Researchers at San Diego Zoo have been studying what has been described as the "secret language" of elephants

http://news.bbc.co.uk/2/hi/science/nature/8527009.stm

….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.…

"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."…