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A model for an incubator with electronically regulated daily temperature cycle 

Victor Loehr

 This article first appeared in "The Tortoise Trust Newsletter - Summer 1997



Several authors have discussed the possible importance of imitating a daily temperature cycle, when incubating eggs of turtles and tortoises successfully (Nijs, 1994; Loehr, 1995). A fluctuation between high temperatures during the day and temperatures that are several degrees Celsius lower during the night, could be beneficial for high hatching rates (in that respect, also the discussion of hatching rates of tortoise eggs by Hoveling (1996) could be seen in the light of daily temperature cycles rather than (or on top of) diapause) or for low percentages of hatchlings with deformations. Furthermore, Nijs (1994) discussed the possibility of deviations in the behaviour of adult turtles that were born from eggs incubated at constant temperatures. It is a fact that eggs of most turtles and tortoises experience fluctuating temperatures under natural conditions (Congdon and Gibbons, 1990; H. Scheffer, pers. comm., pers. obs.). Those few data available on incubation of eggs of turtles and tortoises at fluctuating temperatures show different results. It could have a positive effect in eggs of Pyxis arachnoides (higher hatching rate (H. Zwartepoorte, pers. comm.)) and Testudo hermanni (higher survival rate of hatchlings (H. Scheffer, pers. comm.)). The effect seemed to be negative in eggs of Chinemys reveesii (lower hatching rate (R. van Middelkoop, pers. comm.). It is clear that more experiments have to be conducted (and published) in order to be able to find out more about the importance of fluctuating temperatures during the incubation of eggs of turtles and tortoises.   Nijs (1994) presented a model of an incubator with the possibility of establishing a daily temperature cycle. This incubator was of a humid "au bain marie" type. It included that the incubator was heated by means of (two) aquarium heaters, switched via (two) mechanical thermostats. The environment of the eggs in such a type of incubator is characterised by a high relative humidity, which in my opinion is less suitable for eggs of tortoises from (semi) arid areas. The model as it is presented turns out to be difficult to be transformed into a dryer type of incubator. The first reason for this is that two heaters are being used for heating to day and night temperatures respectively. If a different type of heater is used in a dry type of incubator (light bulbs or heating wire for instance; aquarium heaters are not suitable for heating air), a shortage of space can occur readily if more than one element has to be installed. A larger problem however, is caused by using mechanical thermostats for temperature regulation. If these thermostats are being used for regulating the temperature of the air, a considerable high frequency fluctuation of the temperature can occur. Personally, I have measured a fluctuation from 24 C to 32 C and back within three hours, in an incubator with temperature regulation by means of a mechanical thermostat! Especially because the course of the temperature most often is characterised by a long period of cooling down (because of the isolation of the incubator) and a short period of warming up, in my opinion it must be avoided during the incubation of eggs of tortoises. An additional disadvantage of a mechanical thermostat is the relatively low reliance; in case of a defect the thermostat can stay in "on" or "off" position, thus causing a catastrophe for the eggs in the incubator. Therefore, a better alternative is an electronic thermostat (Highfield, 1990). In this type of thermostats, a high reliance is combined with a high precision. A disadvantage is the high costs most often involved with buying an electronic thermostat. In my opinion these costs should be considered acceptable however, if compared with other hobby expenses such as food, housing and buying of tortoises.


At this moment, a relatively large number of tortoise keepers are using incubators with electronically regulated temperature. If a daily temperature cycle is to be realised in these incubators several problems appear, that are related to the shortage of space for installing more than one heating source and the use of an electronic thermostat. After experimenting, I now think I can present a suitable model for an incubator with electronically regulated daily temperature cycle.

The incubator


The basis of the incubator that is described here is formed by a small refrigerator with the cooling element being removed. In the bottom of one of the sides and at the top in the back, a row of six holes (10 mm) has been bored for a light air circulation. Below, two plastic containers filled with water are placed for some ambient humidity (relative humidity 50-60% during the day and 70-85% during the night). On the two mesh wire planks, the eggs are placed in open containers in vermiculite. At the same height as the planks, two double glass windows are constructed in the front door of the refrigerator, for allowing inspection of the eggs without having to open the incubator. Until recently, heating was provided by means of four light bulbs of 25 Watts in the top of the incubator, switched via one electronic thermostat with the sensor in the middle of the incubator. Between de light bulbs and the eggs, two layers of jute were present in order to dim the bright light. Although in this incubator eggs of M. tornieri and Homopus s. signatus were incubated successfully, it was decided to rebuild the incubator to allow for a daily temperature cycle. The most important reason for this decision was to test whether the constant temperature during incubation could be the cause of the low hatching rates in eggs of M. tornieri, that also were experienced by other keepers of the species (for a review see Hoveling, 1996; H. Zwartepoorte, pers. comm.). Especially in species that lay only few eggs each breeding season such as M. tornieri, a high percentage of developing eggs would be expected in order to maintain the species in the wild. Only in the absence of predation, few hatching eggs would suffice for the maintenance of a population in the long term. Moll and Klemens (1996) however found that predation by different species of mongooses on M. tornieri was actually present in Tanzania.


When rebuilding the incubator, it was chosen to change the heating by light bulbs for heating by a heating wire (55 Watts). This heating wire was spiralled around a metal U-profile (500 x 40 x 45 mm) in order to construct a heating element that could be handled easily. The heating element was fixed in the incubator on two metal corner-clasps on the left and right side. A second electronic thermostat was purchased to allow a daily temperature variation. In order to switch the heating wire via either the first or the second thermostat, a time control unit was provided with a third pole. A bolt was installed in the side of the unit, contacting the switch when it is in "off" position. This allows for connecting one of the poles from an electric point with either the "day" or the "night" thermostat. The second pole of the electric point is connected directly with the heating wire and both thermostats. Finally, a 7-Watts light bulb was installed in the incubator, in order to make inspection of the eggs easier. It provides a dim light through the jute.


The incubator has been switched on for several weeks now and it is functioning very well. The night temperature is adjusted at 25 C and the day temperature at 31 C, both during periods of twelve hours. In the morning, it takes one hour until the temperature has increased to the  maximum temperature and in the evening it takes three hours for the temperature to decrease to the minimum temperature.




Congdon, J.D. and Gibbons, J.W. (1990). Turtle eggs: their ecology and evolution. In: Gibbons, J.W. (red.). Life history and ecology of the slider turtle; pp 109-123. Smithsonian Institution Press; Washington D.C..

Highfield, A.C. (1990). Keeping and breeding tortoises in captivity. R&A Publishing Ltd.; England.

Hoveling, M. (1996). Het kiezen van een broedstooftemperatuur: twee zaken om over na te denken. De Schildpad 22(3): 41-47.

Loehr, V. (1995). Enige punten voor discussie; broedtemperaturen bij schildpaddeneieren in relatie tot misvormingen. De Schildpad21(5); 9-11.

Moll D. and Klemens, M.W. (1996). Ecological characteristics of the pancake tortoise, Malacochersus tornieri, in Tanzania. Chelonian Conservation and Biology 2: 26-35.

Nijs, J. (1994). Een eenvoudige incubator met een dagelijkse temperatuurcyclus. Lacerta 52: 114-119.