Notes on Refrigerator Hibernation for Mediterranean Tortoises with temperature and weight graphs

Kate Bradley

The time was fast approaching when I would have to consider the method of hibernation I would adopt for my two tortoises I had bought in 2002. I purchased Doris, my first tortoise, in May from a pet shop. She was a small (weight, 180 gm), female, marginated, tortoise, hatched in August 1997.

 I bought, privately, a female companion for Doris - Mildred (weight, 610 gm) in July.  Mildred was also hatched in August 1997, but was at least 3 times the size of Doris (Figure 1)!

As British winters are notorious for widely fluctuating outside temperatures, it seemed to make sense to me to hibernate my tortoises in a fridge where I would have the best control of temperature.  I decided to go ahead, having read various articles and from these knew the ideal temperature in which to hibernate your tortoises.

I bought a refrigerated drinks cabinet, a dual-sensor digital thermometer, 2 plastic boxes with lids, play-sand and topsoil.  I sited the cabinet in our garage.  Using the digital thermometer sensor dangling (by means of the closed door) into the empty drinks cabinet I spent a few days adjusting the thermostat of the cabinet so that the temperature did not deviate more than 2°C either side of 5°C.  Next, I prepared the plastic boxes.  I drilled several air holes into the lids.  For the bedding substrate, I mixed the topsoil and play-sand, approximately 50:50.  As the play-sand was rather wet, I dried out the mixture before putting it into the boxes.  I wasn't sure what depth to fill the boxes but guessed at approximately 2/3rds to allow for a bit of burrowing if the tortoises so needed.

I put shredded paper on top of the substrate, and then put the boxes into the refrigeration cabinet with the temperature sensor poking through the lid of the box placed at the lower level (Figure 2).

 Over the next few days, I monitored and adjusted the thermostat until the temperature was stable over a small range, as before.

Meanwhile, it was November, and the tortoises had been without food for approximately a month.  Their usual accommodation consists of a plastic rabbit hutch (grill removed) with an inner compartment, placed inside a greenhouse containing basking and sun lamps. A very small thermostatically-controlled, oil-filled radiator that fits inside the inner hutch provides background heat of a minimum of 12°C overnight (Figure 3).

 Mildred was to be seen occasionally, basking under the heat lamps during the day, but Doris remained inside the inner hutch, rarely venturing out.  Activity of both tortoises was decreasing by the day.

On 4th December, I put Doris in a small depression I had made in the substrate in the plastic box and covered her with shredded paper, leaving her overnight in the hutch.  In the morning, as she had not moved, I put the lid on the box and transferred her to the cabinet.  Similarly, within the next 2 days, Mildred was also transferred to her hibernation box (Figure 4) and into the covered cabinet (Figures 5 and 6).

I monitored the maximum and minimum air temperatures within the cabinet and in the garage for the remainder of the 14-week hibernation period (Figure 7). 

At first I monitored and recorded the temperatures twice a day until I was confident that the temperature in the cabinet was not fluctuating wildly.  Thereafter I monitored the temperature more or less daily, but especially when the outside temperatures were particularly cold.  I did not have to alter the setting on thermostat from that used at the end of the pre-hibernation set-up period. As the garage houses our washing machine, and also a tumble drier (infrequently used), the changes in garage temperature did not always mirror those of the outside garage air temperature.

I examined the tortoises periodically (indicated by 'Ex' on the graph) by removing the box from the cabinet, opening the lid, removing a small amount of shredded paper and gently touching a front limb or tip of nose.  Each time the tortoises responded immediately with a withdrawal of the limb or head (at no time were the head or limbs fully retracted into the shell).  I noted that the tortoises had never moved from the original position in which they had been placed in the substrate, and showed no evidence of burrowing further into the substrate.  At other times I would also open and close the door briefly to ensure some fresh air entered the cabinet.  Whether or not this was necessary is unknown.

On the 14th March I took the tortoises out of their boxes, weighed them, and placed them under the basking lamp in the greenhouse. Within 1 hour Mildred was eating and within 36 hours, Doris was eating.  Within a further day, functioning digestive systems and kidneys were evident.  Doris's weight was unchanged from the start of hibernation (245 gm; summer maximum, 265 gm), while Mildred's weight had fallen by 25 gm to 755 gm (summer maximum, 780 gm).  Both ate ravenously thereafter and at the end of the following week Doris had gained 10 gm, and Mildred 105 gm (Fig. 8)

Summing up, I would say that this method of hibernating your tortoise is very successful and relatively easy, and I will definitely use this method from now on.  I had certainly achieved my goal of maintaining the hibernation temperature at 5°C ±  2°C.  Although some of the equipment may be considered expensive, it will be used for many years to come, and so that initial financial outlay can be thought of as money well spent.   As I write this in August 2003, we are experiencing a heatwave. No doubt these temperatures are usual for Mediterranean tortoises in summer. Doris and Mildred have been very active but pass the hottest times of the day in dappled shade.  They continue to put on weight (Figure 8), Doris now 435 gm and Mildred 1260 gm.  And Doris is showing no ill effects from having been dragged out of her hutch by a fox - but that's another story!