Nesting Sites and Egg F.A.Q. Updated
- tortoisetrust
- 7 days ago
- 11 min read
Updated: 5 days ago
Failure to provide adequate nesting sites in captivity can have serious consequences for heath, including an increased danger of egg-retention leading to possible fatal peritonitis. The lack of acceptable nesting sites can also lead to increased stress, and negatively impact upon captive breeding success.
Tortoise nesting behaviour can be broken down into several distinct phases:
Nest site selection
Preparation of the nest site
Excavating the nest
Laying
Filling in and covering the nest
Different species exhibit markedly different preferences and behaviours in each of these phases, so it is important to understand the particular requirements of the species in question. For example, Mediterranean Testudo species tend to exhibit a strong preference for nesting on gentle slopes, with sandy, well-drained soils. Damp clay soils or soil that is too stony is likely to be rejected as unsuitable, as are nest sites on flat surfaces. By contrast, many tropical species such as Redfoot tortoises (Chelonoidis carbonaria) will accept flat nest sites readily, but typically prefer the soil to be rich in organic content, warm, moist, or even muddy. Mediterranean Testudo species usually prefer to lay in full sun during the day. Other species, however, such as Cheloniodis carbonaria or American Box turtles (Terrapene carolina) frequently lay at dusk, and especially during episodes of light rain when humidity is particularly high.
It is important to be aware of these traits, as the prevailing weather conditions or time of day can give a good general indication as to when nesting may occur. Temperature, humidity and light levels then, are just as important to nesting tortoises as the substrate itself.
Those species that typically prefer to nest in sandy soil frequently prefer to excavate their nests close to shrubs, or in sand that is infiltrated by fine plant roots. It is possible that this provides extra cohesion during excavation, making collapses less likely.
Species from rainforest type environments often utilise leaf litter in the construction of their nests; one species in particular, Manouria emys (the Burmese Brown tortoise) actually constructs a nest mound from leaf litter. The Yellowfoot tortoise (Chelonoidis denticulata) from South America is another species that is sometimes known to nest on the ground, under decaying leaf litter rather than excavate a nest in the usual manner. Unlike Manouria emys, however, it appears to make use of pre-existing accumulations of leaf litter rather than actively constructing a mound specifically for nesting purposes.
The depth of substrate available is a very important factor for all species that excavate nests. If insufficient depth is available, nesting will usually be terminated. In captive situations, therefore, it is necessary to ensure that the laying area provides enough depth of substrate to avoid this. It is only possible to offer general guidelines, as different species do vary in their nest depths.
For most species the depth of substrate should be at least equal to the length of the hind limbs plus 70% of the length of the carapace.
We have found that many terrestrial species will nest in captivity in a substrate comprised of 60% soft (play) sand mixed with 40% loamy compost. If outdoor nesting sites are not possible for some reason, indoor artificial sites based upon such a mixture will often yield good results, especially if positioned under a basking lamp and of sufficient size and depth. We would recommend that for a medium-sized Mediterranean tortoise, an area of at least 1 square meter should be provided, and that this should be of adequate depth according to the formula described above. If the artificial nest site can slope gently, this may well increase the chances that it will be accepted by the female as a suitable and safe location.
For small tortoises, a large plastic or even cardboard box can be filled with suitable substrate and raised to an appropriate temperature. Large tortoises will require an artificial nesting mound constructed of a concrete block or strong wooden enclosure. The temperature of the substrate is certainly important - few tortoises will lay in a cold substrate. You may see the female touching the surface with her nose, as if inspecting it carefully. What she is actually doing is sensing the surface temperature to see if it is suitable for incubating her eggs. Some tortoises also appear to use the hind limbs to check that the subsurface temperature is also satisfactory. We allow artificial nest sites to stabilise to (warm) room temperature before use, and then, as mentioned previously, provide surface heat by means of an overhead basking lamp if a suitable outdoor site cannot be provided.
