Hibernation in Syrian Hamsters
by Grant Forest
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
