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The Field staff came through again. “In this case the scientific potential and the minimal amount of damage justified the procedure,” Dominy says. MacArthur Curator of Mammals (and study co-author) Bruce Patterson (no relation to the hunter John Henry) drilled out a pea-sized piece of bone from each skull. Now Dominy’s team would have a better idea of the lions’ diets during the time leading up to their predation on humans (from the bone samples) compared to their diets during their last few months (from the hair samples). They also had isotopic signatures from the diets of the five modern Tsavo lions that had never eaten people, and from the sort of herbivores that lions normally prey on. What they did not have were samples of the Taita who worked on the railroad. For that, Dominy hoped to rely on what he calls “a fluke of history.”

In 1929, anthropologist Louis Leakey and his first wife, Frida, were honeymooning in Kenya when a missionary alerted him to a cave full of skulls. The skulls belonged to ancestors of the local Taita population. According to their custom, the Taita buried their dead vertically, in a squatting position, with their heads remaining above ground. After scavenging animals picked the skulls clean, the families of the dead would put the skulls in rock-shelter shrines, where they could be consulted about anything from village disputes to failing crops. After talking with local leaders to find out which remains were no longer “in use,” Leakey took 138 skulls back to England. Where were they now?

Dominy first looked to Cambridge, where Leakey had been a student during his early excavations. The Taita skulls weren’t at the university’s Museum of Archaeology and Anthropology, nor were they listed among the thousands of skeletal remains at its Duckworth Laboratory. Knowing that Leakey had later experienced a falling-out with Cambridge, Dominy wondered if the scientist could have given the skulls to another institution. E-mails to other British museums turned up nothing. “I thought they were more or less lost to history,” he says. But a curator at the Duckworth Laboratory eventually sent him a list of every item in its vast collection. On the list was material indexed as “Teita.” Dominy had found the Leakey skulls. Referring to Leakey’s field notes, he requested skull samples from Cave E, where the anthropologist had found specimens that appeared to date back to the 1890s. The lab eventually agreed to provide fragments from 12 skulls, and Dominy’s team ran them through the isotope analysis.

Now it was time to put together the puzzle. By applying what is called an isotopic mixing model, scientists can estimate the probability of specific food contributing to an animal’s diet. “Isotopes mix together to produce a distinctive signature in the same way that pigments of paint can mix to produce a unique color,” Dominy explains. “Using stable isotopes to estimate the dietary behavior of an animal is like going to Home Depot to find the perfect combination of paints to match a swatch of fabric.”

In their “color” matching, Dominy’s team plotted isotopic ratios from all the human and animal samples on a graph, with the X-axis representing carbon values and the Y-axis representing nitrogen values. The Taita ate a combination of foods—corn, a pea-like porridge called mbaazi, and the milk and blood of goats—that placed them on a distinctive spot on the graph. Dominy had what he needed to estimate how much of the lions’ diet had been human. What he found was the diets of the two predators had diverged in their last months.

The second lion to be shot by Patterson appears to have shifted from the traditional lion’s prey of grass-grazing animals, such as zebras and wildebeest, toward leaf-munching browsers, such as gazelles, at the end of its life. In contrast, the first lion shot appears to have eaten browsers for most of its life, until it shifted its diet toward the Taita. During its final three months, humans comprised approximately 30 percent of that lion’s diet, compared to just 13 percent for the other lion. “The lions would attack the camp simultaneously, but one would go after humans and the other would focus on the goats and donkeys,” Dominy says. “They formed a coalition, but they didn’t share.” To translate those percentages to numbers of people eaten, the scientists factored in the average dietary requirements of the Tsavo lions (6 kilograms of food per day) and the amount of human tissues consumed in a typical lion attack on a 150-pound man (25 percent, or 20 kg.). They then extrapolated each lion’s intake over a nine-month period.

Dominy and his team published their findings in November 2009 in Proceedings of the National Academy of Sciences. They estimate that the lions could have eaten as few as four or as many as 72 people, but they believe the most likely toll was 35—slightly more than acknowledged by the railway company, but far fewer than John Henry Patterson had claimed. Dominy thinks that in 1898, a sequence of events helped turn the two lions into man-eaters. First, ivory hunters had killed off most of the region’s elephants, changing the landscape. “We call elephants ecosystem engineers because they push over trees to eat their leaves,” stimulating the growth of grassy savannas, Dominy explains. “When elephants are exterminated, animals that eat grass do poorly,” and that’s what lions typically hunt. Drought and disease had further reduced the population of potential prey. Into this situation came a large population of unfortunate railroad workers. The lion that ate the most people was the one with the most significant dental problems; broken teeth and major abscesses on its jaw likely contributed to its dependency on softer, human prey. “I think it was a unique set of circumstances that drove the Tsavo lions,” Dominy says.

THE NATURE OF THE APPARENT cooperation between the lions contributes to a growing field of ecology known as individual dietary specialization. Dominy says, “No one has ever shown that [members of] a cooperative carnivore species would simultaneously cooperate but eat different things.” Ecologists have known about individual dietary specialization for a few decades, but the field has gained momentum in recent years as dozens of new examples have been identified. “Individuals who to us look pretty much identical as a species nevertheless show pretty different personalities when going about the daily routine of feeding,” explains Dan Bolnick, an associate professor in the integrative biology section at the University of Texas at Austin and a leading expert in the field. “On the one hand, [Dominy’s research] shows one example of this pattern,” Bolnick says. But what makes it so intriguing, he adds, is that it connects to such a “fascinating human-interest story.”

Far from being “noise in the data set,” idiosyncratic feeding behaviors can be “a mechanism for new species to emerge,” Dominy adds. “Traditionally we think new species evolve when animals are isolated from each other. But individual dietary specialization lays a foundation for evolution to occur in sympatry, when animals live together.” Dominy’s findings also prompt questions about the evolutionary basis of cooperation. Lions are among the few social carnivores, but when they hunt together, they typically share the proceeds, he says. Cooperation itself can be risky because the more animals that are involved in a hunt, the more likely they are to be noticed by potential prey.

“What our research shows is that cooperation isn’t simply related to food sharing,” he says. “There must be benefits to cooperation that transcend the benefits of sharing foods. As an ecologist I could reduce it to something like territorial defense. But as an anthropologist, I think maybe they just liked each other. Even if they were not sharing food, it was better to be together than not to be. We can’t be sure, but it’s tempting to say [the Tsavo lions] were related. A band of brothers, if you like.” Because the lions in Tsavo are very closely related to each other, Dominy doesn’t know if the relationship of this pair can ever be determined from the DNA of the specimens. But no one knew a few years ago that so much information could be gleaned from two tufts of hair and a couple of pieces of bone.

In 2004, Bruce Patterson of the Field Museum wrote his own book, The Lions of Tsavo: Exploring the Legacy of Africa’s Notorious Man-Eaters. In it he wrote that “an accurate tally of the lions’ depredations is impossible because detailed records were not kept.” Dominy’s isotope analysis has now largely resolved that question, Patterson says. “It’s amazing that something as incomplete as a trophy rug . . . can tell such a remarkably detailed story a century after an animal’s death.”

Freelancer Susan Frith wrote about Sara Baartman, the “Hottentot Venus,” in June. She lives in Orlando, Florida.