It isn't only children playing with toy cars who make engine noises. Elephants produce a similar roar, though in their case it's the rumble of trucks on an African highway that the animals imitate, scientists say.

　　The experts behind the discovery say elephants are capable of vocal imitation, joining a select group of animals that includes parrots, songbirds, dolphins, and humans.

　　Zoologist Joyce Poole was the first to notice some rather unelephantine noises emanating from a group of semiwild, orphaned elephants in Tsavo National Park, Kenya. She managed to track the sounds to a female named Mlaika. But the ten-year-old's powers of mimicry were so developed that the task wasn't easy.

　　"I was sometimes unable to distinguish between the distant trucks and Mlaika's calling," said Poole, the scientific director of the Amboseli Elephant Research Project. "This is what first made me wonder whether she could possibly be imitating the truck sounds."

　　Poole said others in Mlaika's group have been heard to make a similar noise, which is quite different from any call previously recorded in elephants.

　　Poole suspects Mlaika began mimicking traffic on the busy Nairobi-Mombassa highway because she got bored in her nighttime stockade located two miles (three kilometers) away from the road. "It was a sound she heard every night. Just after sunset sound travels well on the savanna."

　　Another possible explanation is that Mlaika might simply like the rumble of trucks. "Perhaps it was pleasing to her in some way, like humming is to us," Poole speculated.

　　Whatever the motivation, the main point, Poole said, is that elephants can actually produce such a sound.

　　"It shows they are able to come up with novel sounds outside their normal repertoire—some of which they have learned through imitating other animals or machines," Poole said. "This is extremely unusual for mammals."

　　Poole's findings are central to a study published this week in the science journal Nature. Poole and her co-authors suggest elephants use vocal learning to help the group-living animals stay in touch.

　　The study also refers to a 23-year-old male African elephant named Calimero, which spent 18 years living with Asian elephants at the Basel Zoo in Switzerland. Unlike their African counterparts, Asian elephants usually communicate using chirping sounds. Calimero has learned to imitate these calls, using them almost to the point of excluding all other sounds.

　　"Vocal learning enables a flexible and open communication system in which animals may learn to imitate signals that are not typical of the species," the researchers wrote.

　　They say this is the first time vocal learning has been recorded in a nonprimate land mammal.

　　In the wild, elephants often need to communicate over long distances, using low frequency calls to keep tabs on other members of their group. Experiments have shown that elephants can recognize the voices of particular individuals at distances of up to one and a half miles (two and a half kilometers). So having a uniquely recognizable call, like Mlaika's low truck-rumble sound, could be a very useful.

　　"Elephants may well be able to produce unique calls, like a vocal signature, that are particular to that individual or to its family or to very closely bonded individuals," Poole said.

　　"Vocal learning could be used to maintain individual-specific social bonds in the complex and fluid society of elephants, where members of a social group come and go, keeping in contact over long distances and maintaining close social bonds over lifetimes."