Another clue comes from the predatory habits of the closest living relatives of terror birdsseriema. The family of tropical, South American birds prey on lizards, snakes, and small birds from the air.
On the Run
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Understanding the running speed of terror birds can shed light on many aspects of their hunting behavior, said Ernesto Blanco of the Universidad de la República in Montevideo, Uruguay.
Speed is "very important for predators that pursue prey on open land," he said. "It's not only relevant for chasing prey, but also for colliding with more energy to inflict greater damage."
Blanco is the lead author of a new study on terror-bird running speeds recently published in the journal Proceedings of the Royal Society B.
The scientist says high speed not only enables carnivores to catch a wider range of prey but allows them to surprise and confuse their victims more readily. This minimizes the chances of a potentially risky counterattack.
Some of the species that terror birds may have preyed on were heavily armored. The armadillo-like glyptodont, for example, was the size of a small car, covered in plates of body armor, and equipped with a hefty tail club.
To estimate the maximum running speeds of three species of terror bird, Blanco and paleontologist colleague Washington W. Jones used mathematics to model the relationship between bone stress and speed.
Comparing data on the fossil leg bones of various terror-bird species, the researchers estimated maximum running speed as that which puts the bones at considerable risk of breakage.
Blanco had successfully used a similar method to estimate the speed of 30 living mammals and the T-rex-like dinosaur Giganotosaurus. The approach also provided accurate results for ostriches, emus, and rheasthe three living species that are most physically similar to terror birds today.
Mechanical modeling suggested that two terror-bird species may have run at speeds of up to 30 miles an hour (50 kilometers an hour), or about as fast as modern ostriches do.
More striking were the results for Mesembriornis, a 10-million-year-old terror bird species that weighed 150 pounds (70 kilograms). Modeling suggests that the predator may have been able to run at a blazing 60 miles an hour (97 kilometers an hour).
The speed is comparable to that of a cheetah, the fastest land animal today, running at full speed.
Alvarenga, the Brazilian terror bird expert, who was not involved in the study, says he is skeptical of the study's results. He argues that more bone measurements would be necessary to confirm the findings.
He adds that an understanding of terror-bird musculatureparticularly the points of origin of muscles on the thigh and pelvisis necessary to estimate running speeds.
Kung Fu Tactics
Blanco, the lead study author, calculated that Mesembriornis might have run at 60-mile-an-hour speeds to hunt down prey. But he favors an alternate explanation for the superstrong leg bones.
"In some cases the limbs have other functions, which affect their strength. This could cause us to mistakenly estimate extraordinarily high values of running speed," Blanco said. "Our favorite interpretation of the result is actually that the animals used their legs for kicking, and this is why they are so strong."
Blanco found evidence in studies of martial arts to back up evidence for the terror birds' kung fu tactics. He has shown that the leg bones of Mesembriornis would have been able to resist the forces necessary to break the bones of medium-size mammal prey.
"Having the skills to access nutritious and energy-rich bone marrow is a very important adaptation for a carnivore," Blanco said.
Bearded vultures are the only living birds known to access bone marrow, which they do by dashing bones onto rocks from great heights.
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