Why do some domestic pigs have foreheads?

November 30, 2019

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.

References

8 Responses to “Why do some domestic pigs have foreheads?”

  1. LeeB Says:

    I am wondering if the short head high forehead look makes the piglets look cuter; also if it is linked to ears that are large and tend to flop forward over the face.
    LeeB.

  2. Allen Hazen Says:

    Facial skeleton and “cranium” (part of the skull that covers the brain) are under somewhat different genetic?? and/or developmental?? “regimes,” no? Humans with the “achondroplastic dwarfism” syndrome have small faces with — since the cranium is not similarly reduced — bulging foreheads. (Or such is my layman’s impression.) So think of the pig skull as formed by combining originally separate (not necessarily separated– I just mean independent, “built to different blueprints”) facial (mainly snout) and “cranial” components. Now reduce the size of the facial part: not just shortening it in length, but reducing all its dimensions. The bottom plane of the face– the plane of the upper jaw tooth line–
    has to fit onto the bottom of the cranium for the jaw and basicranium to fit together. But then the top of the snout won’t be as high. Automatically, then, we would get a “forehead”: the upper margin of the skull would start at the front similar in shape, though smaller, to that of a wild boar, but it would then have to angle sharply up to accommodate the relatively larger cranium.
    (Am I at least making sense? That is, whether or not what I suggest is true, is it understandable?)

    I don’t know enough about neural crests. But at least in its effect on skull shape, the domestication syndrome seems to have results similar to dwarfism. You don’t want your domestic livestock to run away, so the short limbs characteristic of dwarfism would also be a desirable trait in at least som domesticated species. Does domestication select for … mild or partial?? … dwarfism?

    I’m an amateur; my thoughts may be totally off base. Your (Mike and Matt’s) posts, however, seemed VERY interesting to me.
    (Thanks for the occasional non-sauropod-vertebra topic!)

  3. Allen Hazen Says:

    Hmmm… I note that “weakened ear cartilages” and something involving tail cartilage are aspects of the domestication syndrome (noted on the dog drawing). So whether or not the developmental causes are the same as in achondroplastic dwarfism, there DOES seem to be a bit of “achondroplasia” involved. (And since I really am an ignorant layman, I should shut up now.)


  4. I have always thought that the neural crest hypothesis depends on timing, and is actually a heterochrony. A mutation (and heck, even an initially epigenetic regulation) of a single developmental gene would delay the onset of migration of neural crest cells. So neural crest cells that become melanocytes will start migration too late and not be able to get to the farthest reaches in time, thus white tail tips, paws, era tips, foreheads. Likewise for not enough cells getting to join the adrenal gland, Likewise for not enough cells getting to the parts where they become cartilage (ear tips, snouts). As timing of developmental events is often connected to other types of timing, this may perhaps also affect the circadian clock, thus photoperiodism, thus appearance of the second heat in fall.

    https://blogs.scientificamerican.com/a-blog-around-the-clock/domestication-its-a-matter-of-time-always-is-for-me-thats-my-hammer-for-all-nails/

  5. Matt Wedel Says:

    Allen wrote:

    Now reduce the size of the facial part: not just shortening it in length, but reducing all its dimensions. The bottom plane of the face – the plane of the upper jaw tooth line – has to fit onto the bottom of the cranium for the jaw and basicranium to fit together. But then the top of the snout won’t be as high. Automatically, then, we would get a “forehead”: the upper margin of the skull would start at the front similar in shape, though smaller, to that of a wild boar, but it would then have to angle sharply up to accommodate the relatively larger cranium.

    If I follow your argument, the forehead kink is the junction between the greatly reduced snout and the not-reduced braincase. That all makes good sense to me.

    As for dwarfism, achondroplasic or otherwise, being a result of domestication syndrome, Sanchez-Villagra et al. (2016) discuss shorter limbs and smaller body size as a result of domestication in sheep and goats, citing a 2009 paper by Polák and Frynta (full ref below), but they did not include it in their big table of morphological changes that come with domestication, so evidently they did not consider it a general feature of domestication syndrome. I know that a lot of domesticated lines have been selected for increased body mass, sometimes well beyond that of their wild ancestors, but that’s obviously something that happened well after the domestication event. So reduced body size as an early consequence of domestication might get overprinted by later selection for increased body size, but I haven’t seen it discussed much in the literature on the neural crest hypothesis (as least what little of it I have read — I am a dabbler in this area, and a very recent one).

    Sanchez-Villagra et al. (2016) briefly discuss heterochrony and domestication syndrome — apparently this has been fairly well-studied in the domestication experiments on foxes. It seems logical, but I don’t know how much more broadly it might apply (not doubting here, just airing my ignorance).

    Polák J, Frynta D. 2009. Sexual size dimorphism in domestic goats, sheep, and their wild relatives. Biol. J. Linn. Soc. 98, 872–883. (doi:10.1111/j.1095-8312.2009.01294.x)

  6. LeeB Says:

    Domestication may be a bit like isolation on islands at first; small food animals should be bred to be tame and then larger to supply more food; very large animals should be bred to be tame and also smaller to be safer around.
    Only when they are very reliably tame do you then want to breed for giant sizes like cart horses, Chianini cattle and huge obese pigs.
    Also being a little less bright at first is probably useful; you don’t want the animal outwitting its less clever owners; it is only later when they are reliably controllable do you want to breed for animals that can obey complicated commands like sheep dogs and poodles.

    LeeB.

  7. Matt Wedel Says:

    We’re waaaay past the limits of my knowledge, but that all makes sense.

  8. Allen Hazen Says:

    Re: “If I follow your argument, the forehead kink is the junction between the greatly reduced snout and the not-reduced braincase. That all makes good sense to me.”
    I think you followed my argument more clearly than I wrote it!
    Thanks for reply.


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