(Presented at the Symposium at The University of Bristol, 2nd May 1992)
Captive breeding of tortoises in their country of origin makes sense. The eggs can be incubated under optimum conditions and the hatchlings reared until the critical points in their development have passed, they can then be released in a suitable and safe natural habitat.
If the breeding takes place in a country where the animals are not indigenous, captive breeding and conservation appear to be a contradiction in terms. What are we breeding for, apart from our own satisfaction and satisfying a possible home demand? Sending back the hatchlings to their country of origin is going to be difficult and would need strict controls.
In the UK the main importation of Mediterranean tortoises has been the Spurthighed Tortoise, Testudo graeca graeca from Morocco, but the animals would have been rounded up from all over the country, from the Sousse valley right to desert habitat as well as from neighbouring countries like Algeria and Egypt, In addition the Spur-thighed Tortoises from eastern Europe have been imported. Sub-species of the Spur-thighed Tortoise interbreed quite readily in captivity. The "hybrids" might not have the right genetic make up to survive adequately in habitats of either parents and can therefore not be sent back. Another problem in sending back animals from here concerns disease. The different climate may have brought about alien diseases and although some captive animals do not show signs of infection, they may still be carriers and when returned, infect local populations. This has happened with the Desert Tortoise in the United States.
It looks therefore as if captive bred animals are aliens, cannot be admitted back and are exiles. In spite of this I feel that as their custodians, we have a valuable contribution to make. We are in a unique position to observe them, learn from them and report on then, which can benefit their eventual survival in the wild. Breeding tortoises in captivity is not difficult but doing it well is.
HOW TO OBTAIN HEALTHY EGGS
There appears to be an extremely high rate of infertility with clutches of eggs. The start to healthy eggs is healthy parent animals. Provide a balanced diet, with an appropriate vitamin and mineral supplement. With the formation of calcareous eggshells and supplying nutrients in the egg to sustain development and the formation of skeleton and shell in an embryo tortoise, a high level of calcium is needed in the diet. The digestive system of a tortoise is geared towards digestion of cellulose, so offer a lot of greens in the diet as tortoises need adequate roughage or fibre. Provide sufficient warmth in the form of heatlamps during colder periods as the British climate is cooler than their native lands. Tortoises are ectothermic and need warming up before they can digest their food.
Create the right environment: the run of your garden is splendid, but not if the males constantly molest the females. Stress is an underestimated part of tortoise husbandry and yet a very important one. It is better to keep the males separated from the females, and just unite them for mating purposes; the best times for mating are straight after hibernation and in autumn. In the confines of captivity tortoises have little opportunity to escape excessive shell-butting.
If you have created special tortoise gardens, bring the male to the female's pen. Provide good laying facilities with special mounds of loose earth for the females to dig a nest and only breed from healthy females. Obviously, using the right males and females of the various sub-species is best and has a higher record of fertility (Fenwick 1989); do not allow one of the males of the larger sub-species to mate with a female of one of the smaller sub-species as this can have implications with egg laying.
Assuming we have a healthy male, producing healthy sperm and able to perform satisfactorily and assuming we have a healthy female we can proceed. After mating the gestation period is from thirty days to three years. The female may dig several trial nests and finally will deposit on average between 5- 10 oval or round-shaped eggs. In Britain these will have to be retrieved and placed in an incubator, where they will hatch after 8-12 weeks, depending on temperature.
The egg or ovum of a healthy female tortoise starts as a tiny cell produced in a specialised organ, the ovary. It consists of protoplasma, enclosed in a membrane and containing a nucleus with the genetic material (DNA) (deoxyribonucleic acid). These DNA molecules form a blueprint made up of units called genes. The nucleus of an egg holds the information needed to build the hatchling tortoise. Gametes i.e. eggs and sperm, only hold half the number of chromosomes found in the normal cell of that animal.
When the egg cell, the ovum, is fertilised by a male sperm, their genetic materials form a union cell, called the zygote, which has a mixture of blueprints of both parents. From this mixture. the zygote, the embryo develops. The fertilised egg or zygote has a full complement of two sets of chromosomes, one set of matemal origin and the other of paternal origin.
