For 9 months in 1898, two lions terrorized the southern Kenyan region of Tsavo, killing as many as 135 people by one account. Although the almost mythic tale has spawned three movies, people still debate the final death toll. Now, hair and bone samples from the famed lions have shed light on how many people they devoured and why they did it.
The attacks began in March as the British were building a railway bridge across the Tsavo River, which provided the only water to the parched landscape. The two lions crept into the workers' camp at night, snatching people from their tents, according to some accounts. As the attacks became more frequent, workers erected thorn fences around the camp. Meanwhile, John Patterson, a British colonel, tried to trap and kill the lions, even baiting an empty railroad car. He was finally able to shoot them in December. A book he published in 1907 put the final death toll at 28. But by 1920, Patterson increased the number to 135 for the book's second edition--adding African natives to the count.
Anthropologist Nathaniel Dominy and ecologist Justin Yeakel of the University of California, Santa Cruz, wanted to pin down the death toll. The scientists knew they could piece together the lions' diet from isotopes found in their hair  and bone . That's because isotopes such as carbon-13 and carbon-12 accumulate in an animal's body in a ratio influenced by its food. Zebras typically munch on relatively carbon-13-rich grasses, whereas giraffes chew leaves from trees with lower carbon-13 levels. Thus, lions will acquire isotope ratios in their tissues similar to that of their prey.
Dominy and Yeakel requested bone and hair samples from the Field Museum in Chicago, Illinois, which bought the lions' skulls and skins from Patterson. Other researchers provided samples from possible Tsavo prey--such as zebras and giraffes--as well as from skulls of people who lived in the area. During the lions' last 3 months, humans made up about 30% of their diet, according to the isotope analysis. Combined with the size of an average human and how much a lion eats each day, the researchers estimated that over the lions' 9-month rampage, they ate about 35 people.
The overall sum is well under Patterson's 135 figure, the scientists report online today  in the Proceedings of the National Academy of Sciences. "It's lost in history whether the number was changed to increase book sales," Yeakel says. "I kind of like to give Patterson the benefit of the doubt--he seemed like a stand-up guy."
So why did the lions become man-eaters? In 1898, the Tsavo region had been hit by drought, and a European-introduced virus called rinderpest had killed many of the cats' natural prey, including buffalo and wildebeests. Other researchers have shown that such upheaval can cause individual animals to start transitioning to new foods, even different ones from their neighbors. With the Tsavo lions, Dominy and Yeakel think that they saw this in progress: The isotopic analysis revealed that one lion was the main man-eater with 25 kills, while the other may have still relied on its old diet to some extent. "Lions are … able to switch from one prey to another," Dominy says. "Uncomfortably, that turned out to be us."
The estimate of people eaten by the two lions is "a remarkable convergence on the numbers Patterson first published," says Julian Kerbis Peterhans, an African zoologist at Roosevelt University and the Field Museum, who was not involved with this study. "It's nice to see scientific methods used to reconstruct this historical event." But ecologist Craig Packer of the University of Minnesota, Twin Cities--an expert on lions--says that if people want to figure out why lions eat humans, they don't have to look back 100 years. The large cats continue to attack people in Africa today, he says, and at greater rates than the 1898 Tsavo incident.