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Eagle Ancestors Hunted Early Humans, Skull Study Suggests

James Owen
for National Geographic News
September 5, 2006
 
Ancient relatives of eagles not only hunted and ate early humans but also influenced how our species evolved, a new study suggests.

In addition, the research appears to settle a long-running debate over what killed a prehistoric child in Africa 2.5 million years ago.

The findings are based on an analysis of more than 600 modern-day monkey bones collected beneath the nests of African crowned eagles in the rain forests of Côte d'Ivoire (Ivory Coast).

The monkey remains revealed that the eagles—which typically weigh between 10 and 12 pounds (4.5 and 5 kilograms)—frequently kill and eat primates weighing up to 24 pounds (11 kilograms).

The researchers say that they were surprised to find that large monkey species that live on the ground are a common target for the raptors.

The study suggests that birds of prey "have been a selective force in primate evolution for a long time," says lead study author W. Scott McGraw, an associate professor of anthropology at Ohio State University in Columbus.

The study is now online and will be published in the October issue of the American Journal of Physical Anthropology.

"Before this study, I thought that eagles wouldn't contribute that much to the mortality rate of primates in the forest," McGraw added. "I couldn't have been more wrong."

Targeting Ground Monkeys

Evolutionary biologist Susanne Schultz of the University of Liverpool in England collected prey remains from under eagle nests in the Taï rain forest in the southwestern part of Côte d'Ivoire (see map).

The discarded remains included bones and skulls of mangabeys, large, predominantly ground-living monkeys.

"I thought you would see the eagles eating monkeys that mainly lived in the trees, because the birds are flying around the canopy," Schultz said. "But they were actually hitting animals on the ground harder."

Mangabeys turned up in the eagle nests "more often than chance would predict," according to lead study author McGraw.

"It appears that the [African crowned] eagle specifically targets these large, relatively rare monkeys," he added.

(Related news: "New Monkey Species Discovered in East Africa" [May 2005].)

The new study found that most of the monkeys were dismembered before being taken to the nest.

This, Schultz says, explains how an eagle is able to deal with prey more than twice the bird's body weight.

The behavior also offers an alternate theory for the cause of death of an early human child whose remains were discovered in a cave at Taung, South Africa, in 1924.

The latest research suggests that prehistoric raptors would have been quite capable of successfully attacking a young Australopithecus africanus. The Taung cave specimen weighed around 26 pounds (12 kiolgrams) at death.

"When the Taung baby was discovered, there were several people who proposed that an eagle had perhaps killed it," Schultz said.

"One of the big criticisms of that was the Taung baby would have been way too heavy to be picked up by an eagle. But that's totally misunderstanding the behavior of these eagles."

African crowned eagles have been known to occasionally attack or eat human children, Schultz says.

"There's one report from South Africa of a small child's skull being found in a nest," she added.

Eye Orbits

What's more, the Taung skull showed puncture marks around its eye sockets, or orbits, that are characteristic of eagle attacks, Schultz says.

"We collected quite a few [monkey] skulls with the same patterns of damage on the back of the eye orbits," she added. "There's one particular skull with identical damage to the Taung baby."

Lee Berger is a human evolution expert at the University of the Witwatersrand in Johannesburg, South Africa, who was not involved in the study. He supports the idea that the Taung child was preyed on by an eagle.

At a conference in Johannesburg last January to mark 80 years of debate over the Taung find, Berger revealed that he had reexamined the skull after hearing of the new Côte d'Ivoire research.

"I almost dropped down when I looked into the eyes of the skull and I saw the marks," he recalled. "I couldn't believe my eyes, as thousands of scientists—including myself—had overlooked this critical damage."

The study authors believe that the Taung child was attacked by a raptor that was about the same size as an African crowned eagle. "There were definitely large, modern-looking eagles around at that time," the University of Liverpool's Schultz said.

Eagles, along with other primate predators, might therefore have played an important role in how early humans evolved, the study team says (human evolution interactive map).

The birds of prey may have influenced an evolutionary move toward group living and bigger body size, for example.

"The larger the group you're in, the more protection you have against predators and the more chance you have of detecting them or fighting them off," Schultz said.

"The other thing you'd probably expect to see is an increase in body size," she added. "A way to reduce your risk is to be bigger, and so harder to catch."

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