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Heat shimmers in the air, distorting the tan grass and gray-green, scrubby trees near a family of African elephants. They graze languidly, catching what shade they can from the sparse trees. Suddenly they all lift their heads in unison, flop their big ears forward and begin to march away, as if alerted by an inaudible air raid siren. Miles away they blend with another group.

A bull in musth—physiologically ready to mate and searching for a female—mysteriously avoids other males, but marches miles directly to a female in heat. Old Africa hands used to call both these phenomena “elephant ESP.”

A scientist studying the movements of elephants he has fitted with radio tracking collars documents the odd coordination between families of cows and calves. He repeatedly tracks two separate groups moving in unison, for hours, days and even weeks at a time. They turn together, maintaining parallel tracks miles apart. Sometimes the groups simultaneously change direction, moving directly toward each other and blending. While the elephants likely use their keen sense of smell when they can, the wind often carries odors in the wrong direction, so the scientist concludes smell alone cannot account for these coordinated movements.

Several bulls dip their dusty trunks into a water hole, in Namibia’s Etosha National Park, savoring the stark contrast to the parched air they breathe. Suddenly two look up, spread their ears wide and crunch more than half a mile through the brush to find not a female in estrus, but a pair of biologists and a Volkswagen van with a huge speaker mounted on top. The elephants, possibly taken aback, march on past. The biologists, Loki Osborn and Russel A. Charif of the Bioacoustics Research Program at Cornell University, watch with relief. They had broadcast a call recorded from a female in estrus, but neither they nor the rest of their team, videotaping in a tower near the water hole, heard a thing. The sound, below the lower threshold of human hearing, forms part of the remarkable infrasonic communication system of elephants.

Humans can hear many elephant calls, from the famous shrill trumpets to low groans. But until Katherine B. Payne of Cornell analyzed a tape she’d made of Asian elephants at Portland, Oregon’s Washington Park Zoo, no one knew that the deepest elephant sounds we hear, called grunts or rumbles, were merely the mild overtones of sounds so low and powerful they travel unhampered for miles through Asian forest. African elephants use similar signals.

Elephants live in layered societies, and like any social animal must communicate. These largest of land animals communicate with every sense: touch, taste, smell, vision and hearing. All work at close range, within a small band of elephants browsing together, or between mother and calf, or mating male and female, for example. With their long trunks, elephants can keep track of odors on the ground as they walk head up, and they routinely touch and smell each others’ bodies with their trunks.

But it was their sense of hearing that baffled early naturalists and makes long-distance communication—and therefore elephant society and mating—possible. Small groups of related adult females and their young of both sexes form the basic unit in elephant society, called a family. Females remain in families for life. The family often contains three generations, and may remain stable for decades or even centuries. Families associate with one to five other families, probably consisting of more distant relatives. These so-called bond groups in turn belong to larger groups, called clans.

William Langbauer, of the Pittsburgh Zoo, and several colleagues, including Charif, have characterized several specific infrasonic calls based on when they occur and how elephants hearing these calls react. Elephants appear to produce their extremely low-pitched sounds with a larynx similar to those of all mammals, but much larger.

When individual family members reunite after being separated, they greet each other enthusiastically, and the excitement increases with the length of time separated. They trumpet, scream and touch each other. They also use a greeting rumble, which begins at a low 18 Hz, crests at 25 Hz—just audible to humans—and falls back to 18Hz. An elephant attempting to locate its family uses the contact call, a relatively quiet low tone with a strong overtone audible to humans. Immediately after contact calling, the elephant will lift and spread its ears and rotate its head, as if listening for the response. The contact answer is louder and more abrupt than the greeting call, trailing off at the end. Contact calls and answers may continue for hours until the elephant successfully rejoins her family. At the end of a meal, when it’s time to move on, one member of a family moves to the edge of the group, typically lifts one leg and flaps her ears. She repeats a “let’s go” rumble, which eventually rouses the whole family, who then hit the road.

Unlike the highly social females, males leave their families at about 14 years of age. They travel alone or congregate in small loose groups with other males, occasionally joining a family on a temporary basis. When males come into musth, they wander widely, searching for receptive females.

Females typically come into estrus only once every four years, and then for only four days. So competition is intense, and males must have some way of finding mates from long distances. A male in musth repeats a distinctive set of calls called musth rumbles, listening for a response afterward. Males who hear this sound keep away, as bulls in musth are aggressive and dangerous. Females, however, answer with the so-called female chorus. This consists of several females answering with a call similar to the greeting rumble, but somewhat lower. Females will also give this call when a musth male joins their group or when they smell the strong urine of a musth male. A male homes in on the female chorus, hoping to find a female in estrus. After mating, the female rumbles out the post-copulatory sequence, a group of six grunts with strong overtones. She repeats this sequence several times, continuing for up to half an hour.

All of these calls serve as short-range communication in elephants. Documenting the effectiveness of long-range communication has proved technically difficult, however, even among radio-collared elephants. Despite the difficulties, says Charif, “Elephants may routinely know the whereabouts (and maybe activities) of other elephants that are several miles away from them. When a biologist in the field observes the behavior of a group of elephants, s/he may be missing a lot of subtle long-range interactions.”