From Will’s Skull Page, here.

Here’s a skull of a wild boar. Note the loooong face, practically a straight line from the tip of the snout to the top of the back of the head.

We shall now proceed through a series of pig skulls with increasingly steep foreheads.

From the UCL Museums and Collections blog, here.

Some domestic pigs have a longish snout and nearly straight forehead, like their wild forebears. (Or foreboars, if you will.)

A cast skull from Carolina, available here.

But it seems–from a quick, unscientific, and in-no-way-standardized image search–that the vast majority of domestic pigs have at minimum a more steeply-inclined forehead.

This one was auctioned in New Zealand, at this site.

Foreheadization is becoming undeniable.

From skullbase.info, here.

Is this one any more pronounced than the one before? I’m not sure, and so far I’m too lazy to try superimposing the skulls. But they don’t even look like the same kind of animal as the wild boar shown at top.

From theweirdandwonderful.com, now apparently only available on Pinterest, here.

In my explorations so far, this appears to the ne plus ultra of short-faced, high-forehead domestic pigs, excluding truly pathological cases. The line from the inflection point of the forehead to the occiput is twice the length of the snout!

From theweirdandwonderful.com, now apparently only available on Pinterest, here.

Oddly enough, the high forehead in domestic pigs is not always associated with a super-short snout, as this skull demonstrates.

This figure from Owen et al. (2014) sums up the shape differences between domestic (left) and wild (right) Sus scrofa.

Okay, so domestic pigs have shorter snouts and steeper foreheads than wild pigs of the same species. But y tho? It seems to be part of the “domestication syndrome” present in many domesticated animals, which includes a shortened snout, smaller teeth, piebald coloration, floppy ears, a curly tail, and a host of other morphological and behavioral traits. Interestingly, pigs seem to show more aspects of domestication syndrome than any other domestic animals other than dogs, as shown in the figure below, from Sanchez-Villagra et al. (2016).

Okay, so domestication, but how? It’s not like the Domestication Fairy comes in the night and steals half your snout.

Wilkins et al. (2014: fig. 1)

The various morphological changes that go along with domestication syndrome seemed disconnected until 2014, when Wilkins et al. proposed a pretty nifty hypothesis, which goes like this:

  • Probably the most crucial aspect of domestication is selection for tameness, which is really selection for reduced adrenal gland and sympathetic nervous system activity, so the animals aren’t freaking out all the time.
  • The adrenal glands and sympathetic ganglia are derived from embryonic neural crest, which also influences the growth of the teeth, brain, skull, vertebral column, and ear cartilages, and the distribution of melanocytes in the skin and coat.
  • Selection for increased tameness (= reduced freaking out) is really selection for reduced neural crest activity in early development, and the smaller teeth, shorter snout, floppy ears, curly tail, patchy coloration, and so on, are unselected developmental consequences of reduced neural crest activity.

Wilkins et al. (2014: fig. 2)

So far, so good. The neural crest hypothesis seems to have genuine explanatory power, in that it lassos a disparate set of phenomena and provides a single, logical cause. Of course not everyone is convinced, and the neural crest hypothesis could be true without ruling out other complementary mechanisms and confounding effects. Along those lines, Sanchez-Villagra et al. (2016) is worth a read. It’s free at the link below, as is Wilkins et al. (2014).

The neural crest hypothesis might explain why domestic pigs have shorter snouts than their wild relatives, but I think there must be some other factors in play to explain pig foreheads. Which is fine, domestic dogs have a staggering variety of skull shapes that reflect thousands of years of strong artificial selection, and probably a healthy dose of unintended consequences and other knock-on effects. Given that pigs have been domesticated for a long time, were probably domesticated many times in many places, have had frequent infusions of wild-type genes (from possibly genetically disparate wild populations), and have been canalized into different breeds, it might actually be weirder if they all looked like short-snouted wild boars. All of which is a long way of saying that I’m not surprised that domestic pigs don’t all fall on some morphogenetic monocline from wild boars, but I’m still curious about how they got their foreheads.

I actually started writing this post before the very interesting discussion of pig domestication flared up in the comments on Mike’s pig skull post. Mike’s two skulls nicely illustrate the difference between forehead-less and, er, forehead-ful conditions, and the comment thread touched on a lot of related issues and is worth a read. In particular, I’d like to note again that domestic pig skulls are not notably paedomorphic with respect to wild boars, other than having short snouts–they’re on a different morphogenetic trajectory (Evin et al. 2016).

For a nice comparison of domestic pig and wild boar skulls, see Marcus Bühler’s post at Bestiarium, here.

UPDATE just a few days later: for a skeptical look at the very existence of domestication syndrome, see the new Lord et al. (2019) paper, “The history of farm foxes undermines the animal domestication syndrome”, freely available here.

References

Long-term readers will remember that way back in the pre-history of this blog, I wrote about my experience de-fleshing a pig head, which because the very first part in our ongoing series Things to Make and Do. In a subsequent post with a sheep-skull multiview, I included the multiview of that pig skull, too. Here it is:

Mike’s first pig skull, cranium only. Top row: dorsal view, anterior to right; middle row, from left to right: posterior, right lateral and anterior views; bottom row: ventral view, anterior to right.

As I noted in that sheep-skull post, I no longer own that skull: I donated it to be the first prize for the quiz in the very first TetZooCon, and it was won by Kelvin Britton.

But around the same time, our church hosted a barbecue even in which an entire pig was slow-roasted, and at the end of it I took the head home and prepped the skull out of it. The bone was much more fragile for having been roasted instead of simmered, and was in some danger of crumbling apart, but I stablised it with diluted PVA and it holds together OK.

