Why do some domestic pigs have foreheads?

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

• Evin, A., Owen, J., Larson, G., Debiais-Thibaud, M., Cucchi, T., Vidarsdottir, U.S. and Dobney, K. 2017. A test for paedomorphism in domestic pig cranial morphology. Biology letters 13(8):20170321.
• Owen, J., Dobney, K., Evin, A., Cucchi, T., Larson, G. and Vidarsdottir, U.S. 2014. The zooarchaeological application of quantifying cranial shape differences in wild boar and domestic pigs (Sus scrofa) using 3D geometric morphometrics. Journal of Archaeological Science 43:159-167.
• Sánchez-Villagra, M.R., Geiger, M. and Schneider, R.A. 2016. The taming of the neural crest: a developmental perspective on the origins of morphological covariation in domesticated mammals. Royal Society Open Science, 3(6):160107.
• Wilkins, A.S., Wrangham, R.W. and Fitch, W.T. 2014. The “domestication syndrome” in mammals: a unified explanation based on neural crest cell behavior and genetics. Genetics, 197(3):795-808.