Wild African elephants prefer to live in safer, protected areas like national parks to avoid humans and become stressed when they leave them, researchers say.

Writing in the African Journal of Ecology, scientists reported finding elephants living inside Tanzania's Serengeti National Park showed less stress than those living outside the protected area, suggesting they "know" which areas are safer to live in.

Protected areas in the Serengeti are not fenced, researchers said, meaning elephants can enter areas where they are at greater risk of conflicts with humans.

Scientists tested elephant dung both inside the protected areas and outside for levels of the stress hormone gluccorticoid.

Elephants outside Serengeti National Park may have learned to associate humans and vehicles with the hunting activity and illegal poaching that put them at risk, researchers said.

"Elephants probably remember where they are, and that bad experiences stress them," Eivin Roskaft from the Norwegian University of Science and Technology in Trondheim told the BBC.

"I think elephants know where they are safe or not," he said. "However, sometimes they also are tempted by nice food outside the park which attracts them to such areas."

"The biggest threat to African elephants and other wildlife is the human population increase outside all such parks," he said.