Here it is:

Mike’s second pig skull, cranium and mandible in articulation. Top row: dorsal view, anterior to right; middle row, from left to right: posterior, left lateral (reversed) and anterior views; bottom row: ventral view, anterior to right.

Even allowing that the new skull was photographed with the mandible in place, the difference between the two is shocking. In particular, check out the dorsal views: the zygomatic arches of the first pig protrude way further laterally, and are much more robust than those of the second pig, and the whole shape of the skull roof is different.

I’m not sure what to make of this. I assume what we’re seeing here is variation of different breeds within the single domesticated species Sus domesticus, analogous to the way bulldog and greyhound skulls differer dramatically despite both being breeds of Canis familiaris. There are a lot of pig breeds out there, so perhaps it’s not too surprising. On the other hand, while the different dogs were bred for different purposes, I’d have thought all the pig were bred for the same purpose: to put on weight and provide meat. So I don’t know why such different skulls would have been selected for.

Well, it’s time. Ten years and almost 5 months after Mike kicked off our “Things to Make and Do” section with his post on cleaning a pig skull, I am finally getting around to prepping a pig skull of my own. There will be a complete play-by-play coming, but for now I want to focus on what is usually the least-pleasant step in prepping a skull: extracting the brain. Aside from the relatively small and often tortuous passages for the cranial nerves, the braincase is a cul-de-sac, with a big glob of tissue (the brain and associated meninges and vessels) only accessible through a relatively small hole at the back of the head (the foramen magnum). Virtually every tutorial and how-to on prepping skulls has some section where the author advises you to basically swirl something around in there, get stuff out the best you can, and prepare to deal with a lot of nastiness along the way. So I had my antennae out for anything that might help, and in the local dollar store I ran across the beauty shown above.

I figured drain rooter = brain rooter, and I was only risking a buck, so I picked one up. It worked a trick: by putting the pig’s snout down the drain, running hot water into the foramen magnum to continually flush out the loose bits, and vigorously exploring the cranial cavity with the brain rooter, I was able to get the whole brain out in about 10 minutes. To be clear, all the tissue came out the foramen magnum; there would be no way to get it to come out the nose without breaking the ethmoid bone and destroying the nasal turbinates. I only put the head snout-down for ease of access. I had a great deal of control, and I could tell pretty well which areas were getting emptied out and which still needed work. All I missed was a small glob of meninges and dural venous sinuses, which came out easily after the first simmer.

Some specific advantages of the drain rooter as a brain extractor:

  • backward-pointing teeth to hook out the tissue
  • flexible plastic so you can go pretty hard with it without damaging the bone
  • super long so you’re not going to find a job too big for it, OR you can cut it to length
  • still works for unclogging drains
  • dishwasher safe
  • dirt cheap

Go have fun.

UPDATE: Turns out pigs have an insane amount of cartilage and mucosa in their nasal cavities, and the brain rooter is pretty good at getting that stuff out, too.

Here’s a frozen pig head being hemisected with a band saw.

The head in question, and the other bits we’ll get to later on in this post, both came from Jessie Atterholt’s Thanksgiving pig. As soon as Jessie knew she was cooking a pig for Thanksgiving, she had a plan for the head and the feet: cut ’em in half, skeletonize one half (like Mike did with his pig head), and plastinate the other. Jessie has her own plastination setup and you can see some of her work in her Instagram feed, here.

Here’s the freshly hemisected head. At one time or another, about four of us were involved in checking the alignment of the cut, with the intention of just missing the nasal septum (it can be easier to see some of the internal nasal anatomy if the septum’s all on one side). But we were all wrong–not only did the saw hit the nasal septum dead on, it hemisected the septum itself. Which I guess is the next-best possible outcome. The septum is the big expanse of white cartilage behind the nose and in front of the brain. You have one, too–it separates your left and right nasal cavities–but yours is a lot thinner.

Here’s the left half washed off and cleaned up a bit.

I was completely entranced by the little blood vessels inside the nasal septum, seen here as tiny traceries of red inside the blue-white cartilage. Also notice the frontal sinus above the septum and in front of the brain.

Here’s the right half in a postero-medial oblique view. Shown well here are the first two cervical vertebrae, plus part of the third, and the intervertebral joints. This was a young pig and the remains of growth plates are still visible between the different ossification centers of the vertebrae. If I get inspired (= if I get time) I might do a whole post on that.

It wasn’t my pig or my show, but Jessie made me a gift of two pig feet, and I got a little time on the saw. Here I’m using a plastic tool to push one of the pig’s hind feet through the saw.

We had been dithering over how best to prep the feet but the lure of the band saw proved irresistable: we hemisected all four. We’re planning to do half skeletonized/half plastinated preps for all of them, a forefoot and a hindfoot set for each of us.

Jessie and I were joined by two other WesternU anatomists, Thierra Nalley and Jeremiah Scott. Here Thierra is explaining to Jeremiah, who works on primate dentition and diet, that mammals have more parts than just teeth.

That’s a good segue to this video I shot, in which Thierra gives a quick tour of the hemisected pig head. All four of us have just come off of teaching human head and neck anatomy, so it was cool to see in another mammal the same structures we’ve just been dissecting in humans.

From 1:40 to 1:55 in the video Thierra and I are discussing the prenasal bone, something pigs have that we don’t. It’s the separate bone at the end of the snout in this mounted skeleton:

Darren discusses and illustrates the prenasal bone in this Tetrapod Zoology post.

Parting shots: many thanks to Ken Noriega and Tony Marino of WesternU’s College of Veterinary Medicine for their guidance, assistance, and expertise. Jessie covered this dissection as an Instagram story, here–I believe you have to be signed in to see it. Update: Jessie added a regular stream post, with lots of features labeled, here. I’ll probably have more to say about this pig and its bits in the future. Stay tuned!

For more hemisected heads and skulls, see: