Six million years ago, the Andalgalornis steulleti or “Terror Bird” roamed South America in search of prey. The bird was about half as tall as a modern day ostrich yet its skull was twice as large. There are no counterparts to the terror bird found today – which makes studying this extinct animal all the more interesting to Dr. Witmer, a professor at Ohio University’s College of Osteopathic Medicine, and to the team of international researchers of which he’s a part. To coincide with the team’s publication entitled, Mechanical Analysis of Feeding Behavior in the Extinct “Terror Bird” Andalgalornis steulletti (Gruiformes: Phorushrcidae), Dr. Witmer was kind enough to answer a few questions about himself, his research and his experience with PLoS ONE.
When did you become interested in studying the heads of extinct animals and why do you feel that this type of study is important?
I’ve been working on skulls since I started grad school in 1983. So much of my lab’s research involves using modern-day animals to “flesh out” the past, that is, to figure out what soft tissues clothed and animated the skeleton. The head is where so much of the action is. It’s where the brain and sense organs are located; it’s where all the sustenance (food, water, air) comes in, etc. If we can figure out how the head of an extinct animal worked, we’ll know a lot about the animal in general.
It must be difficult to reconstruct the feeding habits of an extinct animal without a modern day counterpart to compare it with. How did you overcome this hurdle to perform the simulations needed to generate testable hypotheses?
When dealing with extinct animals like terror birds that have no real analogs in the modern realm, we indeed face real challenges. We approached Andalgalornis from two related directions: anatomy and biomechanics. I was chiefly involved in the anatomical side, working closely with lead author Federico (“Dino”) Degrange and his doctoral advisor Claudia Tambussi at the Museo de La Plata. I CT scanned the skulls of Andalgalornis and some other terror bird species, which allowed Dino, Claudia, and I to really get a sense of both the internal and external structure of their skulls. We found that, in terror birds, almost all of the flexible joints we see in other bird skulls had been evolutionarily transformed into rigid structural beams that made the skull very stiff and probably a good bit heavier.
Using a biomechanical method called finite-element analysis, Steve Wroe along with Dino and Karen Moreno tested different feeding hypotheses by seeing how they performed in the simulations. The simulations revealed that that the skull of Andalgalornis was well adapted for striking straight down and then tugging back with the neck, but that shaking the head from side to side would have been potentially catastrophic. Those biomechanical findings meshed perfectly with the anatomy that also showed the skull was very strong and rigid in the fore-aft direction, but weak from side to side, particularly with that hollow beak.
Most carnivorous birds use talons on their feet and the hook on the tip of their beak to capture and kill their prey. Did you find that the Andalgalornis used the same method to kill its prey?
Terror birds were flightless ground birds that didn’t have the deadly talons of a raptor. I’m sure they used their feet as weapons—ostrich and cassowaries do so today—but the powerful skulls of terror birds were their killing weapons. When you combine the information from anatomy and mechanics, it seems pretty clear that Andalgalornis and probably other terror birds used their skulls to kill prey with something like a rapid series of well-targeted, hatchet-like jabs directed straight down into the prey. But their skulls had an “Achilles heel,” if you will, which was that it was so weak from side to side. That meant that terror birds had to be careful hunters, using their speed and agility to dance around like a boxer, repeatedly attacking and retreating. Once the kill was complete, the hooked beak and powerful neck was used to tear the prey apart, pulling straight back, probably much like a modern-day hawk.
During the course of your research on the Terror Bird, was there something surprising that you found which you did not expect?
We were surprised that the force of biting wasn’t greater in Andalgalornis. As we say in the article, our estimate of bite force might be a bit low for some technical reasons, but in general we don’t think that this bird was delivering hugely powerful bites. It seemsbetter adapted for the hatchet-like jabs we envision than for cracking open bones or delivering a killing bite.
What is your next big research project or where would you like to go from here?
I’m not done with terror birds just yet. My colleague Bob Chandler (Georgia College and State University) and I also discovered that terror birds have remarkable, hollow beaks, despite being so tall and narrow. Bob and I are pursuing that project down the road.
Currently, I have some research done and ready to go with Ryan Ridgely in my lab and Bob on the brain and ear structure of terror birds that also sheds light on the behavior of these ancient predators by revealing their sensory capabilities and brain power. Spoiler alert: these birds were no obligate scavengers, but rather had the sensory and neural tools of active pursuit predators. More broadly, the research at WitmerLab at Ohio University is directed at fleshing out the heads of not only terror birds, but dinosaurs in general. We’re integrating information on brains, ears, muscles, sinuses, airflow, skin…and the skull, of course…to understand the heads of dinosaurs, including their feathered descendants.
This is the second time you’ve published with PLoS ONE, what made you decide to submit again?
PLoS ONE is the future. I support their initiative in open-access. Science shouldn’t just be for scientists. PLoS ONE has been a good partner in the outreach we’re doing in conjunction with the article. By being open-access, PLoS ONE allows us to take our science directly to anyone who might be interested. I’ve currently got a manuscript under review at PLoS ONE and another being readied for submission. I’m looking forward to a long-term relationship with PLoS!