Nesting ecology of the Australian freshwater crocodile, Crocodylus Johnstoni, on the McKinlay River
: setting a baseline for assessing the potential impact of the exotic cane toad, Bufo marinus

  • Amaral Machaculeha Chibeba

    Student thesis: Coursework Masters - CDU


    The nesting ecology of the Australian freshwater crocodile, Crocodylus johnstoni, was investigated in 2003 on the McKinlay River, Northern Territory of Australia. The study encompassed the period of time prior to nesting until hatching.

    Altogether, 61 nests were laid in three weeks, from 16 August to 6 September 2003. Coarse sand was the most common substrate type (58.5%) followed by fine sand (30.2%). The majority of nests were laid less than 20m away from permanent water (58.8%), though some nests were laid 200m away from water (5.9%). It appeared that in selecting nesting sites the availability of friable substrates plays the major role with distance of nests from permanent water being determined by availability of suitable substrates in the landscape.

    Analysis of clutch characteristics showed that larger clutches with larger eggs tended to be laid earlier. Larger clutches had wider eggs, suggesting that they were produced by larger females. There was no consistent variation in laying date or other clutch characteristics across sites, indicating that nesting time was synchronised across sites and clutch characteristics did not change with location.

    A comparison of clutch sizes obtained this year to those from 1980, 1982-85 and 2002 showed that while clutch size did not vary significantly amongst years, egg breadth, egg length and egg mass, varied considerably from one year to another. This indicates that should cane toads have an impact on clutch characteristics, it may be difficult to ascertain, unless clutch size it affected.

    Similarly, a companson of time of nesting from 1980, 1982-85,2002 and 2003 showed that commencement of nesting varies noticeably from one season to another. No significant correlation was found between time of nesting and the mean temperature experienced in the three months preceding the nesting season.

    There was extremely high egg mortality during incubation. Varanus panoptes took 52 of the 61 nests (85.3%) found throughout the study. Varanus mertensi was also observed but was never found excavating or investigating C. johnstoni eggs. Water buffaloes, pigs, dingoes and horses were also observed but no evidence was found of pigs or dogs preying on nests, nor was any nest physically destroyed by pugging or trampling by buffaloes or horses.

    A total of ten nests hatched in two weeks, from 4 November to 14 November 2003, 81.3 days after they were laid (N = 4, SD = 4.6, range 75 - 86). Hatching success was estimated at 1.68%, which can be used to derive an estimate of 476 nests laid in this season across the study site. The standard error was very high suggesting that unless large sample sizes, of at least 50 nests found before predation, are obtained, impacts of toads on the reproductive success of C. johnstoni may not be detected statistically.

    There has been an increased invasion of saltwater crocodiles in the McKinlay River area. Saltwater crocodiles were observed in two Sites: in one Site the occurrence of saltwater crocodiles had been suspected yet never confirmed, while the other Site has just been invaded. The arrival of saltwater crocodile in the second Site appears to have displaced the once common and repeatedly observed population of freshwater crocodiles.

    Recommendations for future studies are provided. In evaluating the impact of cane toads upon the nesting ecology of freshwater crocodiles, mark-recapture studies are suggested to ascertain changes in size, sex ratio and population structure attributable to the arrival of cane toads. Concomitantly, monitoring the invasion of saltwater crocodiles and examining the stomach contents of any varanid lizard or crocodile found dead is also suggested.
    Date of AwardDec 2003
    Original languageEnglish
    SupervisorPeter Whitehead (Supervisor) & Grahame Webb (Supervisor)

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