By Pallab Ghosh
Science correspondent, BBC News, San Diego
Matt Anderson of San Diego Zoo's Institute for Conservation Research explains how the research was done
Researchers at San Diego Zoo have been studying what has been described as the "secret language" of elephants.
They have been monitoring communications between animals that cannot be heard by human ears.
The elephant's trumpeting call will be familiar to most people, but the animals also emit growls.
Their growls, however, are only partly audible; two-thirds of the call is at frequencies that are too low to be picked up by our hearing.
To learn more about the inaudible part of the growl, the team attached a microphone sensitive to these low frequencies and a GPS tracking system to eight of the zoo's female elephants.
The researchers could then correlate the noises the animals were making with what they were doing.
Matt Anderson, who led the project, told BBC News: "We're excited to learn of the hierarchy within the female herd and how they interact and intercede with one another."
The team has already learned that pregnant females use this low frequency communication to announce to the rest of their herd that they are about to give birth.
Only a third of an elephant's growl is audible to humans
"We've seen that after their long gestation of over two years, in the last 12 days we see a manipulation of the low part of the growl, the low part that we can't hear.
"This we believe is to announce to the rest of the herd that the baby is imminent," said Dr Anderson.
The researchers believe that this also warns the elephants to look out for predators.
"You may think that a baby calf of about 300 pounds would not be as open to predation as other species," he says. "But packs of hyenas are a big threat in the wild."
Dr Anderson and his team are continuing to analyse data in order to learn more about this secret elephant language.