Bernd Wolff the ground of their tank with slow motions and the neck extended long. Once they come across a female that is out in the open and not in a shelter, the males will carefully and gently bite after her head or front legs. A female that is ready to mate will respond with retracting her head only slightly and not fully fold in her neck laterally. This is a signal for the male to swim around the female quickly and take hold of her carapace firmly with all four feet. While the mating behaviour has until now been calm and gentle, it suddenly turns rather violent, with the male placing a solid bite onto the female’s neck and attempting to copulate. The female responds with trying to flee and reaching the water surface with frantic movements. She will often appear to be in severe distress, and you may be tempted to come to her rescue, especially as her ordeal may then continue for more than fifteen minutes during which she seems to be in danger of drowning. Such interference would be inadequate, however. So far, there have never been any drowning accidents even when two males were holding on to a female at the same time. The male will eventually let go of the female after 15 to 30 minutes. It is interesting to note that females that retreat by fully folding their neck back laterally will not be bothered any further. Oviposition and incubation All mating activity ceases when water temperatures rise and the turtles revert to ignoring each other. Both sexes then either rest in their shelters or swim about slowly, and actually quite gracefully, in search of food. Impending oviposition is eventually heralded by females that remain lying on roots on the water’s surface without shyness from early May. Very rarely, they may even venture on land to bask below a heating lamp. Oviposition eventually takes place under cover of night and will be aborted right away if the female is disturbed. Clutches are often dropped into the water, however. After a spot of the nesting substrate has been warmed by a 40 W lamp during the day, this exact spot is chosen to bury the eggs. Egg sizes are fairly constant at 38 mm long and 22 mm wide on average. They weigh between 19 and 25 g. Fertilization and hatching rates were low in the beginning, which might have been a result of the young age of the breeder specimens. A technical malfunction unfortunately destroyed three clutches in 1999. Table 1 shows a clear rise in hatching rates. Since 2001, all females have been producing clutches, even though some were dropped into the water. These eggs regularly fail to develop. Every female has so far produced only one clutch per year. Oviposition takes place between mid-May and late June. Clutch sizes vary from six to fourteen eggs. The eggs are transferred to two different types of incubators. Incubator I is set to a day temperature of 28.5 °C, whereas a value of 30.5 °C was chosen for incubator II. The heating devices for both incubators are switched off for four hours every night, causing the incubation temperatures to gradually decrease by between two and four degrees Celsius. The eggs are fully buried in moist vermiculite throughout the entire incubation period. Hatchlings emerged after 75 to 90 days of incubation. This process requires the keeper Fig. 4. A fertile clutch in an incubator. 66 RADIATA 17 (3), 2008
Argentine Snake-necked Turtle, Hydromedusa tectifera Year Number of clutches Number of eggs Number of hatchlings Duration of incubation (days) Number of surviving hatchlings (after four weeks) 1997 2 8 3 76-87 1 no result as eggs were dropped into the water 1998 3 9 7 72-89 5 8 3 73-85 1 no result as eggs were dropped into the water 1999 4 9 6 75-86 4 7 spoiled by technical malfunction 8 spoiled by technical malfunction 7 spoiled by technical malfunction 2000 4 12 5 76-88 4 9 2 80-90 1 7 1 72-79 1 no result as eggs were dropped into the water 2001 2 12 laid on 15. May 2001 7 9 14 laid on 21. May 2001 laid on 19. June 2001 Tab. 1. Oviposition and hatching data until June of 2001. Data for subsequent years were not always recorded with precision, but were in any case similar. 3 8 to muster a lot of patience, as the hatchlings often open their eggs when their yolk sacs are still very large. It is therefore best for the baby turtles to leave them undisturbed in their incubators. It can then take up to ten days for them to fully resorb their external yolk reserves and finally exit their eggs on their own. During the years of 2003 through 2006, I tried to optimise hatching ratios by means of introducing diapauses. This did not lead to improved results, although I introduced cooler phases at different temperature ranges for periods of six to ten weeks in duration. In general it may be stated that eggs that were cooled to a minimum of 23 °C were notable only for their extended incubation periods and needed up to 200 days to hatch in extreme cases. This was seen in five eggs, which were cooled to 23 °C for ten weeks in 2003. Three of these hatched and produced what later turned out to be males. Other eggs of the same clutch were incubated under the conditions described above and without diapause and hatched after 78 to 81 (at 30.5 °C) and 89 to 92 days (at 28.5 °C), respectively. Those eggs that had been exposed to temperatures of less than 23 °C failed to hatch. Raising juveniles Once the hatchlings have resorbed their external yolk reserves they are transferred to a bowl with a water level of 4-5 cm that offers some filigree aquatic plants for cover. There were some losses amongst the babies initially as these would not accept all types of food. Larger aquaria with low water levels RADIATA 17 (3), 2008 67
