There is more to the way elephants communicate than the well-known trumpet call.
The creatures also use a low-frequency 'rumble' - like a growl - which includes a component that is not audible to the human ear.
Using specially-developed acoustic software, researchers at San Diego Zoo in the US have discovered that pregnant elephants make a change to their low-frequency sound when they are about to give birth - thereby alerting the rest of the herd.
The other elephants then react by forming a barrier around the mother to protect her from potential predators.
Matt Anderson of San Diego Zoo's Institute for Conservation Research explains.