A model for an incubator with electronically regulated daily temperature cycle
Victor Loehr
Introduction
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
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.
Literature
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;
Highfield, A.C. (1990). Keeping and breeding tortoises in
captivity. R&A Publishing Ltd.;
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
Nijs, J. (1994). Een eenvoudige incubator met een dagelijkse
temperatuurcyclus. Lacerta 52: 114-119.