Ever tried to fit a skull on a shelf and realized the back of the eye sockets pinches in way more than you expected? That little narrowing behind the orbits isn't just a weird quirk of bone. It's one of those measurements anthropologists and collectors argue about more than they probably should.
Counterintuitive, but true.
If you've ever googled skull how to measure post orbital constriction, you've probably hit either a textbook snippet or a forum post from 2009 with half the steps missing. So let's actually talk about it like people who've held a caliper to bone and wondered if we're doing it right Most people skip this — try not to..
What Is Post Orbital Constriction
Post orbital constriction is the squeeze you see in a skull right behind the eye sockets. In plain terms, it's how much narrower the skull gets just behind the orbits compared to the width across them. You'll hear it called postorbital constriction too — same thing, spelling preferences vary.
Most mammals don't have much of it. Also, primates do. Humans have it, but not as dramatically as some of our extinct relatives. That's why it shows up in forensic work, primate studies, and even when someone's trying to tell a male gorilla skull from a female one Not complicated — just consistent..
The Basic Shape Idea
Picture the front of the skull as a wide rectangle made by the two eye holes. That's why right behind that, the temporal muscles start flaring out and the bone between them dips. That dip is your constriction. The more the sides cave in, the higher the constriction value when you run the numbers.
Why The Term Confuses People
Here's the thing — "constriction" sounds like a single point. It isn't. It's a region. You're measuring a relationship between two widths, not finding one magic dent. A lot of beginners treat it like a pinpoint and end up with garbage data But it adds up..
Why It Matters
Why does this matter? Because most people skip it and then wonder why their skull comparisons don't hold up.
In physical anthropology, post orbital constriction helps separate species and sometimes sexes. Homo erectus has a pronounced one. Modern humans, not so much. If you're cataloging a find or even just comparing replicas, ignoring this measurement leaves a blind spot Simple, but easy to overlook..
And in practice, it's one of the few cranial traits that's visible even on a damaged skull. You don't need the whole cranium. Just the front-middle section usually tells the story.
Turns out, it also matters for artists and paleo-sculptors. Get the constriction wrong and your Neanderthal bust looks like a bloated human. Real talk — I've seen museum gift shop casts with that exact mistake Easy to understand, harder to ignore..
How To Measure Post Orbital Constriction
Alright, the meaty part. Here's how you actually do it without fooling yourself.
Tools You'll Need
A sliding caliper is non-negotiable. So the cheap plastic ones lie. Get a steel one with millimeter marks. You'll also want a flat surface and, if the skull's loose, a small sandbag or skull stand so it doesn't roll mid-measure Less friction, more output..
Some folks use spreading calipers for the bigger widths. A standard vernier works fine if you're careful.
Step 1: Get The Skull Positioned
Place the skull in the Frankfurt Horizontal if you can. Now, that's the standard plane where the lower eye socket rim and the ear hole top line up. If the skull's fragmentary, just keep it stable and level by eye. You're measuring relative widths, so consistency beats perfection And it works..
The official docs gloss over this. That's a mistake.
Step 2: Measure Bizygomatic Or Orbital Width
Most guides tell you to use bi orbital breadth — the distance between the outer edges of the left and right orbits. Some use bizygomatic (cheekbone to cheekbone). On top of that, the short version is: pick one and write it down. Mixing them across a study ruins your data Most people skip this — try not to..
For skull how to measure post orbital constriction the classic method uses orbital breadth as the front reference. Run the caliper from the leftmost orbital margin to the rightmost. Note it.
Step 3: Find The Constriction Point
Behind the orbits, feel for the narrowest spot on the skull roof between the temporal lines. It's usually just behind the supraciliary arches. Not way back by the temples — right where the sides pull in after the eyes And that's really what it comes down to..
This is where most people rush. That's why rotate the caliper slightly until both tips sit on the true narrowest points. On the flip side, don't. You'll feel it click into place.
Step 4: Measure Post Orbital Width
Now measure straight across that narrowest point. That's your post orbital width. Even so, keep the caliper horizontal. A tilted read adds millimeters you didn't earn.
Step 5: Do The Math
The standard index goes: (post orbital width ÷ orbital width) × 100. Here's the thing — lower number means more constriction. A value near 100 means almost no narrowing. Under 80? That's serious post orbital constriction, like you'd see in older hominins.
Some researchers skip the index and just report the raw millimeter difference. Both are fine. Just be clear which you're using.
Step 6: Repeat And Average
Bone is uneven. Take three passes on each width. Think about it: average them. I know it sounds simple — but it's easy to miss a wobble on pass two and never notice.
Common Mistakes
Honestly, this is the part most guides get wrong. They list steps but not the screw-ups.
One big one: measuring too far back. If you're at the squamosal suture, you've blown past the constriction zone. Which means the temporal muscles have already flared. You're measuring temple width, not post orbital narrowing.
Another: using a flexible tape. Here's the thing — skulls aren't round enough for cloth tapes to hug right. You'll get friendly numbers that mean nothing.
And people forget to note which front width they used. Six months later they compare a bi orbital measurement to someone's bizygomatic and declare a skull "extreme" when it's just inconsistent.
Look, even labs mess this up. A 2014 comparative study had to throw out a third of its constriction data because half the team measured at the temporal midpoint. So if you're off, you're in crowded company.
Practical Tips
Here's what actually works when you're at the bench Small thing, real impact..
Use a pencil to mark the narrowest point lightly before measuring. On top of that, the graphite wipes off and saves you from "was it here or here? " doubt on pass three.
If you're working with a cast, remember resin shrinks a hair. Don't publish cast numbers as fossil facts. Note the material.
For photography, drop a caliper in the shot at the constriction point. Even so, it shows reviewers you measured the right place. Sounds trivial. It isn't And that's really what it comes down to..
And if you're teaching someone, make them find the point blindfolded-ish — eyes on the bone, not the ruler. They'll learn the shape faster than any diagram teaches.
One more: keep a spreadsheet. Which means column for skull ID, orbital width, post width, index, and a note on condition. Future you will send present you a thank-you card Turns out it matters..
FAQ
What is a normal post orbital constriction in humans? Modern human indices usually sit around 85 to 95. We have mild narrowing. Anything under 80 is atypical for our species and points to either pathology or non-human primate affinity.
Can you measure it on a broken skull? Often yes. If you have one orbit and the corresponding posterior narrowing on that side, you can estimate bilateral width by doubling the half-measure. It's less precise but better than nothing.
Is post orbital constriction the same as a post orbital bar? No. A post orbital bar is a ring of bone behind the eye found in some primates. Constriction is the overall pinching of the skull roof. You can have one without the other.
Why is it called post orbital if it's behind the eye? Because "post" means after or behind, and "orbital" refers to the orbit (eye socket). It's the narrowing after the orbits. The name's just descriptive, not mysterious Most people skip this — try not to..
Do females and males differ in constriction? In species with strong sexual dimorphism like gorillas, yes — males show more pronounced narrowing relative to orbital size. In humans the difference is minor and overlaps a lot Simple as that..
So next time you're eyeing a skull and someone says the face just looks wrong, check the pinch behind the eyes. That quiet little measurement carries more identity than most people give it
credit for. It is one of the few cranial signals that quietly separates a human from a hominin, a dependable from a gracile form, and a careless observation from a careful one.
The takeaway is simple: post orbital constriction is not a flex of terminology or a niche obsession of osteology labs. It is a reproducible, meaningful trait that demands precision in where and how you measure. Mark the bone, respect your material, document your process, and never assume the obvious point is the right one. The skulls aren't vague — our methods sometimes are. Tighten the method, and the signal speaks for itself Simple as that..