One rather odd phenomenon we have noted is that gravid females, who may be reluctant during daytime to use an artificial nesting site, can often be persuaded to accept one if placed on it after dusk, in the dark, under a basking lamp. We do not fully understand why this should be the case, but we have witnessed it often enough to be convinced that there is some merit in this technique! It is possible that this is ancient ancestral trait as sea turtles exhibit the same preferences.
Incubation
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. Various commercial designs are available. Note that incubators intended for use with birds are generally not suitable. Look for a specialist reptile egg incubator instead. We also have a separate article (with video) that describes how to make your own incubator and how to set it up correctly.
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 gravid - although this is really only justified in ''problem'' cases.
WHAT SORT OF NEST-SITE DOES SHE REQUIRE?
Female tortoises can be very selective about what constitutes a suitable nesting site. Others are less discerning. In general however Testudo species 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.
Preferred nest sites for Testudo species are also almost always situated in a sunny position. The most common time for laying is from midday to late afternoon. It is vitally important that a suitable nest-site should always be available to a pregnant female as tortoises may chose to voluntarily retain their eggs unless conditions are right - if this continues for too long, there is a real danger of 'egg binding' or egg peritonitis. A condition which ultimately proves fatal in a high proportion of cases.
HOW MANY EGGS DOES A TYPICAL FEMALE LAY?
It varies widely according to the species. Some species only lay a single egg. Others (such as larger leopard tortoises or sulcata tortoises) can lay over 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 3-4 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.
ETHICAL ASPECTS
Before deliberately setting out to breed tortoises, or to incubate eggs, it is important to consider what will happen to the juveniles. Rescue centres are overwhelmed with unwanted tortoises, and finding enough genuinely suitable and good homes is increasingly a real challenge. Think before you incubate.
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 which carry a risk of accidents, 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 in incubation is one possibility. Incorrect temperatures are another common cause. 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?
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 dystocia (egg-binding) and can prove very difficult or impossible to lay. Veterinary intervention. possiby involving surgery, may be required. This again stresses the importance of providing suitable nesting sites for gravid females.
Conversely, eggs can be under-calcified and can break internally, or externally, when being laid. If internal, this is quite serious as it can result in an inflammatory response and an increased risk of secondary infection. It is always worth getting the female checked by a specialist vet if this occurs. The underlying cause may be due to lack of dietary calcium or UV-B, but can also be the result of hormonal imbalances.
WHAT INCUBATION TEMPERATURES DO YOU SUGGEST?
For most species, the best results are obtained if the incubation temperature is maintained between 30°C and 31.5°C. 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. This varies according to species, however. Consult species-specific references.
HOW LONG DOES INCUBATION NORMALLY TAKE?
As mentioned previously, it depends to a great extent upon the temperature at which the eggs are incubated and upon the species involved. 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 27°C for example, incubation can take 18 weeks or more. At temperatures above 34°C however there is a grave risk of mortality or deformity among the hatchlings. It is best to incubate at the medium range temperatures suggested. Some species, such as leopard tortoises, have very extended incubation periods indeed, often between 8 to 15 months.
IS HUMIDITY OR SUBSTRATE MOISTURE LEVEL 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. The air humidity within the incubator is another factor that can affect the timing of hatching. This is also discussed in our article on building your own incubator.
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. Leave them to emerge in their own time.
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. There is also an effect known as "diapause" where development is arrested for an extended period. This is beyond the scope of this brief article, however. Consult specialist papers.
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. 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 pure sand. It does not allow sufficient gaseous exchange to occur, and as a result anoxia is a real danger. It is better to rest the eggs upon the suface ofan artificial, lighter medium such as 'Vermiculite', or instead any similar non-toxic granular material. Place the probe of a reliable thermometer alongside the eggs to keep a constant check on conditions.
If the eggs are fertile, expect hatchlings at any point from 7 weeks on! Check the incubator daily after 7 weeks.
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