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Refrigerator Hibernation for Tortoises & Turtles

Andy C. Highfield

Refrigerators have been used successfully to artificially hibernate many animals. Snakes, lizards and even bats have been hibernated in domestic refrigerators. Many tortoise and turtle keepers have also discovered the benefits of this system.

Refrigerators can offer a highly stable temperature in precisely the ideal range for chelonian hibernation, which is generally agreed to be in the 3-5 degree Celsius (37.4 to 41 degrees Fahrenheit) range (depending on species). Even if room temperatures rise to above the level at which tortoises begin to activate and use up more energy (around 10 Celsius or 50 degrees F.) the temperature in the refrigerator will remain at safe levels.

What a refrigerator will NOT DO, however, is protect the animals if external temperatures fall BELOW the desired levels. In other words, just because 5 Celsius is the setting on the refrigerator, if the room temperature falls to -3 degrees, or even -1 degree, the TORTOISES WILL STILL FREEZE TO DEATH. It is therefore ABSOLUTELY ESSENTIAL that refrigerators used for hibernation purposes are situated in a room where above-freezing temperatures are guaranteed.  A warm room is fine. A cool room is also fine - but a cold room or garage, or similar, is certainly NOT safe. I would recommend placing the refrigerator in a room that gets NO COLDER THAN 10 Celsius or 50 degrees F. Use a reliable maximum-minimum thermometer to survey the room where the refrigerator is to be placed, and monitor it regularly throughout the hibernation period. 

While it may be thought that a garage or outbuilding would be fine provided it is equipped with a ‘frost-protection’ heater, this is not in fact recommended, as a simple power failure, even for a few hours, could very well result in temperatures within the refrigerator plummeting to lethal sub-zero levels very rapidly.

Choice of refrigerator:

Despite the temptation to use an old or surplus unit, it is firmly advised that instead a new, modern refrigerator should be employed. Old refrigerators often have unreliable thermostats. It is also best to choose a unit without an ‘icebox’ or freezer facility, as temperatures right next to the icebox or freezer section  can be considerably lower than in other areas of the unit. In addition, in the event of a thermostat failure, it is a remote (but real) possibility that any refrigerator which can actually attain freezing point might actually do so… therefore, refrigerators that do not have that capability are inherently safer.

The most suitable types of refrigerators for hibernation purposes are those often described as ‘fridge-larder’ models rather than as ‘fridge-freezer’ models. The size required will of course vary depending upon how many tortoises or turtles you intend to hibernate - for small numbers you may well find that a ‘table-top’ or ‘counter-top’ mini-refrigerator (such as seen in many hotel rooms) will be quite adequate. In fact, the ‘drinks chiller’ type refrigerator (usually supplied with a glass front door) is probably the best of all if you only have to hibernate small numbers of tortoises. These units are specifically designed never to attain  freezing point on their own (although they will still freeze if placed in a sub-zero external environment, of course).

Setting up the refrigerator

First of all, TEST the refrigerator thoroughly when empty. Use a separate thermometer of known accuracy to establish a set temperature and to monitor fluctuations from this point. If using an electronic thermometer with a separate probe, place the probe in FREE AIR (not in contact with any object) to see how much the air temperature varies within the refrigerator. Make a note of the results.  Do not be surprised if there is quite a bit of variation, or if stability is not quite what you expected.

We deal with this problem next!

The second test should be to establish how stable the refrigerator is when fully ‘loaded’. It is a simple rule of physics that the more mass an object has, the more resistant it will be to external changes in temperature (it will equalize more slowly than an object of lower mass). To maximize temperature stability within the refrigerator, then, it is preferable to introduce extra mass within the unit. One easy way to do this is to fill some 2 litre plastic drinks bottles with water, seal them, and position these to fill ‘dead’ space. This will add considerably to the mass within the refrigerator, and hence will dramatically improve overall temperature stability.  One other source of mass when the unit is in use will be the actual hibernation boxes, complete with substrate. For the moment, half fill a 30 X 30 X 10 cm (12 X 12 inch X 4 inch) plastic food store, lunch box, or large ice-cream/margarine type contained with a 50/50 soft sand and topsoil mix. Position this on the middle tray within the refrigerator. Rest the thermometer (or thermometer probe, if an electronic type) on the surface of the substrate. Now repeat the monitoring test for at least 24-48 hours.

You should see a dramatic improvement over the first test.

This second test more accurately reveals the temperatures and temperature variation that a hibernating tortoise or turtle will experience within the unit.

Using the results from this test, make any adjustment to the refrigerator thermostat that are necessary to ensure a stable temperature of 5 Celsius, or 40 °F.  Only when the refrigerator has demonstrated that this temperature can be maintained with no more than a +/- variation of 2 degrees should it be regarded as suitable and safe for actual use.

