Saltwater crocodiles in the Northern Territory, Australia, are eating more feral pigs than in the past, which may be why populations are thriving in the area, a new study in the journal Biology Letters suggests. Researchers compared the bones of museum specimens that lived 40 to 55 years ago with contemporary crocodiles to see how their diet has shifted over the years.
“We measured the naturally occurring carbon and nitrogen isotopes extracted from the crocodile’s bones and other tissue which are derived directly from the animal’s diet,” said Stuart Bunn, director of Griffith’s Australian Rivers Institute, in a statement. “This gives truth to the old adage ‘you are what you eat’.”
They found that modern crocs in the Northern Territory had lower values of carbon-13 and nitrogen-1 isotopes, which indicates they’re eating more terrestrial food, Hamish Campbell, an environmental engineer at Charles Darwin University and leader of the research project, tells the Guardian’s Donna Lu.
"Our results show that they have shifted from a marine-estuarine based diet, such as fish, marine turtles, to a more terrestrial-based diet of feral pigs and buffalo,” Mariana Campbell from Charles Darwin University tells 9News’ Marina Trajkovich. "A reduction in estuarine prey may have also contributed to the crocodiles' diet change."
Saltwater crocodiles have existed on the planet for 65 million years. They can reach up to 23 feet in length, weigh over 2,000 pounds, and hold their breaths for up to seven hours. These predators can ambush prey as large as deer, kangaroos, cattle and wild buffalo.
Crocodile numbers in Australia plummeted to record lows because of unregulated hunting in the mid-20th century, write the authors. By the 1970s, only a few thousand individuals lived in the wild. But after the country put laws in place to protect these animals, they made a remarkable recovery.
“Crocodiles have gone from a population of … probably a few thousand individuals across the top of Australia in the 1970s, to over 100,000 adults in the Northern Territory alone,” Campbell tells the Guardian.
The research team hypothesized that as their numbers grew, increased competition, along with an increased population of ungulates in the floodplains, may have led the crocs to feed mostly on feral hogs.
“There isn’t anything else out there in the floodplain that exists at that high a nutritional value and biomass that could be supporting the crocs,” says Campbell, adding that feral pigs are the “most destructive feral animal right across Australia and we virtually have no idea what the population numbers are.”
The authors write that other parts of Australia have had slower crocodile recovery rates, which may be because those habitats don’t support large feral pig populations. Though feral pig numbers have increased over the past years, crocodiles may be helping suppress their populations, mitigating the pigs’ negative ecological and agricultural impacts, per the authors.
“The extensive recovery of the estuarine crocodile population in the study area appears to have been supported by access to an increased abundance of invasive terrestrial prey (i.e. feral pigs),” the authors conclude. “The study highlights the significance of prey availability in contributing to large carnivore population recovery.”