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Casting was carried out using a thermocuring polyurethane foam (diphenyl-methane-4,4'-diisocyanate) usually used for house insulation. This foam has a great plasticity (allowing exact reproduction of the shape) and a good homogeneity (enabling further use of the casts). In addition, as a spray, this material is very handy to use in the field.

Casting was carried out either at the end of digging, immediately prior to oviposition (n=9) or immediately after the laying process was completed (n=2). In the former instance, the female is easily disturbed by touching her tail, in the latter the female must be moved by 4-5 individuals after affixing a harness to the turtle's body. Before casting begins, the sand around the opening and inside the cavity is meticulously packed with the hands to prevent any collapsing. Inside the cavity, to avoid altering the natural shape of the nest, this process is carried out with very light pressure of the palm of a hand. A metal rod, with one end bent at right angle, is set in the nest; the angled extremity is driven in the sand at the bottom of the nest and the axis is held vertically by an assistant using a plumb-line. The foam is then injected inside the cavity. When the nest is full of foam, a Plexiglas plate with a central hole is slipped on down the metal axis and kept horizontally at the top of the nest. After two hours, the cast is removed by excavating all surrounding sand. The cast is then sawed into successive slices, 25-50mm thick. Thinner segments are taken from the lower part of the cast as variation in morphology is greater in this region. Each slice is scanned and digitized and through a process of rectilinear extrusion, a three-dimensional computer representation is generated, which can be viewed from many different angles (Figure 1).

Our casts refute the initial descriptions of the nests as being boot-shaped in this species. Fretey (1975) indicated an average total depth of leatherback turtle nests recorded on Organabo beach, French Guiana of 76 cm, comprised of a "wash-bowl" above the eggs (27 cm deep), a "laying well" (31 cm deep) and an incubation chamber (18 cm deep), with the "sole" of the boot being 36 cm long. Bruno (1978), in Sicily, reported a total depth of 75 cm, and a "sole" length of 35 cm.

The mean depth of nests of unmutilated leatherback turtles in our study was 70 cm (sd=7.84, range=60-84, n=23). The nest can be divided into three different parts: the washbowl (ca. 40% of nest depth), the laying well (ca. 15% of nest depth) and the incubation chamber (ca. 45% of nest depth; Figure 1). Using graphical software, mean volume of the two lower sections of the nest could be calculated, with the mean volume of the "laying well" being 6 litres (sd=1.5, 4.2-9.2, n=11) and the mean volume of the incubation chamber 15 litres (sd=5.7, 6.1-24, n=11).

Figure 1. Various views of the cast of a typical nest of Dermochelys coriacea (the first 15 cm of the upper part of the nest do not appear in the cast). a: front view of the whole cast . b: the cast split into three different parts. c: cross sections made in the middle of each part of the nest; measures taken in natural nests on Yalimapo beach are shown on those 3 sections. d: the cast is viewed from different sides. The axis and degree of rotation applied to the cast are shown along the arrow joining one view to the next.

Ackerman (1997) suggested that current knowledge of the relationship between eggs and incubation environment is not extensive. These casts of the nests have allowed a better visualization of the morphology of the nests of leatherback turtles and could allow the development of models which might be useful in extending our knowledge about the climate of the incubation chamber, its evolution during incubation and the coupling between development and the incubation development. While transplanting leatherback turtle nests to enclosures, it will now be possible to more accurately mimic natural conditions in the man-made nest.

Acknowledgements: We are grateful to the people who participated in the Kawana project in French Guiana in 1995 and 1996; the inhabitants of Awala and Yalimapo who welcomed us in their villages, special thanks to the Tiouka family, William family, Michel Thérèse and Paul Henri. This study was financed by the WWF and the National Veterinary School of Lyon. The manuscript greatly benefited from the corrections and enhancements proposed by Ray Carthy, Matthew Godfrey and the editors.

ACKERMAN, R. A. 1997. The nest environment and the embryonic development of sea turtles. In : P.L. Lutz & J. A. Musick (Eds.). The biology of sea turtles. CRC Press, Boca Raton. pp. 83-106.

BRUNO, S. 1978. Le tartarughe nei mari italiani e nel Mediterraneo. Natura e montagna, 3: 5-17.

BUSTARD, R. 1972. Sea Turtles. Natural History and Conservation. Collins London-Sydney 220 pp.

CARTHY, R. R. 1996. The role of the eggshell and nest chamber in loggerhead turtle (Caretta caretta) egg incubation, Ph. D. Dissertation. University of Florida, Gainesville, 121 pp.

FRETEY, J. 1975. Guide des Reptiles et Batraciens de France. Lib. Hatier, 239 pp.

FRETEY, J. 1981. Tortues marines de Guyane. Eds Léopard d'Or, 136 pp.