When a herd of prehistoric elephants walked through mud in the Arabian Desert about 7 million years ago, its members unwittingly left their footprints—and clues about their social lives—behind. Those prints now reveal how the herd behaved: Just like modern elephants, mature males meandered on their own while the rest of the herd apparently followed a female leader.
Researchers have long wondered when modern elephants began living in matriarchal groups in which females and young follow a female leader while males disperse when they reach sexual maturity. Today, both African and Asian elephants live in complex matriarchal groups, so scientists have hypothesized that their common ancestor 5 million to 7 million years ago also lived in a female-led herd of Proboscidea (the order that includes living elephants and several extinct families), says paleontologist William Sanders of the University of Michigan, Ann Arbor, who was not involved in the new work.
With the discovery of a remarkable 260-meter-long trackway, researchers can now get a glimpse of how some prehistoric elephants moved across the landscape at the site of Mleisa 1 in the Al Gharbia region of Abu Dhabi. The tracks, which are the most extensive ever recorded for mammals, were made by at least 13 proboscidians of different sizes, according to a study published today in Biology Letters. Using a kite-mounted camera to take aerial photographs of the footprints, an international team of researchers, with the support of the Abu Dhabi Tourism and Culture Authority, analyzed the prints' stride lengths and depths. They also were able to use stride length to work out the proboscidians' body weight and, in some cases, sex (by comparing the stride lengths with those of living elephants). This helped them deduce that one solitary elephant was a male moving in a direction totally different from the other elephant ancestors, including a group with several young individuals that left behind faint tracks.
This behavior suggests that 7 million years ago, the elephants lived in sex-segregated groups as they do today, says team member Faysal Bibi of the Museum fur Naturkunde in Berlin. And if mature males rejoined the herd only to mate, that suggests that herds followed female leaders, also as they do today.
The contrast in size and mass deduced for the male elephant that walked alone "does suggest that the herd was matriarchal," says Sanders. "These footprints are truly wonderful," adds ethologist Phyllis Lee of the University of Stirling in the United Kingdom, who was not involved in the work. "They capture the behavior of large animals at one moment in time from millions of years ago and which we can clearly interpret in the context of well-known modern animal behavior."
A key question now is what species of Proboscidea made these footprints, says evolutionary biomechanicist John Hutchinson of the University of London. This time period 7 million years ago was "chock-full of different kinds of proboscidians." It would be interesting to test whether the ancestors of African and Asian elephants lived in a matriarchal herds and try to trace how far those behaviors stretch into prehistory. If the footprints were made by particularly primitive proboscidians, such as so-called shovel-tuskers, it would suggest that such matriarchal behavior "was not a new behavior only of elephants," Sanders says. If so, it might be an ancient, primitive social behavior for proboscidians, including such distant cousins of elephants as mastodons.
In the meantime, though, the paper demonstrates that researchers can use tracks to reconstruct prehistoric behavior, along with fossils and teeth, Bibi says. "Most of the time in paleontology one doesn't get such a direct behavioral window onto the biology of an animal."