A C Highfield
Every year the Tortoise Trust receives a large number of queries concerning eggs and hatchlings. This short article covers some of the more frequently raised questions.
In order to develop properly tortoise eggs need to be incubated within a specific temperature and humidity range. The incubation temperature is especially critical. If the eggs are incubated at too low a temperature development will be very slow or the eggs may fail to hatch. Excessively high temperatures can lead to deformity. For best results always use a reliable thermostat and thermometer when incubating eggs. Solid-state (electronic) units are by far the most effective in this application. It is not a good idea to incubate eggs on a ''hit or miss'' basis. Not only is it unlikely to prove consistently successful in the long run, but it is hardly fair on the eggs themselves (which may eventually contain baby tortoises) or to the female who went to a great deal of trouble to lay them.
HOW CAN I TELL IF A FEMALE IS GOING TO HAVE EGGS?
Most females will begin to behave somewhat strangely in the period immediately before they are due to lay. Typical behaviour includes aggression displays towards other females, hyper-activity and climbing over obstacles, even attempts to 'mate' other tortoises (male or female) including making the high pitched vocalisations usually only heard from males. Females carrying eggs may also reduce their food intake, and sometimes may stop feeding altogether just prior to laying. Another way to tell is to ask your veterinary surgeon to X-ray a tortoise you suspect may be pregnant - although this is really only justified in ''problem'' cases.
WHAT SORT OF NEST-SITE DOES SHE REQUIRE?
Some female tortoises can be very selective about what constitutes a suitable nesting site. Others are less discerning. In general however nearly all prefer a site which is on a slope, is well drained and has light but relatively well textured soil which is easy to excavate. Very sandy sites, especially those without plant roots to bind the sand together, are often not favoured. The female must be able to dig a bell-shaped excavation without too much difficulty, but at the same time the soil must not be so dry or so loose that the nest collapses in on itself.
HOW MANY EGGS DOES A TYPICAL FEMALE LAY?
It varies according to the species. Some species only lay a single egg. Others lay up to 30. Even within some species there can be considerable variation - for example, Testudo hermanni boettgeri (the eastern or balkan Hermann's tortoise) usually lays between 6-10 eggs. By comparison, Testudo hermanni hermanni (the western Hermann's tortoise) typically only lays 3 eggs per clutch. On average, most Mediterranean tortoises lay 5 or 6 eggs per clutch, and many are capable of laying 2 or more clutches per season.
IS IT IMPORTANT TO KEEP THE EGGS THE SAME WAY UP AS THEY WERE LAID?
No, it isn't. At least not initially. Once embryonic development is underway however orientation should not be disturbed. Always handle eggs very carefully- at whatever stage of development.
CAN I 'CANDLE' EGGS DURING INCUBATION TO CHECK IF THEY ARE VIABLE?
Yes, you can. But we do not recommend that you should. It involves unnecessary additional handling and does not actually make any difference to the final outcome. Rather than subjecting eggs to such examinations, we recommend patience!
IF EGGS ARE LATE IN HATCHING AT WHAT STAGE SHOULD I CRACK THEM OPEN TO HELP ANY HATCHLINGS OUT?
Our view is that this sort of intervention is rarely successful and is in addition highly dangerous - how can you be sure that the eggs really are 'late'?. If hatchlings are so weak that they are unable to leave the egg unaided then something is already going drastically wrong somewhere. We would look to a genetic incompatibility or poor incubation technique. Anoxia, or oxygen starvation is one possibility. Prevention is infinitely better than the 'cure' in this instance.
CAN FEMALE TORTOISES HAVE EGGS EVEN IN THE ABSENCE OF CONTACT WITH A MALE?
The short answer is yes, they can, but are in fact less likely to.
IS THERE A SAFE MAXIMUM AGE FOR BREEDING FEMALES?
Not exactly, but if a female has not had eggs for many years we would be very wary about subjecting her to breeding attempts. There is a high incidence of (fatal) egg-binding in such cases. It is much safer, and considerably more successful, to employ only healthy young females in captive breeding exercises. These very rarely experience any problems and fertility rates are also likely to be much higher.
