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Artificial incubation techniques in relation to Testudo graeca & T. hermanni with notes on embryonic anoxia as a possible factor in hatchling mortality in captive breeding programs.

A C Highfield

Incubator Design

The incubators used by the Tortoise Trust and in the Tortoise Survival Project captive breeding program are based upon the design shown in plate 1. This comprises a faced chipboard box measuring approximately 18" X 24" X 12". Heat is provided by a 60W industrial quality ceramic heating element located at one end, and temperature control is provided by a proportional electronic module which avoids the major fluctuations experienced with simple on-off type thermostats (Highfield, 1987). 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 a high-low warning if required.
The incubators are 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. 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.

Comparative egg morphology - Testudo species

There are considerable morphological differences in the construction of eggs between various species. These differences are consistent and can be of use in taxonomic diagnosis (Highfield, in press). For example, the eggs of the east European Testudo hermanni boettgeri MOJSISOVICS 1889 (Bour, 1987) typically measure some 38mm X 30mm and are markedly elongate. By comparison the eggs of Testudo ibera PALLAS 1814 typically measure 36mm X 29mm and those of the true Testudo graeca LINNAEUS 1758 from Oran, Algeria are a diminutive 29mm X 28mm and are roundish rather than obviously elongate. The more easterly Algerian Testudo whitei BENNETT 1836 (Highfield & Martin, 1989) produces eggs which are very much larger and a different shape than those of T. graeca at 36mm X 28mm. There are similar differences to be seen in the hatchlings, with true Testudo graeca L. 1758 typically emerging at 27mm/8g, Testudo ibera PALLAS 1814 at 33m/13g, T. hermanni boettgeri MOJSISOVICS 1889 at 33mm/12g and Testudo whitei at 34mm/14g. Early phase growth is also variable between species, but very consistent within species (Highfield, in press). A recently discovered species of land tortoise endemic to Tunisia lays eggs which are the smallest of any known terrestrial chelonian at only 15mm long by 13mm broad.

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 (it seems very probable that a rise in blood Co2 levels may act as the 'trigger' for emergence). 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 too 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 genetic mismatches) 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 good and permeability comparatively high.

Hatchling diet

To avoid secondary nutritional osteodystrophy and osteoporosis it is extremely important that the chemical profile of the diet conforms as closely as possible to that experienced in the wild. In respect of Testudo species (and virtually all other herbivorous terrestrial chelonians) this essentially means that it should be low in protein, high in fibre, low in fat, rich in minerals and adequate in vitamins. The Ca:P ratio should be 5:1 or higher. Artificially high protein diets result in enhanced blood urea levels and subsequent renal problems, fatty infiltration of the liver (steatitis) and excessive growth of keratin resulting in severely 'lumpy' and deformed carapaces (Highfield, 1989). All of these problems are 100% preventable if a sensible dietary regime is adhered to. Our own hatchlings receive a diet based largely upon wild plant foods supplemented with additional green leaf vegetables and fruits. 'Vionate' boosted with additional calcium is provided at every meal. No meat products (e.g dog food or cat food) are given either to the hatchlings or the adults - it is detrimental to their health and development and is not required. The average protein content of most Testudo species diets in the wild is circa 4%. Anything consistently in excess of this can and does cause serious developmental problems.
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 higher levels of ambient humidity, but the same design of incubator and incubation environment has proved successful with many Geochelone species. At present, eggs from Malacochersus tornieri, Chelonoidis carbonaria and T. horsfieldi are incubating under similar conditions.


  • Bellairs, A. (1969) The Life of Reptiles. Weidenfeld & Nicholson. London.
  • Bour, R. (1987) L'identite' des Tortues terrestres europeennes: Specimens-types et localite-types. Revue fr. Aquariol 13 (4):111-122
  • Highfield, A. C. (In press) Studies in Comparative Morphology of the eggs of Tortoises, genus Testudo and their relevance as Characters in Diagnostic Taxonomy.
  • Highfield, A. C. (In press) Comparative early phase growth in Palearctic land tortoises genus: Testudo.
  • Highfield, A. C. (1987) Electronic temperature Measurement and Control for Incubators and Vivaria. The Herptile. Journ. International. Herpet. Soc. Vol.12 (4):130-133
  • Highfield, A. C. (1989) Notes on dietary constituents for Herbivorous terrestrial chelonia and their effects on Growth and Development. ASRA (U.K) Journal Vol.3 (3):7-20
  • Highfield, A. C & Martin, J. (1989) Testudo whitei BENNETT 1836; New light on an old carapace - Gilbert White's Selborne tortoise re-discovered. Journ. Chelonian Herpetology 1(1):13-22