Indian / Sri Lankan star tortoise (Geochelone elegans) eggs can be incubated at a constant temperature, but Burmese star (Geochelone platynota) eggs benefit from, or require, a cooling period before incubation.
Generally, eggs are placed on dry or slightly damp vermiculite, or other suitable incubation medium, and partially embedded (1/3 - 1/2 egg) into the substrate. Humidity in the incubator should be above 60% and up to 80+%.
If the incubator has no built-in water troughs on the bottom, you can place a shallow dish of warm water into the incubator to raise the ambient humidity level.
The trick is to get the substrate dampness and the air humidity just right inside the incubator. If the substrate is too wet, the 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 without wetting the eggs.
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 (TDSD, TSD), if done with accurate incubators and thermometers, has been observed to be quite reliable in Indian and Burmese star tortoises. The threshold (pivotal) incubation temperature is said to be around 86.9 °F (30.5 °C). Roughly equal number of males and females and produced at that temperature.
Based on TDSD results in Indian 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 Peter 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, see below).
According to Dr Zovickian, a premier US radiated and 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 and Burmese star tortoises. Based on his extensive experience, correctly done TDSD is almost 100% accurate in Indian and Burmese star tortoises (Kingsnake forum, 2010).
Caution! Higher incubation temperatures, especially above 90 °F (32.2 °C), are more likely to introduce birth defects.
Hatchlings incubated at a specific temperature are often sold as "temperature sexed females" (TSF's) or as "temperature sexed males" (TSM's). However, this is NOT a guarantee of sex. Scroll down to the last section on this page for more info.
Burmese star tortoise mom nesting in the early evening hours.
A close-up of her eggs in the nest. She has laid as many as 12 eggs and as few as 6 eggs in a clutch.
This Burmese star cutie decided to hatch upside down and bottom first, but he was ok. :0)
Much of the Indian star incubation info below applies to Burmese star tortoises as well, but their eggs benefit from an initial cooling period. This will help break the the arrested development of the eggs known as diapause.
Per Fife, cooling 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 Peter 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 lose up to 20% of their body weight after nesting (Dr Liu, 2000, website gone).
Below are some examples of published incubation temperatures for Burmese star tortoise eggs. For more details, please read the original publications.
1.) Jerry D. Fife, Drew Rheinhardt (The Batagur #3, 2013)
Burmese star tortoise eggs are incubated at room temperature (75-79 °F, 23.9-26.1 °C) for a week and then cooled for 30 days at 65-70 °F (18.3-21.1 °C). After the cooling period, eggs are again kept at room temperature for a week. Then, the eggs are placed into the incubator for a constant incubation temperature of 82-89 °F (27.8-31.7 °C). Lower temperature is used for males and higher for females. Eggs will hatch in about 90 days.
2.) Andreas S. Hennig (Incubating Chelonian Eggs, 2020)
This book provides six different sources for Burmese star tortoise egg incubation. Recorded temperatures range from 20 °C (68 °F) to 31 °C 87.8 °F). Perlite, peat moss, vermiculite, and gravel are listed as incubation substrates. No humidity info given.
3.) Gerald Kuchling, Eric Goode, 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).
4.) Chris Leone (GardenStateTortoise.com, 2013)
Eggs are first kept at room temperature (around 75 °F, 23.9 C°) for about a week and then they are cooled for about a month at 65-70 °F (18.3-21.1 °C). After cooling, they are again kept at room temperature for a week. Finally, they are incubated at 86-90 °F (30-32.2 °C). Incubation lasts about 90 days.
5.) Chris Leone (Reptiles Magazine, Nov/Dec 2016)
First, eggs are put into deli cups filled with dry vermiculite and cooled at around 65 °F (18.3 °C) with 70-80% humidity for 30 days. Then eggs are placed into new deli cups with damp vermiculite (1:1 vermiculite to water by weight) and incubated at 86-89.6 °F (30-32 °C) with 80-90% humidity. During both cooling and incubation, the deli cup have lids with air holes. Incubation lasts 88-122 days.
6.) 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.
