Indian / Sri Lankan & Burmese Star Tortoises
Hatching Indian Star tortoise. Photo courtesy of Andrew Lewis, UK.
A. INDIAN / SRI LANKAN STARS (Geochelone elegans)
Incubation of eggs
Incubation period for Indian / Sri Lankan Star tortoise eggs is variable. Reports range from 47 to 257 days. In captivity, the average is around 90-120 days depending on the incubation temperature.
Indian / Sri Lankan Star tortoise eggs can be incubated at a constant temperature. Burmese Star eggs on the other hand benefit from, or require, a cooling period before incubation.
Eggs are placed on slightly damp vermiculite, or other suitable incubation medium, and can be partially embedded (1/3 - 1/2 egg) into the substrate. This way eggs will stay dry enough, but benefit from the surrounding humidity.
If the incubator has no built-in water troughs on the bottom, you can place a shallow dish of warm water in the incubator to increase the humidity. Humidity in the incubator should be above 60% and up to 80+%.
The trick is to get the substrate dampness and the air humidity just right. If the conditions in the incubator are too wet, eggs can absorb too much water, swell, and crack. It the air is too dry, the eggs can dehydrate or become too tough for the hatchlings to break through. When in doubt, it's safer to keep the substrate a bit too dry than too wet.
Some Star tortoise breeders place eggs on dry vermiculite and then lightly spray the eggs with water as needed. Others strongly advice against egg spraying because this can cause eggs to crack. Instead, small amounts of warm water can be added directly into the incubation substrate.
Introducing polluted water into the incubator can contaminate eggs and subsequently kill them. Distilled water is safest to use. It has been boiled to steam, and major bacteria, fungi, and other contaminants have been removed in the process.
Temperature dependent sex determination
Temperature dependent sex determination (TSD, TDSD) has been observed to be quite reliable in Indian / Sri Lankan and Burmese Star tortoises. The threshold (pivotal) temperature is said to be around 86.9 °F (30.5 °C).
Based on TDSD results in Indian / Sri Lankan Star tortoises, incubation temperatures of 88-89 °F (31.1-31.7 °C) produce more females and 84-85 °F (28.9-29.4 °C) more males. This probably applies to Burmese Stars as well (Dr Liu, 2000, website gone).
Per Fife, eggs incubated at 85-87 °F (29.4-30.6 °C) will result for mostly boys being hatched and eggs incubated at 88-90 °F (31.1-32.2 °C) for mostly girls (Star Tortoises, 2007).
According to Dr Zovickian, a premier US Star tortoise breeder, incubation temperatures of 88-90 °F (31.1-32.2 °C) produce females and 84-85 °F (28.9-29.4 °C) produce males in both Indian / Sri Lankan and Burmese Star tortoises. Based on his extensive experience, TDSD is almost 100% accurate in Indian / Sri Lankan and Burmese Star tortoises (Kingsnake forum, 2010).
However, higher incubation temperatures, especially above 90 °F (32.2 °C), are more likely to introduce birth defects.
Update 2012: See #5 below for some additional notes on the TDSD in Indian Stars (Ivanchev). Also, Turtle Conservancy's Behler Chelonian Center (TC/BCC) is currently conducting research on TDSD. For example, Burmese Star tortoise eggs were incubated under specific conditions geared towards gender selection, and then the resulting hatchlings were endoscopically sexed. Data from this research will be published in the future (BCC blog, Jul/Aug 2012).
Examples - Incubation temp & humidity (G. elegans)
Incubation temperatures for Indian / Sri Lankan Star tortoise (Geochelone elegans) eggs vary somewhat among breeders. Same for the vermiculite dampness, i.e., how much water is mixed with it. Vermiculite to water ratios by weight vary from 1:1 to 3:1. Many breeders use the "squeeze the water out of the vermiculite" method instead of weighing the amounts.
Below are some examples of published incubation temperatures and humidity levels for Indian / Sri Lankan Star tortoise eggs. For more details, please read the original publications.
1.) Hans J. Bidmon (Turtles, Proceedings: International Turtle & Tortoise Symposium Vienna 2002, 2006)
Egg box is filled with vermiculite that has been covered with fine gravel. Eggs are embedded 1/3 into the substrate. A small amount of water is injected into the vermiculite with a syringe. This is repeated every 3rd week. Eggs are incubated at 28 °C, or are first incubated at 33-34 °C for 80 days and then at 28 °C for the rest of the incubation period. Humidity above gravel is 86%. Incubation takes 86-123 days.
2.) Anslem de Silva (The Biology and Status of the Star Tortoise in Sri Lanka, 2003)
Eggs are half buried in wet vermiculite (1:1 vermiculite to water) and incubated at 28.5 °C, or 30.5 °C, or 31.5 °C. Incubation takes 84-166 days.
3.) Simon Girling, BVMS CertZooMed MRCVS (Pet Owner's Guide to the Tortoise, 2002)
Eggs are laid 60-90 days after mating in a 12-20 cm (5"-7") deep nest. Eggs are incubated at 29-31°C (83-86 °F). Average incubation lasts 90-120 days.
4.) Jerry D. Fife (Star Tortoises, 2007)
Eggs are incubated either at 85-87 °F (29-31 °C) for more males being hatched or at 88-90 °F (31-32 °C) for more females. Humidity in the incubator is maintained above 60%. Eggs are placed on damp vermiculite (2:1 vermiculite to water).