The ovaries are paired and suspended in the posterior body cavity. They contain a large number of follicles. each of which contains an immature ovum (Smith 1986).
At ovulation the follicle ruptures and the ovum drops into the oviduct. The oviducts are long paired tubes each about 40 cm long and it is in the oviducts that the egg yolk is formed. (Smith, 1986). Tortoise eggs are supplied with enough yolk to nourish the developing embryo through its development. Yolk is manufactured in the liver of the female tortoise and transported via the bloodstream to the oviducts. (Never breed from a tortoise that has had veterinary treatment for liver problems, as the chances are that neither the female nor the offspring will survive.)
Yolks contain combined fats, proteins and all necessary nutrients, minerals and vitamins. In the egg of a mature Spur-thighed tortoise the yolk makes up about 40% of the total weight the albumen (the egg white) 44% and the shell 16% (Smith 1986).
The yolks move along the tube of the oviduct at a considerable speed, helped by beating, hair-like cilia and muscular action and on their way become coated with albumen, membranes and eggshell; eventually they pass into the cloaca on the way to being laid. Because the fertilised egg is surrounded by these secretions on its passage down the oviduct, fertilisation therefore must take place at the top of the reproductive tract.
There are four major regions recognisable in a mature female reproductive system. The INFUNDIBULUM on the top. It is thought that this may be the site of fertilisation. The second part is the MAGNUM, where the yolks are coated by albumen, which will be providing the fluid support in the egg. Then follows the UTERUS where the yolk, surrounded by albumen, is provided with a fibrous shell membrane. On the outer surface of the membrane are small spheres acting as initial sites for deposits of the aragonite form of calcium carbonate, (birds' eggs are of the calcite form) (Smith, 1986). More deposition to form the thick and rigid shell occurs later. The eggs now stay in the uterus until they are ready to be laid, when they are forced through the VAGINA (into the urogenital sinus) and the CLOACA and out through the vent.
OBSTETRIC PROBLEMS WITH THE FEMALE TORTOISE
Egg retention: Providing good mounds of loose earth is essential, so is temperature. If there are no mounds, or if the female is too cold, egg retention is common. Try heating the animal up; give a warm bath. If all else fails, have the eggs induced by oxytocin injections via your veterinary surgeon. Tortoises are often seen to lay eggs in hot weather.
Other problems: Too many eggs can cause weakness especially if the animal has been unable to lay through lack of good facilities or the right temperature and has not been feeding well. Sometimes the hind legs cannot move because a nerve is pinched and a veterinary surgeon will need to remove the eggs surgically.
Eggs can be too hard (double-layered) or too large (which highlights again the need for breeding with the right sub-species). You can aid the passage of eggs by lubricating with some vegetable oil or yogurt.
A prolapse of the uterus can occur and this requires urgent veterinary attention.
The female can suffer from dehydration or even starvation. It is normal for gravid females to refuse food.
On the way out, an egg has to pass the opening to the bladder and can accidentally enter the bladder, giving the animal an infection as it decomposes; it can even cause peritonitis or lead to a septicaemia. Eggs may also accidentally enter the general body cavity. An X-ray will reveal problems of this kind.
But if all goes well, you will have a clutch of eggs, which will have to be retrieved for artificial incubation. The incubator can be of the bought variety, or of the home made variety. I use a water incubator in the shape of two aquaria, a large one with water heated by thermostatic control, containing a smaller one with a suitable substratum (in our case cat litter, the dried stone variety, which we have found ideal).
Incubation temperature is vital as the sex of the hatchling is determined by it. Lower temperatures give males, average (80-85°F/24-29°C) give a mixed clutch and over 85°F/29°C give female hatchlings. This is called Environmental Sex Determination (ESD).
EGGS AND THEIR DEVELOPMENT
An egg is the repository of genetic material, it carries the opportunity for evolution and adaptation to changes in the species' environment. It is one of nature's most interesting inventions and can be compared with a small space ship, a capsule, containing everything necessary for the creation of new life.
Reptile eggs are provided with a more or less impermeable shell, which protects the tissue of the egg. The reptile eggs is called "cleidoic", meaning box like. The shape is usually spherical oval or elongated.
