An Examination of Crocodylus porosus Nests in Two Northern Australian Freshwater Swamps, with an Analysis of Embryo Mortality

Abstract

Crocodylus porosus nesting was examined in a perennial freshwater swamp (Melacca Swamp) adjacent to a tidal river, and in perennial floodplain river channels where floating mats of vegetation overlie fresh water (Finniss R. and Reynolds R.). Time of nesting was quantified. Vegetation at nest sites was identified and ranked according to importance indices, which are used as a descriptive tool. Most nests are exposed during mid-morning and afternoon, but variously shaded during early morning and late afternoon. Nests in Melacca Swamp are more shaded, and have lower nest temperatures, than those in the Finniss-Reynolds region. Nest visibility from the air is influenced by species-specific plant regrowth on nests. In Melacca Swamp the height at which clutches are deposited is related to swamp water level. Mean clutch depth, the distance between top and bottom eggs, was 22.6 ± 8.3 cm (SD); the distance between the top egg and water level at the time of laying was 34.7 ± 8.0 cm. Mean clutch size was 53.1 ± 9.4 eggs; formulae relating egg and hatchling dimensions are presented. A highly variable but significant increase in egg size accompanied increased clutch size, but no relationship was found between clutch or egg size and time of nesting. A high correlation between egg widths of four pairs of clutches laid at the same sites strongly suggested multiple nesting by some females. Nest temperatures, embryo development rates and total incubation times were highly variable, both within and between nests. Of the 2712 eggs examined, 31.6% (856) produced live, apparently normal hatchlings. Flooding was the major cause of mortality, accounting for 36.3% of Melacca eggs and 40.6% of Finniss-Reynolds eggs. Other causes of egg failure included: infertility (M 9.4%, F-R 5.0%), high-temperature incubation (2.0%, 0.4%), low-temperature incubation (6.3%, 3.2%), mechanical damage to eggs by adult crocodiles (O.6%, 2.4%) and undetermined development failures (9.8%, 18.0%). No instance of dehydration of eggs within a nest, or predation on eggs, was recorded. A model for simulating Melacca nest flooding predicted a 33.2% loss of eggs in 1980-81 (estimated real loss was 36.3%), and indicated possible losses of O-54.9% between 1960 and 1980 if the same numbers of nests had been made; mean loss was 26.3%. Three double-yolked eggs (0.1% of eggs examined) were recorded. Developmental anomalies and possible causes (incubation temperature-genetic), have been tabulated. In attempting to analyse the siting of C, porosus nests and explain variation in nesting vegetation, nest site locations, nest mounds and embryo mortality rates, insights were gained by examining nest site selection from the point of view of the female's well-being rather than that of the nest. Resulting criteria considered important in nest site selection are listed and discussed. If the numbers of nests in freshwater swamps are an index of the total population size in such areas, there are clearly many more C. porosus in such swamps than have hitherto been estimated. Nest surveying may be the only practical method of estimating the total population. Regardless of high mortality rates, an egg-collection strategy may not prove a practical method of incorporating sustained-yield harvesting into an overall C. porosus management program.