Ecology and demography of freshwater crocodiles (Crocodylus Johnstoni) in the Lynd River of north Queensland

Tucker, Anton Dean (1997). Ecology and demography of freshwater crocodiles (Crocodylus Johnstoni) in the Lynd River of north Queensland PhD Thesis, School of Biological Sciences, The University of Queensland.

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Author Tucker, Anton Dean
Thesis Title Ecology and demography of freshwater crocodiles (Crocodylus Johnstoni) in the Lynd River of north Queensland
School, Centre or Institute School of Biological Sciences
Institution The University of Queensland
Publication date 1997
Thesis type PhD Thesis
Supervisor Hamish McCallum
Colin Limpus
Total pages 232
Language eng
Subjects 06 Biological Sciences
Formatted abstract

Mark-recapture studies of the endemic freshwater crocodile (Crocodylus johnstoni) were conducted in the Lynd River drainage of north central Queensland between 1976-1995. The collective data set provided a basis for ecological and demographic analyses to resolve many poorly known aspects of crocodile biology.


Size-related dietary patterns were examined for crocodiles in the dry season. In three consecutive dry seasons, the stomach contents of crocodiles were flushed to record prey items. Numerical frequency and percent occurrence of prey were recorded. Prey categories included spiders, aquatic insects, terrestrial insects, shrimp, fish, frogs, turtles, snakes, mammal s and birds. Diets of C. johnstoni showed ontogenetic shifts as the cranium broadened and once body size exceeded 60 cm SVL. With increasing crocodile size, the ingestion of spiders, insects and frogs declined strongly whereas the consumption of fish, turtles and snakes increased strongly. Shrimp were eaten at low and variable levels by all size classes of crocodile. The low overall prevalence of mammals and birds suggested that they were consumed opportunistically by the larger crocodiles. With increasing crocodile size, there were overall increases in prey richness and significant declines in realised dietary niche, dietary breadth, and mean number of prey items per crocodile, but no significant changes in dietary diversity, evenness or number of equally common prey species.


Habitat preferences were inferred from relative frequency of capture locations from pools, margins, rapids and burrows. Three major habitat shifts were identified that corresponded with ontogenetic changes in body size. Small crocodiles exhibited a strong negative preference for pools and strong preference for magins and rapids. With increasing size, crocodiles shifted away from margins and rapids to pool and burrow habitats. A primary change in habitat use related to maturity status. Immature animals used margin, rapids, and pools with equal consistency. In contrast, adult females were encountered more often in pools, while adult males split time between pools and burrows. Size specific habitat associations do not appear to change significantly across seasons. Foraging preferences and behavioural interactions are likely to promote the observed patterns of habitat preference.


Movements patterns for C. johnstoni were examined from the mark-recapture study. After adjustment for detection bias, there was a minor upstream direction to movements. Seasonal changes of location were not evident from field trips taken only twice yearly. Annual movements averaged less than 1 km except for those of pubescent males, which appeared to be nomadic. Creche dispersal was randomly directed but associated with a threshold in mass/length ratio. On average, males were found further from previous capture sites than were females. Adults of both sexes moved shorter distances than did immature crocodiles with a clear reduction in movements occurring as mass /length ratios approached 0.17 kg/cm snout-vent length. Reduced movement at that general size ratio probably indicated the onset of territoriality associated with maturity. Females usually remained near breeding sites even in years when they did not breed. Nomadic tendencies of pubescent males are probably associated with unsuccessful attempts at entering local dominance hierarchies. Linear home ranges were estimated to be 1.5 to 1.9 km for immature animals, 1.2 km for pubescent females, 30.3 km for pubescent males, 0.6 km for mature females and 1.6 km for adult males.


Studies of natal dispersal seldom consider ectotherms, long-lived animals, large spatial scales, or non-terrestrial habitats. These shortcomings were addressed by a study of natal dispersal for a long-lived polygynous reptile. The natal dispersals of freshwater crocodiles (Crocodylus johnstoni) were compared against the prediction of male dispersal bias for a polygynous mating system. The crocodiles inhabited a linear series of pools and the net distances were calculated from natal pools to recapture locations some 12-18 yr later at maturity. Philopatry was assessed in terms of adult social distances. A female social distance was 0.46 pools and a male social distance was 1.0 pools. By these criteria, both sexes showed low levels of philopatry (7- 12%). However, individuals of both sexes dispersed from the natal site long before they were sexually mature. Divergence in dispersal patterns by sex occurred after the maturity threshold as males dispersed 2-3 times farther than females. Intrasexual competition by males is resolved by a size-based hierarchy. The displacement of small males from local mating access is a probable cause of the longer dispersals undertaken by males. Competition. rather than inbreeding avoidance, is driving dispersal in this population of freshwater crocodiles. Age estimates were determined for free-ranging crocodiles by skeletochronology studies of the postoccipital osteoderms. Growth marks were distinct and counted reliably in unstained calcified thin sections (60-80 μm) of osteoderm viewed by Nomarski interference microscopy. The periodicity of annulus formation was validated directly from crocodiles of known age (up to 19 years) and individuals sampled sequentially in different seasons. Growth marks comprised annuli deposited in winter and zones deposited in summer. Errors in age estimation for small crocodiles resulted from difficulty in assigning the growing edge as either a zone or an annulus. No osseous remodelling was noted in immature crocodiles.