Hibernation boxes:

You can use cardboard, plywood or plastic boxes as containers for tortoises and turtles within the refrigerator. Obviously, cardboard and wood are unsuitable for any species that requires a moist substrate. For species that require a dry substrate, they are useable, however.  Boxes should not be too small. Allow enough room for the animal to turn around if it wishes to. Sides should be deep enough to prevent easy escapes (although it is strongly recommended that a lid is fitted to completely preclude this possibility). I prefer to use polythene food-grade ‘lunch boxes’ and I drill multiple holes in the lid to assist with ventilation. The only purpose of the lid is to prevent escape and injury - so I drill as many holes in it as I can without weakening it too much.

Substrates:

Most of the comments in our article on ‘Vivarium Substrates’ apply equally in this context, as do the comments in our ‘Microclimate’ article. For arid habitat species, a dry substrate is appropriate. We use a 50/50 mixture of soft (play) sand and topsoil. You can also use shredded paper, although I personally feel that the advantages of a natural sand/soil substrate are considerable. Certainly, such a substrate does tend to reduce ‘scratching and scraping’ and appears to allow the tortoise to ‘settle in’ rather faster than when shredded paper is used.

For moist habitat temperate species, such as American box and Wood turtles, I would recommend a hibernation substrate composed of one third soft sand, one third peat-based compost, and one third moist sphagnum moss.

This substrate should not be too deep - just deep enough to permit the animal to bury itself. This a) helps to stabilize temperatures around the tortoise and b) helps to prevent excess fluid loss from respiration and cutaneous (skin) evaporation. The humidity within a refrigerator can fall very low indeed - it is worth monitoring this, especially with sensitive species. If it gets too low, a shallow tray of water with added sponge material can help restore it to acceptable levels. For most species humidity in the 50-60% range is fine. Some very arid species such as Testudo horsfieldii can do well at a Relative Humidity of 40% or less. Box turtles tend to require RH to be maintained in the 65-75% range or higher.

Ventilation:

This topic probably causes the most concern to those unused to the concept of refrigerator hibernation. The first thing to be aware of is that hibernating tortoises and turtles have a very low oxygen demand. Many species completely bury themselves to a depth of a foot (30cm) or more in the earth to hibernate, where available oxygen is pretty low.

In practice, provided the following guidelines are followed, there will be no problem due to anoxia within a refrigerator hibernation environment:

  • Install a separate aquarium air-pump inlet and a separate outlet opening to allow for exhaust. Chose a pump that moves approximately 200 litres of air per hour for an average-sized refrigerator unit (this data is indicated on most air pump boxes). The better models of air pump also feature a charcoal/carbon inlet filter, to remove undesirable contaminants from the air supplied. One model we have used with success is the Interpet Pond-Air system. You will need to carefully trim two sections of the door seal, about 12 mm or one quarter-inch in both cases, one to allow for the inlet tube to enter without being squeezed or compressed, the other to act as the exhaust outlet. This will allow for proper ventilation, introduction of fresh air, and voiding of CO2 or stale air.
  • Open the refrigerator door at least 3 times a week for a minute or two.

Both methods have proven entirely successful.

In most other respects, refrigerator hibernation is the same as for other methods (hibernation pits, hibernation boxes in uncontrolled environments, etc.). All of the same general considerations apply. See our article ‘Safer Hibernation and Your Tortoise’ for further details.

A member of the Tortoise Trust Mailing List discussed his method of hibernating Russian tortoises (Testudo horsfieldii) using refrigerators. As this is a personal account of this method in use, you may find these comments very helpful:

I have used a refrigerator to hibernate my Russians for the past 3 years. I stop feeding them about 4-6 weeks before hibernation and shorten the time the lights are on. They have weekly soaks in warm water. About 3 weeks before hibernation I turn off the heat lamp resulting in cooler daytime temperatures. During the last week I soak them multiple times until they no longer void in the water

About 2-3 weeks before hibernation I get the fridge setup.  I use a small aquarium air pump with 2 air hoses that are feed into the fridge.  I tape the door shut as the air tubes tend to cause the door to want to open (note: this can be avoided if you cut away the door seals where the tube enters) .  I have a min-max thermometer and keep track of temperatures so I know the setting for 5 degrees C.  Then I turn the temp back up to about 15 degrees C for the start of hibernation. I put each tortoise in it's own box. The box is slightly bigger than the tortoise and I place lots of shredded blank newspaper on top. Once they are in the fridge at 15 C I slowly turn down the temp over the 1st week of hibernation until it reaches 5 C.

