A male saltwater crocodile approached a female saltie — as they’re known in Australia — in the same enclosure at Australia Zoo. He snapped at her aggressively.
But then in a change of heart that wasn’t what you’d expect from one of Australia’s most fearsome predators, he appeared to think better of it.
“He went down under the water and started blowing bubbles at her,” said Sonnie Flores, a crocodile researcher at the University of the Sunshine Coast who observed the interaction. “It was kind of sweet. It was almost like he was blowing her a kiss.”
Trying to decipher what crocodiles like that one are saying is at the center of ongoing research by Ms. Flores and her colleagues to create the world’s first crocodile dictionary. Such a gator glossary would catalog different forms of crocodilian communication and unlock their meanings. If successful, it could even help prevent conflict between humans and crocodiles.
Like all reptiles, crocodiles and alligators don’t possess a larynx and their vocal cords are rudimentary. And unlike those of most mammals, crocodilian lung muscles can’t regulate the vibrations of those vocal cords.
But crocodiles and alligators have overcome their physical limitations to become the most vocal of all reptile species.
After studying recordings and video footage from captive crocodiles at Australia Zoo, and from wild crocodiles on the Daintree River and Cape York Peninsula in the northern Australian state of Queensland, Ms. Flores has identified 13 categories of crocodile sounds.
These include growls, bellows, coughs, hisses and roars. But there are also nonvocal forms of “speaking,” like head slaps on the water, narial geysering (when a crocodile dips its nose beneath the water and spouts water into the air), narial toots, and, yes, blowing bubbles.
A crocodile can even vibrate its back so that its scale-like scutes move up and down like pistons, spraying water.
Vladimir Dinets of the University of Tennessee has studied American alligators from Texas to South Carolina and described a ritual in which alligators gather to swim in circles “like an old-fashioned village dance.” He has also observed what he calls “alligator choruses” during the spring mating season in Everglades National Park in Florida.
“They get together and bellow in groups,” he said. “You can sometimes see 200 alligators calling together.”
Most intriguingly, crocodilian species communicate using vibrations at very low frequencies known as infrasound, which Dr. Dinets said “should be physically impossible.”
“Their ability to produce infrasound is interesting because usually you have to be the size of a big whale to produce infrasound underwater,” said Dr. Dinets, who is not involved in the crocodile dictionary. “And yet crocodilians have found some physical mechanism that allows them to do it.”
Having identified how crocodiles are communicating, scientists are now trying to unlock what they’re actually saying.
“In popular fiction, people can talk to animals all the time. But we don’t actually find it that easy,” said Dominique Potvin, a behavioral and acoustic ecologist involved in creating the crocodile dictionary. “The assumption with crocodiles has always been that they don’t have much to say other than what might be a warning to us.”
Filling in gaps in our knowledge about crocodile communications — about crocodiles in general — can be extremely difficult: Most wild crocodiles hide when humans approach. Getting too close to them can be dangerous, and wild crocodiles live in challenging environments.
“Most crocodile behavior takes place below the surface of the water, and the waters of most northern Australian rivers are like milky coffee,” said Ross Dwyer, who is Ms. Flores’s academic supervisor.
Even studying captive crocodiles has its complications: The crocodiles at Australia Zoo kept eating the microphones.
David White, who owns Solar Whisper Wildlife Cruises, has been observing wild crocodiles on the Daintree River for 26 years, and his local knowledge is helping scientists to match crocodile sounds to actual behavior.
“We know all the crocodiles here,” he said. “Some actions might look like a fight, but we know from experience that it’s courtship.”
Scientists hope that a crocodile dictionary could help improve human-crocodile relations.
Northern Australia has an estimated 130,000 wild crocodiles and, with crocodile numbers rising, more salties are moving into landscapes dominated by humans. Detecting and understanding crocodile sounds could help the authorities set up early warning systems whenever a crocodile is active in an area. It may even be possible to drive potentially threatening crocodiles away using playback of certain sounds underwater.
A crocodile dictionary could also facilitate crocodile conservation, by detecting when crocodiles are distressed or hungry. Unlocking crocodile communications could even help to change popular attitudes.
“Crocodilian behavior is much more complicated, and they’re much more intelligent than most people realize,” Dr. Dinets said. “My hope is that once this becomes more widely known, people will start to see them in a different light, and not just as something that tries to eat everything that moves.”