The Bite of the Terror Bird

Figure 1. Skull of Andalgalornis steulleti (FMNH P1435).

So you're flipping through various websites and journal table of contents. You come across an article about the feeding behavior of the ancient "terror bird." I think it's pretty clear that you must stop and read it.

Andalgalornis steulleti is a member of an extinct group of large, flightless birds known as phorusrhacids. This group were predominatly a South American radiation of gruiform birds from the middle to lower Paleocene and are most closely related to extant seriemas. Phorusrhacids are called "terror birds" due to their gigantic body sizes, large skulls, and carnivorous lifestyles. The members of this group were ground predators or scavengers that were likely apex predators that dominated their environment in the absence of large carnivorous mammals. A. steulleti lived approximately 6 million years ago in Argentina, it weighed about 40kg, stood 1.4m high, and had a skull length of 370mm. This large, rigid skull was capped with a hawk-like hooked, yet curiously hollow, beak. The feeding behavior of these birds has only been speculated at up till now. This new study performed a biomechanical analysis of the skull using comparative anatomy and engineering (Finite Element Analysis [FEA]) to predict the behavior of the skull. Basically, they looked at the skull itself, compared it to other skulls, and ran it though a CT scanner for analysis.

Figure 2. Stress (Von Mises) distribution of FE models.

Now, if you look at most birds you'll notice that their skulls allow for a lot of mobility between their bones. This gives them light but strong skulls. A steulleti, on the other hand, showed rigid beams in these normally mobile areas. This gave the bird a very strong skull, particularly in the fore-aft direction. The FEA analysis worked with the 3D models created by the CT scans to simulate and compare the biomechanics of biting straight down, pulling back with the neck, and shaking the skull side to side. These are all attack and dismembering motions (lovely). One of the neat things about FEA analysis is that it gives you color images that show areas of low stress as cool-blue and high stress as white-hot (image left). The results from this analysis show that the terror bird was well adapted for driving its beak in and the pulling back. Its marks weren't so high in the shaking side to side motion. Some more comparative anatomy came in when the researchers tested bite force. They had an eagle bite down on a bite meter - bet that was fun to try to do - and they used that information to compare it to the bite of A. steulleti. Pretty close, as they are both large birds with big, hooked beaks. The results showed that the terror bird had a lower than expected bite force that was weaker than the bite of similarly sized carnivorous mammals. This weaker bite force was likely supplimented by the driving down and pulling back motion that they found with the previous test. Overall, behaviorally speaking, the bird probably located prey, stabbed it with its beak and then implimented a repeated attack-and-retreat strategy, puncturing its prey until it was eatable.

Here's the article:
Degrange, Federico J., Claudia P. Tambussi, Karen Moreno, Lawrence M. Witmer, and Stephen Wroe. (2010) Mechanical Analysis of Feeding Behavior in the Extinct 'Terror Bird' Andalgalornis steulleti (Gruiformes: Phorusrhacidae). PLoS ONE: 5(8), e11856. (DOI: 10.1371/journal.pone.0011856)

and...
http://www.sciencedaily.com/releases/2010/08/100818171916.htm