Australian researchers say the continent's saltwater crocodiles are the most aggressive, much more so than six other species from around the world. A study conducted by Charles Darwin University in the Northern Territory found Crocodylus porosus, colloquially known in Australia as the saltie, was much more aggressive any species from the United State or South America. The saltwater crocodile exhibited vastly more aggressive behavior than species from Papua New Guinea, South America and south-east Asia, the researchers said, and the freshwater crocodile, also found in Australia, was ranked fifth, ahead of the American alligator and the Indian gharial. "They are certainly the ugliest when it comes to aggression," university wildlife biologist Matthew Brien told Britain's The Guardian. "They get in an agitated state then wind themselves up and swing their heads into other crocodiles. When you see larger salties, especially males during mating season, it's quite fearsome. It's like a sledgehammer that would certainly shatter your head." Other species of crocodile are social and comparatively gentle with each other, said Brien said, going so far as to term the Indian gharial "a real sweetheart," with a delicate, narrow snout and pleasant disposition. Aggression in saltwater crocodiles is "hardwired from hatching," he said, but it is unknown why the species is so short-tempered. "A two-meter (6-foot) croc won't eat a human, but once you get an animal that is four meters (13 feet) long, humans are certainly on the menu," Brien said. "You really do have to take precautions in croc habitat."