Whereas a number of commercial incubator designs sutiable for incubating bird's eggs are available, none of these are entrely suitable for incubating tortoise's eggs. Making your own is an option. There are four major components to an incubator:
- Cabinet or box
- Heating Element
We will shortly examine the components and their installation in detail, but before doing so it is important to understand the biology of the incubation process and the specific requirements of tortoise eggs in particular. Most of the following observations are techniques are equally applicable to snake and lizard eggs.
This comprises a faced chipboard or plywood box measuring 18" x 24" x 12". Heat is provided by a 40W tubular heater located in the base of the enclosure (although heat mats of a similar rating can also be used). Temperature control is provided by a proportional thermostat, which avoids the major fluctuations experienced with simple on-off type thermostats. The design is capable of maintaining temperatures to +/- 0.25°C if required (this facility can prove useful in environmental sex determination experiments). The probe from the temperature module is placed in free air at exactly the same height as the egg trays, and a second probe, resting next to the eggs and in contact with the substrate, leads to a digital thermometer located on the front panel.
Incubators are fitted with two lids, one made of heat resistant transparent plastic and a second of lined plywood. To inspect the eggs only the outer lid need be lifted, thus preserving temperature stability during extended periods of observation (but see notes concerning ventilation). The eggs are not uried but rest in a plastic ice cream or margarine container on a safe substrate of moist vermiculite. Sand is not a good or safe material to use as a substrate.
The time required for incubation is related to temperature, higher temperatures producing progressively shorter times. At 31°C most Testudo species hatch in 55 - 68 days. Very high temperatures, although producing shorter incubation times can result in severely deformed hatchlings. Tortoise eggs, like crocodile eggs, are subject to environmental sex determination (ESD). The precise points at which this operates are not known for all species y any means, but in the case of a T. hermanni, for example, eggs incubated at or below 29.5°C tend to result in male hatchlings and those incubated at or above 32°C tend to result in females
What incubation temperatures?
For most species, the best result is obtained if the incubation temperature is maintained between 30°C and 31.5°C. Also in this temperature range a mixed sex brood is most likely result. Tortoise eggs are subject to ESD or Environmental Sex Determination. In most cases, lower temperatures produce males and higher temperatures produce females.
How long does incubation normally take?
As mentioned previously, it depends to a great extent upon the temperature at which they are incubated. In a temperature range suggested, between 55 - 75 days is about average for most species. The lower the temperature the longer it takes. At around 27°C it takes about 126 days or so. At temperatures above 34°C there is a risk of deformation and mortality. For best results incubate at a medium range temperature.
Is humidity important?
The importance varies with species. Those species that produce short soft-shelled eggs are much more critical in this respect. For example the American Box turtle eggs need to be incubated in a very high ambient humidity level or they will dehydrate. For most hard-shelled eggs, a medium humidity is usually adequate (65-80%). The best way to achieve this is to place a wet sponge in a tray of water somewhere in the incubator. For eggs that require higher levels, use a secondary enclosure in the main incubator.
How long do hatchlings take to leave the egg?
From the time they initially peirce the egg it usually takes 8 - 24 hours. The first small fracture is to permit air breathing to begin - prior to his time the embryos oxygen demand has been met via permeation through the eggshell. The first small hole is gradually enlarged over the next few hours. The hatchlings may then sit in the egg for some time whilst its egg sac is absorbed. Until this is absorbed, the hatchling remains especially vulnerable, as movement is seriously impaired.
Do all the eggs from a single clutch hatch at a time, or are some later than others in leaving the egg?
There can certainly be a considerable delay between the emergence of the first hatchling to the last - in the case of the Mediterranean Tortoises we have experienced periods of upto 18 days and even longer may be possible. With some tropical species the time scale can be very extended indeed (This effect is noted even in highly accurate and stable incubators, and only seems to happen occaisonally).
Should I bury the eggs in sand during incubation?
Definately avoid sand. It does not allow sufficient gaseous exchange to occur, and as a result anoxia is a real danger. It is better to use an artifical, lighter medium such as "Vermiculite", or instead any non-toxic granular material. The eggs only need to be rested on the surface of the substrate, and not buried in a good quality incubator. Bury the probe of the thermometer alongside the eggs to keep a constant check on conditions.
The electronic section of the suggested tortoise incubator system comprises three components:
The main temperature controller module
This is an extremely accurate, high reliability electronic proportionate thermostat. It simply plugs into the wall. There are two wires leading from it. One of these is the temperature sensor; the other goes to the heating element. Various models are available, and may be equally suitable. Just ensure that it is a "proportional controller" with a seperate sensor probe, and covers the range 27 - 35°C. You can use less sophisticated thermostats of course - but you will sacrifice both accuracy and reliability.
The heating element
A heat mat is designed especially for vivarium use. Alternatively, a tubular heater or industrial heater can be used. For an outer box of the dimensions suggested, a power rating of 25 - 40 Watts is adequate. Whatever heater is used, you must ensure that it is fixed securely and is a type approved for use in a confined space. Follow all electrical instructions carefully, and see qualified advice if in doubt. We strongly recommend you fit fuses of an appropriate rating and ELCB or earth-fault detector be used (to ensure that fatal electrocution does not occur under any circumstances).
The digital temperature module
This has one wire leading from it to the probe. This unit gives a very accurate readout in degrees centigrade.
Build a suitable box. This can be made from chipboard or plywood. If you use a 40W element, we would suggest that your box is no bigger than 18" x 18" x 12". If you decide you want a larger box then we suggest that you use a 60W element. Polystyrene tiles glued to the outer panes are a good way to insulate your incubator tank. If your incubator is stored on a cold garage then the element also needs to be more powerful than if it were stored in a warm room.
Position the heating element in the bottom of the box. The more evenly it covers the floor area the more even the heat distribution will be. All eggs must rest at the same level. Therefore, construct a wooden slatted shelf at about 1/3 the height of the incubator, above the element. The eggs should be placed in plastic containers. The best substrate is damp vermiculite. The temperature probe from the digital thermometer should rest immeditately next to the eggs.
The thermostat probe (from the control unit) should be positioned at the same height as the eggs but should not be placed in the egg trays. The best position will depend on the design of the incubator box, but generally it should be in the air and not in direct contact with any object (if it touches anything it will reduce the sensitivity and speed of response)
Oxygenation & INCUBATION CONDITIONS
One potential problem area often overlooked by those involved in captive breeding testudines is the requirement of developing eggs for oxygen; admittedly this demand is low in the early phase of development but it does increase. Where anoxia of eggs is allowed to occur an increased number of "dead-in-shell" incidents will be noted. Embryonic anoxia can also result in hatchlings leaving the egg early. Where eggs are incubated in conditions where oxygenation and gas-exchange potentials are limited, hatchlings will be seen to leave the eggs in a weak, feeble condition often bearing unusually large egg sacs.