The shell is composed of layers of fibres at right angles to each other, soft and rubbery in the case of turtles and some of the terrapins, hard in the cases of tortoises due to be impregnated with calcium salts.
The reptilian embryo develops as a disc of cells lying on top of a mass of yolk. A network of blood vessels is formed, connecting with a tiny pulsating heart. You can sometimes see this when you candle the egg (holding it upto the light) after a few days of incubation. Tortoise eggs lack the fibrous spirals (the chelazae) of birds' eggs holding the embryo in place, so care must be taken not to shake the tortoise egg and dislodge the embryo. Blood is pumped round to pick up nutrients from the yolk and carry them back to the embryo.
Apart from the yolk sac the most important features of the reptile egg are the three membranes surrounding the embryo, the AMNION, CHORION and ALLANTOIS. These membranes are the life-support machine for the embryo: they act as lungs for exchanging oxygen and carbon dioxide with the atmosphere and form a reservoir for waste products.
The amnion forms a sac enveloping the embryo.
Reptiles excrete nitrogenous waste as uric acid or urates, which is filtered out of the bloodstream by the kidneys. It is flushed out, but some water is re-absorbed (an adaptation to living in a dry environment). The uric acid is left as solid crystals in a little bag outside the embryo's body: the allantois.
The wall of the allantois, where it meets with the chorion, helps to form a double membrane around the inner surface of the eggshell and develops a system of blood vessels which acts as a lung.
The embryo completes its development: the beating heart will be brought to its final position the gut will be forming and then the limbs and organ formation will take place.
During the last stages of incubation the eggshell has been weakened as calcium and other minerals have been removed to help build the skeleton of the tortoise.
The egg shell is strong in compression and because of its shape, squash resilient. It has no tensile strength though and only slight pressure from within causes enough tension in the shell for the hatchling tortoise to crack a tiny hole with its egg tooth, a small carbuncle on its upper jaw.
There are no specialized hatching mechanisms; the young tortoise uses the same movements as it will later use for locomotion. (Burton, 1986). Once the hole has been pierced and the hatchling has plenty of fresh air, the membrane action as lungs dries up and blood beings to circulate through the tortoise's lungs, which fill with air and begin their appropriate function. The tortoise then simply bursts outof the egg looking a miniature replica of the adult.Th eweight of a hatchling Spur-thighed Tortoise varies from 8-18 grammes, depending on the sub-species and the size and age of the mother.
The egg shell can be too hard, or too thin. If too hard a little help is needed, although the hardness, which can be a double layer of egg shell, sometimes has prevented the hatchling from breathing properly and it may be found to be dead in the egg. Too thin an egg shell is usually due to a lack of calcium in the females.
- The egg yolk can be deficient as a percentage to white.
- The egg tooth can be missing from the hatchling's upper jaw.
- The substratum can be too dry and the embryo dies through dehydration.
- The substratum can be too wet and the embryo dies through suffocation.
- Hatchlings can he deformed, with feet the wrong way. they can be twinned, or even born without eyes.
- Should the hatchling emerge with a very large egg sac. protect it by placing it in a cleaned out chicken egg shell with smoothed edges.
Acknowledgement: Thanks are due to Mrs. Chris Tilley, M.R.C.V.S. who commented on the text.
REFERENCES AND FURTHER READING
Adrian C. 1981 Schildpadden, BV W.J. Thieme & Co. Zuthpen, Holland
Arnld E.N., Burton J.A., Ovenden D. 1978. A Field Guide to the Reptiles and Amphibians of Britain and Europe. Collins, London.
Burton R. 1987. Egg. Nature's miracle of packaging. Collins, London.
Smith R.N. 1986. Anatomy of male and female genitalia of tortoises. Testudo Vol. 2, No. 4, pp. 1-7. Orbis Publishing Ltd. London 1989. The Illustrated Encyclopedia of Wildlife, Vol. 26.
Wilke H. 1979. Schildkroten, Grafe und Unzer GmbH, Munich, Germany.
Testudo Volume Three Number Four 1992