ARE EGGS LEFT LYING ON THE SURFACE EVER FERTILE?
Just occasionally. More often than not they are infertile. If no attempt at all was made to nest them properly, it is probable that the female was merely discarding them.
SOMETIMES THE EGGS PRODUCED BY MY FEMALES HAVE A ROUGH TEXTURE, ALMOST LIKE THE SURFACE OF A GOLF-BALL. WHAT DOES THIS INDICATE?
Such eggs have usually been retained for a much longer time than is normal - or safe. Investigate nest-site availability. This sort of egg usually has a much thicker than usual shell, and in our experience is never fertile. Such eggs are often implicated in egg-binding and can prove very difficult to lay.
WHAT INCUBATION TEMPERATURES DO YOU SUGGEST?
For most species, the best results are obtained if the incubation temperature is maintained between 30C and 31.5C. Also in this temperature range, a mixed sex brood is most likely to result. Tortoise eggs are subject to ESD or Environmental Sex Determination. In most cases, lower temperatures produce males, higher temperatures result in females.
HOW LONG DOES INCUBATION NORMALLY TAKE?
As mentioned previously, it depends to a great extent upon the temperature at which the eggs are incubated. In the temperature range suggested, between 8-11 weeks is about average for most species. The lower the temperature the longer it takes. At around 27C for example, incubation can take 18 weeks or more. At temperatures above 34C however there is a grave risk of mortality or deformity among the hatchlings. It is best to incubate at the medium range temperatures suggested.
IS HUMIDITY IMPORTANT?
The importance varies with species. Those species which produce softer shelled eggs are much more critical in this respect than those which produce hard-shelled eggs. A good example of the former would be American Box turtle eggs which need to be incubated at a very high ambient humidity if they are not to dehydrate. For most hard-shelled eggs, a medium level of humidity is generally adequate. The easiest way to achieve this is usually just to place a damp sponge in a tray of water somewhere with in the incubator itself.
HOW LONG DO BABY TORTOISES TAKE TO LEAVE THE EGG?
From the time when they initially pierce the egg, it usually takes between 8-24 hours. The first small fracture is to permit air breathing to begin - prior to this time the embryos oxygen demand has been met via permeation through the egg shell. This first small hole is gradually enlarged over the next few hours. The hatchling may then sit in the egg for quite some time whilst its egg sac is absorbed. Until this is absorbed, the hatchling remains especially vulnerable as movement is seriously impaired.
DO ALL THE EGGS FROM A SINGLE CLUTCH HATCH AT ABOUT THE SAME TIME, OR ARE SOME LATER THAN OTHERS IN LEAVING THE EGG?
There can certainly be a considerable delay between emergence of the first hatchling and the last - in the case of Mediterranean tortoises we have experienced periods of up to 18 days and even longer may be possible. With some tropical species the time scale can be very extended indeed. This effect is noted even in highly accurate and stable incubators, and only seems to happen occasionally.
WHAT SORT OF INCUBATOR DO YOU RECOMMEND?
By far the best sort of incubator uses an electronic proportional thermostat and ultra-reliable heating element combination. Commercial reptile-egg incubators are available. You can however make do with less. What we do not recommend is relying upon airing cupboards or light bulbs as your heat source. Whilst these can work, they are in reality far too unpredictable and unreliable to guarantee a secure incubation environment for on average a continuous period of operation lasting 8-10 weeks or more. When light bulbs are used as the heat source they invariably fail during the most critical phase, just as the egg is about to hatch!. Stability and reliability are the two key precepts to adhere to when incubating tortoise eggs.
SHOULD I BURY THE EGGS IN SAND DURING INCUBATION?
We suggest you definitely avoid sand. It does not allow sufficient gaseous exchange to occur, and as a result anoxia is a real danger. It is better to use an artificial, lighter medium such as 'Vermiculite', or instead any non-toxic granular material. Bury the probe of a thermometer alongside the eggs to keep a constant check on conditions.
For comprehensive data on all aspects of captive breeding, including advanced level guidance on incubation and captive care you are advised to consult our book and video lists.