Age estimates for adult females were unreliable because previous annuli were obscured by osseous remodel ling. presumably resulting from calcium mobilisation by egg-laying females. Old adult males continued to deposit annuli, but these were more difficult to distinguish. being more closely spaced than in young crocodiles. Skeletochronology permitted reliable (coefficient of variation = 3.3%) and accurate (± 1 yr) age estimation to about 20 years. Osteoderm growth dimensions were strongly correlated with average snout-vent length, mass, and age and have acceptable potential as an indirect means of age estimation for this crocodile.


Growth, maturity and sexual dimorphism were investigated for the Lynd River population of freshwater crocodiles. A von Bertalanffy growth equation provided the best fit of several growth models tested with recapture data and age-size data. The estimated asymptotic sizes (snout-vent length of males, 1 25.3 cm; females 97.4 cm) agreed well with average size of the ten largest males and females in the population. There were no sex differences in the earliest age at maturity (12 yr) although mean age at maturity was later for males (23 yr) than for females ( 19 yr). Similar outcomes were identified in snout-vent length at maturity; smallest adults of each sex were about 70 cm but the mean size of at maturity was 88 cm for males and 79 cm for females. Sexual size dimorphism in C. johnstoni arises from a combination of smaller mean size at maturity of females and subsequent slowing of growth. Breakpoint regression plots of changes in growth rate detected the youngest age and smallest size at maturity but were not robust estimators of maturity because of variance in size (from 70-85 cm) and age (from 12-24 yr) at maturity. Sexual selection appears to promote individual trade-offs in age versus size at maturity that result in size dimorphism.


Diagnostic ultrasound was used to determine the reproductive status, clutch size, and egg size of gravid female crocodiles. Eggs or vitellogenic follicles were easily detected and counted by ultrasound but corpora lutea and previtellogenic follicles were below the resolution levels of the equipment. A positive relationship exists between reproductive output and female mass or snout-vent length.


A life table is derived from data that spanned the two decades of the study. Age-specific data on fecundity, sex ratio, and survivorship were estimated to understand how variation in these demographic parameters affects population dynamics of this crocodile. The mean annual proportion of reproductively active females was 84.7% and mean clutch size was 12.8 eggs. An estimated 10% of nests was assigned to combined effects of nonviable eggs, nest flooding, or goanna predation . Hatchling sex ratios were 1 : 1 in most years but in years of El Nino episodes, an average of 1 8 % more males were produced. Average age of female maturity was 1 9 years but ranged from 12-22 years. Average age of male maturity was 21 years but ranged from 12-24 years.


The annual survival rates of males and females differ with females having higher overall survivorship but with males having greater longevity. Beyond the first decade, survivorship was high in both males (96.2%) and females (95.2%). Male survivorship was below the predicted average for years between 1 8-30 when most are maturing and seeking breeding access to females. Reduced survivorship in this period may have resulted from injuries or mortality from male combat. Under average conditions, cohort generation time (Tc) was 26.3 years and the rate of population increase was 3 .0%. Under favourable conditions of rapid growth and early breeding, cohort generation time dropped to 20.7 years and the rate of population increase was 7.9%. Unfavourable conditions of slowed growth and average age at maturity left cohort generation time at 26.4 years but the rate of population growth was essentially zero (-0.1 %). High adult survivorship is the key life history trait that enables populations of C. johnstoni to persist through periods of adverse environmental conditions by demographic stasis. Population recruitment appears to be stochastic when sporadic episodes of favourable environmental conditions enhance nest survival and juvenile survivorship to support strong cohorts.


A life table response experiment contrasts the demographic traits of a Queensland population of freshwater crocodiles with those of N. T. freshwater crocodiles, estuarine crocodiles, American alligators, American crocodiles and spectacled caimans. Published life table information for each species' survival, fecundity, age at maturity and longevity were compared by matrix projection models. Comparisons included various combinations of survivorship, early or late age-at maturity and large or small fecundity. Sensitivity and elasticity values were calculated to determine whether age at maturity, fecundity, or survivorship had a greater proportional effect on λ, the instantaneous rate of increase.


In general, crocodile populations offset low and variable offspring survival by high adult survival. However, the parameters displayed in C. johnstoni demography (a long life, high adult survival, lower annual reproductive effort, and in particular, delayed maturity) exhibited traits of a bet-hedging strategy more so than the other crocodilians that were examined. In C. johnstoni, two realised life history types were detectable, one in which λ depended on delayed maturity (typified by a Qld. population) and another in which λ depended on early maturity (typified by a N. T. population). The differences were inversely related to nest survivorship, with the Qld population having lower nest mortality than the N. T. population.

Keyword Crocodiles -- Ecology -- Queensland

Document type: Thesis
Collection: UQ Theses (RHD) - UQ staff and students only
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