Natural and Artificial Nest Sites for Terrestrial Tortoises & Semi-terrestrial Turtles
A. C. Highfield
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 behavior can be broken down into several distinct phases:
Different species exhibit markedly different preferences and behaviors 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 (Geochelone carbonaria) will accept flat nest sites readily, but typically prefer the soil to be rich in organic content, moist, or even muddy. Mediterranean Testudo species usually prefer to lay in full sun, on a dry day, from mid-day to late afternoon; other species, such as Geochelone 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 utilize 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 (Geochelone denticulata) from
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 does provide 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, 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, and 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 stabilize to (war) room temperature before use, and then, as mentioned previously, provide surface heat by means of an overhead basking lamp.
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, 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!
(c) 2003/4 A. C. Highfield