5.) Ivo E. Ivanchev (Schildkröten im Fokus Online, 2012:3)
Eggs are placed half buried in plastic containers filled with 3-4 cm (1.2"-1.6") deep vermiculite. Humidity is maintained at 70-80%. Hatching time varies depending on the incubation temperature: 97-101 days at 31-32 °C (87.8-89.6 °F) and 104-124 days at 29-29.5 °C (84.2-85.1 °F). Interestingly, the higher incubation temperature has produced at least one male! According to the author, other breeders have had females hatch at 28 °C (82.4 °F). These findings differ from the common thinking that higher temperatures always produce female Stars. Thus, either the pivotal temperature (equal numbers of males and females) varies, or other factors may be affecting the gender of hatchlings.
6.) Monika & Johannes Janssen (Radiata No. 4, 2009)
Egg container is filled with very moist vermiculite that is covered with a layer of small pebbles. Eggs are place on top and incubated at 32-33 °C for the first two months, and then temperature is reduced to 29-30 °C. Humidity is kept at 70-80%.
7.) Gunther Koehler (Incubation of Reptile Eggs, 2004)
Eggs are incubated at 26-30 °C. Incubation lasts 109-147 days.
8.) Antonio Sanz & Francisco Javier Valverde (Reptilia No. 9, 1999)
Eggs are half buried in damp vermiculite (3:1 vermiculite to water) and incubated at 31-32 °C to produce more females. Average incubation lasts 90 days.
Hatching Burmese Star tortoise. Photo courtesy of Royden Lepp.
B. BURMESE STARS (Geochelone platynota)
Incubation of eggs
Much of the above incubation info applies to Burmese Star tortoises as well, but their eggs may benefit from an initial cooling period. This will help break the diapause (arrested development).
Per Fife, keeping the Burmese Star eggs at 65-70 °F (18-21 °C) for about 30 days before incubation increases the hatch rate (Star Tortoises, 2007).
According to Dr Liu, Burmese Star tortoises lay eggs 90-120 days after mating. Clutches are laid at 30-50 day intervals. Eggs are deposited in flask-shaped, 15 cm (5.9") deep nests. Females can loose up to 20% of their body weight after nesting (Dr Liu, 2000, website gone).
Examples - Incubation temperature (G. platynota)
Below are some examples of published incubation temperatures for Burmese Star tortoise eggs. For more details, please read the original publications.
1.) Gerald Kuchling, Eric Goode and Peter Praschag (Endoscopic Imaging of Gonads, Sex Ratio and Temperature Dependent Sex Determination in Captive Bred Juvenile Burmese Star Tortoises Geochelone platynota, Asian Herpetological Research, 2011, 2:240-244)
Burmese Star tortoise eggs were incubated on dampened, chunky vermiculite with a 2:1 vermiculite to water ratio by weight. For the first 6-8 weeks, the eggs were kept in room temperature to provide a diapause. The temperature varied from 21 °C (69.8 °F) to 28 °C (82.4 °F) depending on the time of day. After the diapause, the eggs were incubated at either 28.9 °C (84.02 °F) or 30 °C (86 °F). Babies hatched after 115-124 days. Later, these youngsters were endoscoped to determine the genders. Endoscopic sexing is 100% accurate when done by a qualified practitioner.
Eggs incubated at 28.9 °C (84.02 °F) resulted in 10% females and 90% males. Eggs incubated at 30 °C (86 °F) resulted in 46.5% females and 50% males. Based on this, the pivotal temperature is crudely estimated to be close to or just above 30 °C (86 °F).
Update 2012: Turtle Conservancy's Behler Chelonian Center (TC/BCC) is currently conducting more research on TDSD. Burmese Star tortoise eggs were incubated under controlled conditions geared towards gender selection, and then the resulting hatchlings were endoscopically sexed during summer 2012. Data from this research will be published in the future (BCC blog, Update Jul/Aug 2012).
2.) Dr Peter Liu (2000, website gone)
Dr Liu incubates Burmese Star tortoise eggs at around 30 °C (86 °F). Eggs are placed in a plastic box filled with slightly damp vermiculite (1:1 vermiculite to water by weight). Holes are punched to the egg box lid, and the box is then placed into the incubator. Eggs are sprinkled with small amounts of water once a week. If the vermiculite is kept too wet, eggs will crack.
3.) TSA newsletter, Aug 2003
Wildlife Conservation Society's Wildlife Survival Center hatched their first Burmese Star tortoises at the St. Catherines Island tortoise facility. First, eggs were kept on dry vermiculite at 70 °F room temperature for 38-40 days. Then eggs were placed into the incubator and incubated at 84 °F (28.9 °C). Eggs were lightly misted weekly. Incubation lasted 108-116 days.
4.) TSA newsletter, Aug 2008
San Diego Zoo hatched their first Burmese Star tortoises in 2008. First, these eggs were kept at 18 °C (64.4 °F) for one month. Then, they were incubated at 30 °C (86 °F). Baby tortoises hatched at 128-135 days.
5.) TSA newsletter, Aug 2011
At the Wildlife Rescue Center of the Taipei Zoo, Burmese Star tortoise eggs were incubated between 28 °C (82.4 °F) and 30 °C (86 °F). Babies hatched at 184-186 days.
6.) TSA newsletter, Aug 2011
The Rotterdam Zoo incubated their Burmese Star tortoise eggs at 31 °C (87.8 °F). Babies hatched after 150-152 days of incubation.
Hatching Burmese Star tortoise. Photo courtesy of Ken Siffert.
See the incubators page for info on small home incubators and incubation substrates.
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