Supporting the recovery of large carnivores is a popular yet challenging endeavour. Estuarine crocodiles in Australia are a large carnivore conservation success story, with the population having extensively recovered from past heavy exploitation. Here, we explored if dietary changes had accompanied this large population recovery by comparing the isotopes δ¹³C and δ¹⁵N in bones of crocodiles sampled 40 to 55 years ago (small population) with bones from contemporary individuals (large population). We found that δ¹³C and δ¹⁵N values were significantly lower in contemporary crocodiles compared to the historical cohort, inferring a shift in prey preference away from marine and into terrestrial food webs. We propose that an increase in intraspecific competition within the recovering crocodile population, alongside an increased abundance of feral ungulates occupying the floodplains, may have resulted in the crocodile population shifting to feed predominantly upon terrestrial food sources. The number of feral pigs consumed to sustain and grow crocodile biomass may help suppress pig population growth and increase the flow of terrestrially derived nutrients into aquatic ecosystems. The study highlights the significance of prey availability in contributing to large carnivore population recovery. 

Estuarine crocodile bone samples were collected from north-western areas of the Northern Territory (Australia) between Darwin Harbour and the East Alligator River (~300km apart). Historical bone samples were collected from the Museum and Art Gallery of the Northern Territory (MAGNT, Darwin, NT, Australia). Those bones were from crocodiles caught and killed between 1968 and 1986 from various parts of the Northern Territory (n = 22). Some crocodiles were very large and were likely older than 50 years of age upon death. Sampled crocodile bones were from animals with total body lengths ranging between 120 and 513 cm. The sampled bones were not treated or preserved and cleaned by macerating the bones in water. Samples were taken from the left front leg humerus. First, an ~ 0.2 mm outer bone layer was scraped away using a scalpel, and then ~ 0.2 g of bone was scraped out and placed in a sample vial for the analysis. 

The contemporary cohort of bone samples was collected from crocodiles trapped and removed from around Darwin as part of the crocodile management program in 2016. These crocodiles ranged from 115 to 330 cm total body length (n = 24). A small section from the left humerus bone was removed and prepared as the museum specimens. The bones were not acidified prior to analysis as carbonate removal has been shown to have minimal effect upon reptile bone δ¹³C, but instead a correction factor was used. The mean C/N ratios for the bone samples was 3.4 ± 0.2 S.D. (range = 3.1 to 4.3), and only 3 out of 48 samples had C/N ratios above a proposed threshold of 3.6 for intact collagen.

Bone samples from both cohorts were freeze-dried at -40ºC (Dynavac Freeze Dryer) for 48h and then homogenised to a fine powder using an electric ball-mill grinder (RETSCH Mixer Mill MM400). A small sub-sample from each was weighed (0.8 – 1.0 mg) into tin capsules and analysed for stable isotopes (δ¹³C and δ¹⁵N) at the Stable Isotope Core Laboratory at Washington State University (WA, USA). The δ¹³C and δ¹⁵N values from those bone samples were corrected using diet-tissue discrimination factors (1.4 ‰ and 3.0 ‰, respectively). The carbon isotope values were corrected for the Suess effect by deducting 0.4 ‰ from the “historic” cohort.

Body size is known to influence δ¹³C and δ¹⁵N values in crocodilians. Our “contemporary” cohort had a reduced range of body sizes compared to the “historic” cohort. To provide a more comparative range of body sizes, we sourced additional stable isotope values from Adame et al., 2018. These estuarine crocodiles (n = 41, size range = 83.5 to 420 cm) were captured from the same region between 2012 and 2014 (details on scute tissue isotopic discrimination in Adame et al., 2018). We first tested if it was a valid assumption to combine these two groups of crocodiles, whose collagen had been sampled from different tissues (bone and scute). 

Data uploaded  as csv file, containing the raw isotopic values for δ13C and δ15N (prior to the correction factor, subtraction of discrimination factor and correction for Suess effect, see Methodology). Other variables in the file are: crocodile total body lenght (cm), site (location where crocodile sample was originated from), experimental cohort, sample/data source, year that crocodile skeleton was received by the Museum and Art Gallery of the Northern Territoty - MAGNT, and year of tissue sample collection.