Incubation of Mediterranean Tortoise Eggs

One from our 'Historic' series! This was originally released on VHS (remember that?) and then DVD way back in 1999! Hence, the video quality is not close to what we can do today, but despite that, the information and designs it includes are still accurate and relevant. We hope you find it useful. As we gradually excavate our deepest vaults, we will bring you some more ancient history in due course!

Also, some additional details on the self-built incubators shown from a very early paper written by A. C. Highield in 1987 :

The incubators used in our Mediterranean tortoise captive breeding program were based upon a fairly simple design. This comprises a faced chipboard box measuring approximately 18" X 24" X 12" (45 X 86 X 30 cm). Heat is provided by a 60W industrial quality ceramic heating element located in the base (although a similarly powered heat-pad or small greenhouse heater is just as effective), and temperature control is provided by a proportional electronic module which avoids the major fluctuations experienced with simple on-off type thermostats . The design is capable of maintaining temperatures to +/- 0.25 degrees centigrade if required (this facility can prove useful in environmental sex determination experiments). The probe from the temperature module is placed at the same height as the egg trays, and a second probe leads to a digital thermometer located on the front panel. This can include an audible high or low temperature warning if required. Small electronic digital temperature modules are now fairly widely available from electronic stores such as Radio Shack (more ancient history, remember them too?) at very reduced prices compared with a few years ago. An even more useful development is that of combined humidity-temperature modules. These are absolutely ideal for incorporation into incubators and allow a degree of control previously unattainable in anything outside laboratory settings.

The incubators are each fitted with two lids, one made of heat resistant transparent plastic and the second of cork-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 re; ventilation). The eggs are not buried but rest in plastic ice-cream or margarine containers on a substrate of sterile peat, light soil and gravel. Contrary to much published advice, sand is not a good material in which to bury eggs during incubation. I have encountered many cases involving the suffocation of emerging hatchlings where this has been used; a lighter mixture of sandy soil or peat is much safer.

A third transparent plastic lid rests gently on each (to prevent unexpected hatchlings climbing out and injuring themselves). This is perforated to permit air to reach the eggs.

Humidity is provided by placing a tray of water containing sponges in the incubator alongside the egg trays. The level of humidity does not appear to be especially critical with the hard-shelled eggs of Testudo species, but we usually maintain it at around 70-80%. Each incubator holds approximately 30 eggs - we prefer to use multiples of these smaller incubators rather than use fewer larger incubators and (literally) place all our eggs in one basket!. So far, an incubator has never failed but it is obviously far safer to distribute any risk in this way. It also facilitates incubating batches at different temperatures. As a general starting point, 31 degrees C. is suggested for most Testudo species.

The time required for incubation is related to temperature, higher temperatures producing progressively shorter times. At 31 degrees C most Testudo species hatch in 8-10 weeks. Very high temperatures, although producing shorter incubation times can result in severely deformed hatchlings. For maximum success and safety, it is best to keep incubation temperatures within the range 29-32 °C. 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 by any means, but in the case of 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.

Oxygenation & incubation conditions

One potential problem area often overlooked by those involved in captive breeding chelonians is the requirement of developing eggs for oxygen; admittedly this demand is low in the early phase of development but it does increase (Bellairs, 1969). 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 (a rise in blood Co2 levels almost certainly acts as the 'trigger' for emergence in tortoises as it does in birds). 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.

Incubation in sealed incubators or in inadequately ventilated containers can drastically increase both pre-hatching and post-hatching mortalities. It may also contribute to neo-natal deformities. The practice of incubating eggs in sealed or almost-sealed containers is therefore to be strongly discouraged.

Conversely, the airflow of most bird egg incubators is far too high for the successful incubation of chelonian eggs and can easily lead to excessive drying and even to embryonic dehydration. Infertile eggs dehydrate most rapidly due to the lack of formation of protective internal membranes, but even fertile eggs can suffer fatal dehydration if subjected to extended periods of high airflow. We have found that if the incubator is opened once per day for a 30 second ventilation period this is quite adequate to flush any CO2 build-up and prevents any problems of this nature manifesting.

Whilst it is easier to maintain high levels of humidity in a sealed environment tests have shown that CO2 levels accumulate rapidly in such situations. Embryonic anoxia is quite possibly the hidden factor responsible for many 'dead-in-shell' and premature hatchlings encountered by captive breeders where other explanations (e.g incubator failure or parental genetic incompatibility) can be discounted.

Where a sealed incubation environment has been used and high levels of mortality encountered, a change to a more natural and aerated incubation method will often produce an immediate and dramatic improvement in survivorship. It is a myth that because eggs are buried underground they require no oxygen or do not need to ventilate waste gasses - in fact, soil oxygen levels of nesting sites are usually quite good and permeability comparatively high.

Hatching

Hatchlings pierce the eggshell using an egg-tooth like appendage ( actually an egg-caruncle) gradually enlarging this opening by biting small pieces from the eggshell and pushing with the front legs.

Immediately hatching begins the eggs should be kept under continuous observation. Hatching can take some time; between 2-5 hours is average for T. graeca, T. hermanni and T. marginata. Once access to air has been gained, the young tortoise will often stay in the egg for a day or more gradually gaining in strength and allowing time for the egg-sac to be properly absorbed. If a hatchling is in obvious trouble and is clearly weakening then careful assistance can be given. Provided that hatching is in full progress giving such aid will not do any harm.

Do not ever initiate hatching artificially - even if it appears that the 'normal' incubation period is long past and hatching is overdue. Tortoises have been hatching without assistance from humans for millions of years and really don't need our unwelcome (and usually untimely) interference. If you find yourself having to break open eggs to get live hatchlings then look again at your incubation technique. You are going badly wrong somewhere along the line.

Although primarily concerning the incubation of eggs from Testudo species, most (if not all) of the above observations apply equally to other species. Obviously soft-shelled eggs frequently require considerably higher levels of ambient humidity, but the same design of incubator and incubation environment has proved successful with many aquatic turtles and semi-terrestrial tropical species. At present, we also have eggs from Malacochersus tornieri, Chelonoidis carbonaria and Terrapene carolina incubating in these incubators.

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(C) 199-2025 A. C. Highfield/Tortoise Trust