7.) Ken Siffert (Tortoise Forum, Mar 2014)
First, eggs are put through a diapause for 30 days at 65 °F (18.3 °C). Then eggs are incubated at 85 °F (29.4 °) - 89 °F (31.7 °C) depending on the sex preference for the babies. If you want to be safe, use 86 °F (30 °C) that will result in a 50/50 mix of genders. The higher incubating temperature of 89° F (31.7 °C) will produce females, but can also induce split scutes. Eggs are incubated on vermiculite mixed with water 1:1 by weight.
8.) Toronto Zoo, Canada (TorontoZoo.com, Aug 2016)
Toronto Zoo hatched their first Burmese star tortoise in July 2014 (first in Canada) and their second in July 2016. Eggs were initially cooled at 18-20 °C (64.4-68 °F) for four weeks to break the diapause, and then they were incubated at 28-30 ° C (82.4-86 °F). The first egg hatched after 135 days of incubation and the second one hatched 230 days after it was laid.
9.) 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.
10.) 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.
11.) 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.
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. After pipping, I place hatching babies in small deli little cups lined with wet paper towels.
Just hatched Burmese star tortoise with a small residual yolk sac. He will stay in the incubator inside the deli cup until his belly is sealed and flat.
Incubation temperatures for Indian / Sri Lankan star tortoise 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. Alternately, some breeders use dry vermiculite with no water added.
Below are some examples of published incubation temperatures and humidity levels for Indian 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 (82.4 °F), or are first incubated at 33-34 °C (91.4-93.2 °F) for 80 days and then at 28 °C (82.4 °F) 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 (83.3 °F), or 30.5 °C (86.9 °F), or 31.5 °C (88.7 °F). 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.) Andreas S. Hennig (Incubating Chelonian Eggs, 2020)
This book provides twelve different sources for Indian star tortoise egg incubation. Recorded temperatures range from 22 °C (71.6 °F) to 34 °C (93.2 °F); around 28-32 °C (82.4-89.6 °F) being common. Vermiculite is the most commonly used incubation substrate, but sand/soil mix, river sand, soil, and pebbles are also listed. Relative humidity varies from 45% to 80%.
6.) Ivo E. Ivanchev (Schildkröten im Fokus Online, 2012:3)
Eggs are placed half buried in plastic containers filled 3-4 cm (1.2"-1.6") deep with 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.
7.) Monika & Johannes Janssen (Radiata #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 (89.6- 91.4 °F) for the first two months, and then temperature is reduced to 29-30 °C (84.2-86 °F). Humidity is kept at 70-80%.
8.) Gunther Koehler (Incubation of Reptile Eggs, 2004)
Eggs are incubated at 26-30 °C (78.8-86 °F). Incubation lasts 109-147 days.
9.) Antonio Sanz & Francisco Javier Valverde (Reptilia #9, 1999)
Eggs are half buried in damp vermiculite (3:1 vermiculite to water) and incubated at 31-32 °C (87.8-89.6 °F) to produce more females. Average incubation lasts 90 days.
Caution! Just because you buy tortoise hatchlings that were supposedly incubated for female, aka as temperature sexed females or TSF's, it does NOT guarantee that the hatchlings will actually be females. Over the years, I have heard of many cases where Burmese star tortoise hobbyists have bought TSF's that turned out to be males. This has happened to me as well. Always very disappointing if hoping to add females to your breeding crew.
Numerous reasons could explain why temp sexed females are actually males. Maybe the temperature dependent sex determination is not quite as accurate as it's considered to be, or there are other factors influencing the gender development.
Temperature inside the incubator tends to fluctuate somewhat following the room temperature ups and downs. Also, the location of the eggs inside the incubator can make a difference. The closer the eggs are to the heating element, the higher they are incubated.
Most breeders do not use super accurate scientific incubators, so the incubation temperature could have been be more or less off the target. Or maybe the thermometers used were not very reliable. The breeder may not know the actual incubation temperature, and in worst cases may not even tell the whole truth to make a sale. Some breeders may also prefer to incubate for males in stock that's intended for sale.
By the way, there's nothing wrong about having male tortoises. In fact, they usually make excellent pets. Many times males are bolder, more active, and more personable than females. Plus, you don't have to fuss about suitable nesting sites or worry about egg binding (retained eggs can cause serious illness or death).