Terror birds — boxers wielding hatchets

The world seems to have finally discovered terror birds, the sometimes gigantic, extinct, flightless predatory birds that roamed South American landscapes for much of the last 60 million years. Of course we’ve known about them for more than a century, and they’ve often made it into the “dinosaurs and animals of the past” genre of books, but terror birds never seemed to break through to the big time. But terror birds must have hired an agent, because they’re downright hot. The “Big Three” US television cable networks that run science documentaries—the National Geographic, Discovery, and History Channels—all ran terror bird shows.

A new article hits the tubes of the interwebs
And today, an article appeared in PLoS ONE that presents research conducted by a multinational team on skull structure and function in the terror bird Andalgalornis. I (Witmer) was part of that team (and appeared on all three of the documentaries…skip to the end of the post to watch clips), and so it seems appropriate to blog about it here.

Find out more about it
I’m not going to say too much about the details of the study itself. Since it’s published in the open-source online journal PLOS ONE, you can freely download the article itself. You can also find links to the press release and media graphics on the Ohio University WitmerLab site, as well as a common language summary. NSF also did a news release.

Animated GIF of stresses on Andalgalornis skull during a normal bite (click to activate animation).

The science-y bits, stripped to the core
In a nutshell, our studies combined CT-scan-based anatomical studies with 3D engineering simulations using finite-element analysis (FEA) to test hypotheses of feeding behavior in the 6-million-year-old terror bird Andalgalornis. The anatomical and biomechanical studies came up with the same result: the tall narrow skull was very strong and rigid sagittally (vertically and fore-aft), but the beak was pretty weak transversely (side to side). Moreover, the reconstructed bite forces were lower than we expected and were moderate at best (though you still wouldn’t want to wave your hands near its mouth!).

Killing strategies — more Ali than Frazier
The bottom line for feeding and killing strategies is that Andalgalornis (and maybe terror birds in general) was no slugger or wrestler, grappling with prey to deliver a killing bite—its beak couldn’t handle the twisting and side-to-side bending that that strategy would require. Instead, it was more of a boxer delivering repeated hatchet-like jabs straight down on the prey driven by the powerful neck muscles. It presumably used its speed and agility to attack and retreat, over and over, delivering well-targeted blows with surgical precision, being careful not to twist or bend its beak. Once the prey was killed, the terror bird either swallowed it whole or ripped off bite-sized chunks tugging straight back with its hooked beak, a behavior shown by the FEA simulations to be safe and reasonable. This scenario seems pretty detailed, but we’re confident that it’s basically correct. Maybe surprisingly, the weaknesses in the skull told us as much as its strengths, allowing us to really dial in the behavior. In fact, there aren’t really too many other killing options for this beast.

I came up with the metaphor of Muhammad Ali versus Joe Frazier for the new release: Andalgalornis couldn’t have been a slugger like Frazier and must have been an elegant jabbing boxer like Ali. I thought I was pretty clever. This Ali-Frazier metaphor is even used in politics and business to depict agile and adaptable versus brutish and slow. When I ran it past my coauthors, Steve Wroe (an Aussie) loved it, but it sadly left the South Americans (Dino Degrange, Claudia Tambussi, and Karen Moreno) scratching their heads. Oh well…  We kept it in the release but toned it down.

How did this study come about?

I’ve been working on terror birds since 2007 with Bob Chandler (Georgia College and State University). I got several skulls on loan from the Field Museum of Natural History (Chicago) and American Museum of Natural History (NYC) for CT scanning, including Andalgalornis, Andrewsornis and a couple skulls of Psilopterus. In conjunction with Ryan Ridgely, the WitmerLab 3D viz guru and co-PI on my NSF grants, Bob and I started working on the brain, inner ear, and sensory systems. We also discovered a remarkable hollow cavity within the beaks of all the terror birds for which we had CT data. Very interesting findings, but a topic for another day.

Witmer with the terror bird skulls CT-scanned for various projects. Andalgalornis at left. Andrewsornis at rear. Two Psilopterus skulls in the foreground.

It turns out that the National Geographic Channel was filming a terror bird episode for its “Prehistoric Predator” series. They contacted Bob, because he’s the world expert on Titanis, the gigantic terror bird that wandered north into Texas and Florida once the land bridge opened up 2–3 million years ago. Bob suggested they talk to me, too.

In February 2008, Darryl Rehr, the director of the documentary, pointed out to me that there was this guy, Steve Wroe, in Australia who was also part of the show and was doing engineering simulations on the skull Andalgalornis. I was intrigued because, as far as I knew, the only decent skull of this terror bird was in my lab, and I was the first to scan it! I contacted Steve, whom I knew pretty well, and he mentioned that he and Karen were working with this great grad student from La Plata named Dino Degrange…but they were just using a cast of the skull I had in my lab.

Witmer and article lead author Dino Degrange during his visit to WitmerLab in October 2008

And the rest is history. I offered them the original CT data, my career-long expertise with avian skull anatomy, and my growing expertise in terror birds…and we had a wonderful collaboration that finally came to fruition today!

In fact, this post has given me a spark of an idea for a future blog post about how my involvement in TV documentaries has spawned a number of new research projects, new collaborations, new analyses, and whole new research directions.

–Larry Witmer

Below are the clips from the TV documentaries about terror birds in which I have appeared.

National Geographic Channel: “Prehistoric Predators: Terror Bird”

Discovery Channel: “MegaBeasts: Terror Bird” (later “Monsters Resurrected: Terror Bird”)

History Channel: “Prehistoric Monsters Revealed”