Throughout hibernation I open the door of the fridge about once a day just to let fresh air in. I weight them at least 2 times during hibernation to ensure that they are not losing too much weight. I also take them out and examine them about once a week. The most amazing thing for me is when they come out of hibernation. I follow Andy's recommendations from his guide and place them in bright light immediately after taking them out of the fridge.  I soak them multiple times over the first few days until they have voided and start drinking.  I feed them immediately and they typically eat the same day or next day after hibernation.

Airpumps

Let me clarify what I use.  It is a simple pump used for aquariums.  Cost is about $10-20 here. The pump sits outside on the floor and I just open the door, run the air tubes, in and tape the tubes onto the back and sides of the fridge to stop them from moving.

As for boxes, I have used sturdy cardboard boxes with a piece of thick indoor-outdoor carpet on the bottom.  I fold the tops over just to make sure they can't climb out. With the tops folded there are still large spaces open. So far this has worked. I usually need new boxes each year as they do tend to scratch almost right through the box.  This is where weekly checks are important. This year I am going to build boxes out of plywood. I place only 1 tortoise per box. They can be active (some scratching etc.) even at less than 10 C and I would be concerned that if one animal was active it might disturb another if in the same box. I try to cool the turtles down gradually rather than taking them from room temp right into the fridge. To do this you need to test the fridge before the turtles go in to know what the fridge temp setting should be for 5 degrees.  Thus, when you first put the turtles in at say 10 C they will still move about.  Just wanted to mention this as it might be distressing to hear them scratching around if you haven't hibernated Russians before.  Russian tortoises are also more cold tolerant than other tortoises and are more active at lower temps. If unaware of this, some might be tempted to pull the turtles out of hibernation thinking that it wasn't working when in fact some activity even at 5C is normal.

Graham Reid, Ontatrio, Canada.

REFRIGERATOR HIBERNATION Q&A

Q. Can I hibernate aquatic turtles using this system?

A. In terms of temperature, yes - but there is a real possibility of problems arising from lack of oxygen in the water, or from other chemicals building up in the water around the turtle. The fact is that unless the refrigerator being used is extremely large, the water volume around an aquatic turtle is likely to prove inadequate to ensure safety. WE THEREFORE DO NOT RECOMMEND YOU SHOULD ATTEMPT TO HIBERNATE AQUATIC TURTLES UNDER WATER IN A REFRIGERATOR. We would be interested to hear from anyone who has had success (or failure) in this regard.  Certain species of aquatic turtles do hibernate out of water, however, often buried in river banks or under tree roots. These species may be candidates for refrigerator hibernation in the same kind of moist substrate used for semi-terrestrial species.

Q. Can I hibernate semi-terrestrial turtles, such as American box turtles this way?

A. Yes, certainly. No problems have been reported. Use a substrate as described above.

Q. How does this system work with very small juveniles or hatchlings?

A. Perfectly. They should be treated as for adults. Do be sure to use a suitable substrate to avoid undue risk of dehydration when hibernating juveniles.

Q. Can I go away on vacation if I use the airpump method of ventilation?

A. We absolutely do NOT recommend leaving the animals in a refrigerator unattended for any kind of extended period. What if there was a power failure?

Q. What kind of thermometer should I use to check temperatures?

A. We recommend using two: one should be a mechanical (old fashioned!) minimum-maximum horticultural type, and the other can be a modern LCD electronic type with remote sensing probe. These are useful in that the internal temperatures within the ‘fridge can be seen constantly without having to open the door. Many models can also be switched between two probes (one of which is usually built into the thermometer body) to provide indoor/outdoor readings. In this application, the ‘indoor’ reading will reveal the ambient temperature around the refrigerator, and the ‘outdoor’ reading (remote probe) should be resting in contact with the substrate in the tortoise’s box to reveal the actual temperature in the hibernation container. You can even get models with programmable high-low audible alarm points. If available, we recommend these highly. Try aquarium supplies or electronics stores such as Radio Shack/Tandy for a complete range of suitable thermometers.

Q. Can I use the same refrigerator I use to keep food in?

A. No. Absolutely not. There is a SERIOUS danger that food could become contaminated with potentially LETHAL pathogens, including salmonella. No refrigerator used for tortoise hiberbation should EVER be used to keep food or drink in.

We wish you and your tortoises or turtles a safe hibernation!

If you are using the refrigerator method, and have any photos or experiences, or advice for other keepers you would like to share, please let us know.

REFRIGERATOR HIBERNATION VIDEO

A detailed video giving step-by-step instruction on the refrigerator hibernation method

Now available from Carapace Press

 More useful hibernation information on this site:

Safer Hibernation and Your Tortoise

Hibernation Hints

Post-Hibernation Problems

Incorrect Use of Jackson Ratio

Hibernating Juvenile Tortoises