Computer Models Show Terror Birds Hunted by Sound

Prehistoric killer used low frequency sounds to follow its prey's footsteps

Photo: M. Taglioretti and F. Scaglia
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Terror birds, one of South America's most feared prehistoric predators, didn't use super sharp eyesight to catch prey, as do eagles, hawks, and other modern day raptors. Using sophisticated computed tomography X-ray scans and 3-D modelling software, researchers were able to show that these flightless giants actually hunted by listening to their quarry’s footsteps.

Terror birds, or phorusrhacids, to give them their scientific name, were top predators in South America for fifty million years after the dinosaurs died out, finally going extinct 1.8 million years ago. Terror bird fossils have also been discovered in North America, Africa, and Europe. These feathered killers stood from1 to 3 meters tall. They had huge hooked beaks, and the biggest ones could use them to kill with one gigantic stab.

In 2010, paleontologists discovered the almost complete skeleton of a new species of terror bird, near Mar del Plata in Argentina. They named it Llallawavis scagliai, after Galileo Juan Scaglia, one of Argentina's most celebrated naturalists. Scaglia’s grandson, Fernando, was part of the discovery team.

Subsequent analysis revealed that this bird would have stood 1.2 metres high and weighed around 18 kilograms. The remains were so well preserved, however, that a team led by Federico Degrange, assistant researcher of vertebrate paleontology at the Universidad Nacional de Córdoba, was also able to study the bird’s auditory system.

Degrange used a hi-speed medical CT scanner to produce detailed 2-D images of the bird’s inner ear. CT scans enable scientists to see minute detail inside fossils. These images were then transformed into 3-D segmented models using Materialise Mimics, 3-D imaging software. Mimics (Materialise Interactive Medical Image Control System) uses a special medical imaging aglorithm called “marching cube”  that takes partial volume into account to produce more accurate 3-D models.

The results were published in the Journal of Vertebrate Paleontology. Degrange explains that he based his deductions about Llallawavis’ hearing capabilities on the length of the terror bird’s cochlea.

“We calculated that it would have had a mean hearing range of approximately 3800 Hz and a mean hearing sensitivity of approximately 2300 Hz,” he says. Mean hearing range is the average difference between the highest and lowest frequencies that the bird could have heard. Hearing sensitivity, is the frequency at which the bird's hearing was the most acute.

Llallawavis would have been much better at hearing low frequency sounds than are humans. We hear best between 4,000 and 5,000 Hz. The ability to hear low frequencies would have also helped it locate and track the small mammals and birds it preyed upon. The bird would have been able to hear footsteps, even if the prey animal was hidden in the undergrowth. Crocodiles hear low frequency sounds. So too, did Tyrannosaurus Rex.

Degrange also thinks that Llallawavis would have had a deep booming voice, a bit like an ostrich. “This is plausible to hypothesize because the vocalization range of most birds falls within the lower half of their hearing sensitivity range,” he says.

Degrange admits that he doesn’t know about the bird’s voice for sure, though. The tracheobronchial syrinx, the structure that produces sound in birds, was missing from the remains. This structure is made mainly of cartilage and didn’t survive 2.5 million years in the ground.

Degrange is currently studying the terror bird’s eye bones, brain case, and skull. He intends to discover more about its vision and senses, and from this deduce whether Llallawavis was active during the day, at night or during twilight hours. This he hopes, could provide clues as to why terror birds died out.

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