Wildlife Research Wildlife Research Society
Ecology, management and conservation in natural and modified habitats

Crocodylus johnstoni in the McKinlay River Area N. T, VI.* Nesting Biology

GJW Webb, SC Manolis and R Buckworth

Australian Wildlife Research 10(3) 607 - 637
Published: 1983


In the McKinlay River area most female C. johnstoni mature at 74-78 cm snout-vent length (SVL), and 11-14 years of age, whereas maies are about 87 cm SYL and 16-17 years of age. Adult sex ratios in the field are lM:3-4F. There is a well defined, brief nesting period in late August and early September, and the nests are typical 'hole' nests excavated in sand or other friable substrates close to permanent water. Mean clutch size is 13.2 ± 3.2 eggs (± SD) and mean egg dimensions are: length 6.64 ± 0.26 cm, breadth 4.19 ± 0.19 cm and weight 68.2 ± 8.0 g. There is a variable but significant increase in egg size with clutch size, and large clutches of large eggs tend to be laid earlier in the nesting season than small clutches of small eggs. From 2 to 4% of eggs are infertile. Nest temperatures show considerable daily variation and baseline levels differ between shaded and exposed nest sites. Incubation times are temperaturedependent and range from 9 to 14 weeks. Females excavate nests at hatching time, but attend the nests little, ifat all, during incubation. Hatchlings are 11.2 ± 0.5 cm SVL and weigh 42.0 ± 6.1 g. Formulae for predicting egg and hatchling dimensions from each other are presented. A preliminary method for aging C. johnstoni embryos is described. Artificial incubation at 26°, 30° and 34°C, respectively, resulted in 0, 63% and 21% survival; at 26 and 34°C physically deformed embryos were common. Hatching success is correlated with the age at which eggs are transported to incubators. Sex determinantion is influenced by incubation temperature, and at temperatures we tested between 26 and 34°C, females predominated; males were produced at 31-32°C. Histological examination of females from high-temperature incubation (34°C) indicated slight hermaphroditic tendencies in two of 35 animals examined; their status remains to be clarified. The temperature-sensitive period for sex determination appears to be between 20 and 57 days of age (30°C equivalent ages), but this may well vary with incubation temperature, as in Alligator mississippiensis. Egg losses due to predators (particular varanid lizards) were estimated as 64%, although they may have been increased by our interference with nests. Eggs are also lost to flooding and overheating, and a 60-70% annual egg mortality may be common. In one experiment, hatchling mortality was estimated at 98% within the first year. An egg and/or hatchling harvest balanced by a proportional return to the wild of raised 1-year-olds (5% and 10% of eggs and hatchlings collected respectively) is a potential strategy through which sustained-yield harvesting could be introduced into a conservation-management program.


© CSIRO 1983

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