Radiated Tortoise eggs are developmental surprise eggs

The hatching of ray turtle eggs is incredibly exciting and requires some patience, because it is not possible to make a reliable prediction about the hatching time. The eggs in a clutch can take different lengths of time to develop (bimodal development). This variable in the breeding cycle could be an adaptation to an extremely dry habitat, with different development times possibly intended to favor hatching during food-rich rainy periods. It is also quite possible that embryo development is stimulus-driven, i.e., controlled by the rise or fall of temperature and humidity. Therefore, incubation according to Malagasy seasons is particularly popular and also recommended (in the Emys issue volume 9 / issue 2 this method is described in detail).

After egg laying, the eggs are carefully dug out, cleaned with warm water without turning them, marked on the top (e.g. with the laying date and name of the female) and transferred to plastic trays filled with vermiculite. Alternatively, other coarse-grained substrates such as expanded clay, sand, etc. are suitable. Substrates that absorb too much moisture are unsuitable. Burying the eggs in the substrate has also not proven successful in incubators due to the formation of waterlogging. Too much humidity and water drops on the egg will cause the embryo to suffocate and the egg will rot or mold. We simply place the eggs on coarse vermiculite and refrigerate them only slightly so that they cannot roll away.

For incubating ray turtle eggs, the following two incubator types have proven to be most effective. Both models have a day-night temperature function, and high temperature peaks can also be programmed.

  • Jaeger Bruttechnik Model FB 80 (with built-in temperature night setback)

    + manual setting of a natural day-night temperature curve
    + relatively high temperature peaks can be generated
    + large capacity for several trays with clutches
    + easy operation and setting
    + easy cleaning of the entire device
    + robust equipment
    + Water reservoir can be operated with both distilled and normal tap water

    - Viewing lid must be darkened with a cloth
    - the precise adjustment, measurement and verification of the desired incubation model can take several days
    - no digital system monitoring with signal of temperature and humidity
    - older models do not have digital display of temperature and humidity


    (The Jaeger Bruttechnik models FB 80 with built-in temperature night reduction, are unfortunately currently no longer manufactured. However, there are still some second-hand models and spare parts available).

  • Rcom Pro 230 Digital Reptile Incubator

    + digital setting of a natural day-night temperature curve
    + relatively high temperature peaks can be generated
    + easy operation, setting and commissioning of the desired incubator model
    + digital system monitoring with visual and audible signal of temperature and humidity.
    + digital display of temperature and humidity

    - Viewing lid must be darkened with a cloth
    - small capacity with several clutches
    - Relatively cumbersome cleaning of the individual parts
    - monotonous sound of the fan
    - system-monitored water reservoir may only be operated with distilled water due to the sensitive inner workings


Basically, different temperature models are possible when incubating Radiated Tortoise eggs. Timing at relatively moderate, constant temperatures of 29 - 31°C, during 24 hours, is possible as well as incubation schemes with a clear day-night temperature gradient. For example, successful incubation occurs without problems at 30°C - 33°C for 10 hours and 24°C - 28°C for 14 hours. Humidity is always about 70 - 80%. The water reservoirs of the respective incubators always remain completely filled. The eggs are not sprayed and the substrate is not moistened. Water drops, which can accumulate around the eggs, should be avoided at all costs.

Eggs that have been fertilized may show the beginning of a light developmental band when candled after only 14 days. This makes the eggshell of fertilized eggs appear much whiter than that of unfertilized eggs. Eggs that appear completely pale yellow and pale when candled or in which the yolk has sunk and is dark in color are unfertilized and already show signs of decomposition. Eggs that are hollow and light to the touch or begin to smell can be tapped with a clear conscience to check the contents and discarded. However, eggs that show inconspicuous, indifferent and normal coloration when candled and show no signs of mold or decay still have the potential for further development and should definitely be incubated further.

Radiated Tortoise eggs that show no signs of development after months in the incubator can be experimentally stored in the dark, dry and cool at about 10 - 15°C for about 2 - 3 months and then transferred back to a regular incubation scheme. We hypothesize that diapause occurs more frequently when the temperature curves during incubation are rather flat and approximately constant. This is completely independent of whether the temperatures are high, medium, or low. We suspect that diapause does not occur in clutches incubated at distinct temperature curves with a distinct day and night gradient. The same principle applies to humidity. The more clearly a trigger stimulus in the form of pronounced humidity and/or temperature is initiated, the less likely a diapause will occur. Some breeders use an artificial diapause with cooling of the eggs at the beginning of each breeding cycle preventively and successfully.

Fertilized but not yet developed eggs of Radiated Tortoises show a surprisingly high tolerance to strong temperature fluctuations during the day and night as well as seasonally constant cool temperatures. Despite extreme conditions, eggs can still have the potential to develop after more than a year (!). This astonishing ability is probably due to completely unexplored biochemical processes inside the egg, which prevent premature spoilage.

Sex specific breeding

Due to the acute shortage of females in many stocks, we now only breed with high-temperature incubation schemes in order to produce as many females as possible. Experimental approaches with long, high temperatures led to increased drying out and death of the embryos in the eggs. The question arose, how does a balanced sex distribution actually arise in nature, and how can we change the model in favor of a high proportion of females? Constant, high temperatures as in incubators of over 33°C for 24 hours do not occur near the ground and nesting pits in nature and yet the sexes always hatch relatively reliably equilibrium in the wild, how can this be? Females are known to be timed at high temperatures, which in nature can only occur over midday, when the sun is at its peak. In fact, the temperature in egg pits is often well above 34°C during midday, but only for a short time. In nature, therefore, the primary conditions for the timing of female embryos are high temperatures. Temperature peaks of +34°C over a period of about 3 hours is necessary to already allow a relatively balanced sex distribution in a clutch. If the high temperature peaks of +34°C are extended to 5-6 hours per day, the sex distribution automatically shifts in favor of female emryons. As in nature, the following longer temperature drops during the night obviously have no decisive influence on the sex determination. Only the reaching of relatively high and short temperature peaks during the day are responsible for this. Sexual development is already completed in the first third of the breeding cycle. A strong temperature reduction during the night to 24°C is even desired to initiate embryonic development at all and to avoid a diapause. In nature, however, other decisive factors are responsible for the sex-determining breeding temperature in nesting pits: cloudy days, the depth of the nesting pit and the position of the eggs in the nest, but also the shadows cast by neighboring vegetation or objects (e.g. trees, bushes, stones, roots, mounds, etc.), depending on the position of the sun, can have a decisive influence on the climate in turtle nests.

We recommend the following incubation schemes for the timing of the sexes:

  • Scheme for the incubation of females: +34°C (max. +34.6°C) for 5 to 6 hours with subsequent slow temperature reduction to +26°C to +24°C, humidity 70-80%. To verify this incubation scheme, we had random samples of juveniles from different vintages that were timed with this incubation scheme endoscopically examined for sex. All examined animals were confirmed as females.

  • Scheme for the incubation of males: +30°C to a maximum of +32°C for 5 to 6 hours with subsequent slow temperature reduction to +26°C to +24°C, humidity 70-80%.

We suspect the apex of sexual incubation in Radiated Tortoises to be 32.8°C. If incubation temperatures are long and high, the small tortoises often hatch too early, so that the yolk sac is not yet completely depleted. If incubation temperatures are too high and prolonged, embryos die halfway through development in the egg due to desiccation. Carapace abnormalities seem to be more common with unnatural, constant, and high temperatures often used in incubators.