Head Butting Goats: Part I

This post is slowly getting written. . .SVP talk preparation and other commitments take away blogging time! Anyhow, here's part I. . .

Farke, A. A. 2008. Frontal sinuses and head butting in goats: a finite element analysis. Journal of Experimental Biology 211: 3085-3094. doi: 10.1242/jeb.019042

To get a PDF of this paper, try this link first. If it doesn't work, email me at andyfarke [at] hotmail [dot] com, and I'll send you a (legal) link for a free download.

You're a Paleontologist - Why Head Butting in Goats?
This paper is all about the dinosaurs, really! Years ago (when I was in high school, in fact, back in 1996), I read an article by Cathy Forster on skull anatomy in the horned dinosaur Triceratops. She speculated on the function of these sinuses (hollow spaces above the brain and below the horns), noting some similarities with horned mammals such as bighorn sheep. Sheep and many of their relatives also have these massive sinuses in their skulls (see image at left) - some researchers posit that the sinuses serve as shock absorbers. The sinuses then protect the brain from being rattled around during horn-to-horn combat. Cathy (and others) thus inferred that because sheep have sinuses, and the sinuses are shock absorbers in these animals, then the sinuses of Triceratops are also probably related to shock absorption. Cool idea, huh?

But, I noted one problem: nobody has ever demonstrated that the sinuses of sheep, goats, and their relatives actually act as shock absorbers! It's one of these nice "truths" that remained untried. So, I decided to test this as one small part of my dissertation work on skull function in horned dinosaurs.

Methods to the Madness
How do you investigate head butting in a living animal? One path is to wire up the bone of the skull with strain gages, which measure the deformation of the bone during an activity. This didn't appeal to me for a few reasons: 1) It would be messy and invasive in living animals; 2) it would be just plain messy in dead animals; and 3) there was really no good way to experimentally manipulate the skull anatomy to test the effect of adding or removing sinuses. In all seriousness, it was point 3 that proved the most problematic.

The solution: computer modeling. Specifically, I used a technique called finite element analysis, or FEA for short. In brief, FEA allows you to model the physical "behavior" of a complexly-shaped structure under given conditions. For this study, it was a goat head under a load to the horns. So, what's so good about a computer analysis, over a "real-world" experimental approach? Most importantly, I could really, truly manipulate the skull anatomy. In order to measure the effect of sinuses, I made goat skulls with big sinuses. Goat skulls with small sinuses. Goat skulls with no sinuses at all. You just can't do this in real life!

I chose goats rather than other horned mammals because they were cheap, easily accessible, and a well-studied lab animal already. An archaeologist colleague got me a fresh goat head, which I then CT scanned. From the CT scan, I developed a 3D model of the skull itself. This skull was then imported into commercial FEA software (Algor FEMpro). Finally, I told the software to "pretend" that the goat's horns were being loaded in various directions, to simulate the forces of head butting. I hit the "analyze" button, and waited the half hour or so for the results. . .

Remember, now, that I made models of goat heads with and without sinuses (see the image below left for external views of two of these models, modified from a figure in the original paper; you can't see the sinus region here). If sinuses truly protect the brain, I would expect 1) that strains in the bone surrounding the brain should be greatly reduced for models with sinuses; and 2) lots of energy should be absorbed in the walls of the sinuses before reaching the brain.

What did I find? Stay tuned for the thrilling sequel to this post!

Head Butting Goats and FEM: Teaser

Finally! The first paper from my dissertation has made it into press:

Farke, A. A. 2008. Frontal sinuses and head butting in goats: a finite element analysis. Journal of Experimental Biology 211: 3085-3094. doi: 10.1242/jeb.019042

Abstract: Frontal sinuses in goats and other mammals have been hypothesized to function as shock absorbers, protecting the brain from blows during intraspecific combat. Furthermore, sinuses are thought to form through removal of `structurally unnecessary' bone. These hypotheses were tested using finite element modeling. Three-dimensional models of domesticated goat (Capra hircus) skulls were constructed, with variable frontal bone and frontal sinus morphology, and loaded to simulate various head-butting behaviors. In general, models with sinuses experienced higher strain energy values (a proxy for shock absorption) than did models with unvaulted frontal bones, and the latter often had higher magnitudes than models with solid vaulted frontal bones. Furthermore, vaulted frontal bones did not reduce magnitudes of principal strain on the surface of the endocranial cavity relative to models with unvaulted frontal bones under most loading conditions. Thus, these results were only partially consistent with sinuses, or the bone that walls the sinuses, acting as shock absorbers. It is hypothesized that the keratinous horn sheaths and cranial sutures are probably more important for absorbing blows to the head. Models with sinuses did exhibit a more `efficient' distribution of stresses, as visualized by histograms in which models with solid frontal bones had numerous unloaded elements. This is consistent with the hypothesis that sinuses result at least in part from the removal of mechanically unnecessary bone.

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To get a PDF of this paper, try this link first. If you don't have institutional access via the link, email me at andyfarke [at] hotmail [dot] com, and I'll send you a (legal) link for a free download (more on this below).

Within the next few days, I'll have a post summarizing this research. For now, I'll just talk a little about. . .

JEB and Open Access
Journal of Experimental Biology is not an open access journal - although it does allow that option for a healthy (unaffordable, in my case) fee. But, they present an admirable compromise - all papers become freely available 6 months after initial publication. Although a full open access model would be ideal, I think the publishers have found a good middle ground. The publishers get their due priority, and folks who are willing to wait a few months will get full access to all papers (or can email the authors for a reprint). If only more upper tier journals were to follow this route!

Authors get a link that allows up to 50 free downloads of the PDF (for use before the PDF goes free in six months). As mentioned above, anyone who would like this link should email me at andyfarke [at] hotmail [dot] com. Out of respect for JEB (because I think they're one of the few commercial journals that might have researchers' interests at heart), I won't be posting the PDF outright at this time. But, don't be afraid to email me if you want a copy!