grant Forrest

Hamsters are among the many animals that hibernate. Hibernation is defined as "The dormant state in which some animal species pass the winter. It is characterised by narcosis, and by sharp reduction in body temperature and metabolic activity and by a depression of vital signs. It is a natural physiological process in many warm-blooded animals. NOTE: summer dormancy corresponding to this winter dormancy is known as AESTIVATION."

There are 2 main theories about the evolution of hibernation. The traditional view was that primitive animals needed to hibernate to survive harsh environmental conditions and the ability to do so has been inherited by modern animals from their ancestors. An alternative theory says that it is size and diet that are the main determinants of hibernating ability. Modern bird groups and ancient mammal groups contain mainly small species that often rely on fluctuating food supply, whereas modern mammalian orders and ancient bird orders contain the largest species with low energy requirements for maintenance of body temperature. Hibernation in birds has apparently evolved separately from that in mammals, which tends to favour the size and diet theory.

2 types of hibernators are described - permissive and obligatory. Syrian hamsters fall into the permissive category; they have the option to hibernate depending on conditions of temperature and food supply. Obligatory hibernators do so regardless of conditions and are generally obese before entering hibernation.

Although temperature is undoubtedly an important factor in inducing winter hibernation, hamsters have been known to hibernate (or possibly aestivate) at temperatures of 14-20°C. Why would they do this? The natural range of the Syrian hamster exposes it to a yearly mean temperature of about 18°C, but this varies from 4-6°C in January to 28-30°C in July. The annual precipitation is about 400mm with partial vegetation in the autumn and winter months in conditions of lower temperature and higher rainfall. Drought is the norm between May and October and the hamster has a kidney which is adapted to prevent dehydration. These factors together suggest that the hamster may be an aestivator as well as a hibernator or that it may have been a hibernator originally in a more northerly range but migrated south and became an aestivator.

In either case, the ability to hibernate has been retained but the factors responsible for inducing hibernation in captivity are still not understood fully. Temperature is probably the most important factor. In experiments, the duration of cold required to induce hibernation decreases during November, December and January. In the spring, up to 3 months exposure to cold may be required to induce hibernation. Cold induces weight loss (in contrast to obligatory hibernators) and food storage. Prevention of hoarding is said to inhibit hibernation and is dangerous as the hamster may waken between hibernation ‘bouts’ and have no food supply. Although in a sense hamsters are among the most cold- resistant rodent species because of their ability to hibernate, they are not good hibernators! Nor is the hibernation response present in every hamster. When exposed to cold, some hamsters will eventually hibernate, some will never hibernate and some will hibernate but die of hypothermia. It’s very likely that the ability to hibernate is determined genetically and can be inherited like any other characteristic. Another factor that may be important in inducing hibernation is light. The effects of reduced light intensity and duration are additive with those of temperature - a hamster kept in the cold and dark is more likely to hibernate than one kept cold but in strong light for more than 12 hours out of 24. Other factors that may be important are food supply, isolation and stress but it’s difficult to assess the contribution of these because of the variability in the response to the main factor, cold. All that can be said is that the harsher the environment, the more likely hibernation becomes. The changes in an animal’s physiology during hibernation have attracted a great deal of interest among scientists. Doctors especially would dearly like to be able to induce a state of hibernation in humans for procedures such as open heart surgery. The organs of a hibernating animal are able to withstand extreme conditions, in particular a lack of oxygen that would cause cell death at normal body temperatures. They can also increase their temperature and survive the flood of toxic substances that are released when the cells become active again. Despite years of intensive research it’s not known precisely how they manage this.

From a practical point of view there are 3 main issues that may concern a hamster breeder. Firstly, how do I prevent my hamsters from hibernating, as time spent hibernating is time that could be spent breeding. Secondly, how do I awaken a hibernating hamster and lastly, how do I know when a hamster has died in hibernation?

The first point is addressed easily. Keep your hamsters warm and this means keep the temperature above 15°C, keep them well-lit for at least 12 hours a day, provide plenty of food and handle them regularly. Despite your best efforts, they may still hibernate. If it happens in the summer, they are probably aestivating (see above for details).

The second point is a matter of opinion. If left alone, hamsters will often hibernate for 2-3 days but if the temperature is very low then they may remain in hibernation for up to a week. One option is to leave them alone, provide plenty of food and water for when they waken up and try some of the measures outlined above to prevent them from re-hibernating. Alternatively, they can be stimulated just by picking them up and stroking them gently. They are very sensitive to tactile stimulation and this is said to speed up arousal. Forcing their body temperature up by applying heat or putting them in temperatures above 20°C can (at least in theory) have adverse effects on the natural arousal process and can’t be recommended. Spontaneous arousal and return to hibernation can occur within 12 hours and one technique to detect this is to put a couple of wood shavings on the hamster’s body. If, at the next check, the shavings are gone then the hamster must have woken up at some point.

One of the commonest stories told about hamsters is how they can miraculously "rise from the dead" and everyone knows someone whose hamster was dead and buried and then found at large in the garden or house some days later. The origin of these stories must be the hamster that is given up for dead but is in reality in deep hibernation. It can be very difficult to tell when a hamster has died in hypothermia. During deep hibernation the heart rate slows to as little as 4 beats per minute and the respiration can fall to only one breath every 2 minutes. The temperature of the cheek pouch will remain slightly above the ambient temperature. On examination, the hamster is curled up and the limbs feel more rigid than normal, but can still be extended slightly by gentle traction. On close inspection and following a gentle stroke the whiskers will be seen to twitch. This is the most consistent finding in a hibernator. Features that suggest that the animal has died are board-like rigidity of the limbs indicating rigor mortis and failure to waken or show any sign of life after 24 hours in an ambient temperature above 20°C. Low weight at entry into hibernation is associated with death in hypothermia.

If you are still unsure whether your hamster is dead or in hibernation, always consult your vet. He or she is likely to have experience of this scenario and can provide a specialist opinion.

References: The Golden Hamster - its Biology and Use in Medical Research Chapter 3 Hibernation and Effects of Temperature Roger A. Hoffman

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