An interesting and rather peculiar rendition of a group of warthogs by Joseph Wolf, dating to 1850. Image in public domain.

Warthogs are African members of the pig family (Suidae), famous for their long, upcurved tusks and facial ‘warts’. They look mostly naked-skinned*, possess a dorsal crest that’s longest over the neck and shoulders, and are specialised grazers that ‘kneel’ on their wrists in order to bring the mouth close to the ground. Warthogs are so unusual relative to other pigs that they’ve sometimes been given their own ‘subfamily’, Phacochoerinae.

* Yes, yes, they're actually covered in bristles.

For much of the 20th century it was widely thought that living warthogs all belong to the same variable species (Phacochoerus aethiopicus). However, it has become increasingly realised in recent decades that two species are present (Ewer 1956, Grubb & Oliver 1991, Randi et al. 2002, D’Huart & Grubb 2005, Grubb & d’Huart 2010). Here I will gloss over an interesting and complicated story that involves fossils, recently extinct populations and arguments about lumping and splitting, and simply say that these two are the dry-adapted Desert warthog P. aethiopicus, and the more widespread and polytypic Common warthog P. africanus. As much as I’d like to talk about warthogs at length, we’re here because of the skeleton and, specifically, just a few things about it.

Desert warthog photographed at the Cambridge University Museum of Zoology, UK. Lots of interesing things to note here. Photo by Darren Naish.

Those are some tall neural spines. It will be obvious to you from the picture shown here that warthogs have pretty respectable neural spines – they form a great ridge along the shoulder region and part of the back, surely meaning that warthogs have mighty sails, or humps, right? Well, no, they really don't. In fact, pigs of all sorts have tall neural spines in the same region, though those of warthogs are especially tall and slender relative to those of wild boars, domestic pigs and their relatives. Muscles and ligaments connected to these spines also attach to the back of the skull, and it follows that mammals with big, heavy heads often tall have neural spines in the shoulder region. However, quite why warthog neural spines are so slender compared to those of other pigs is a good question and I don’t know if an answer is yet available.

Dorsally located orbits. It’s a typical feature of grazing mammals that the eyes are located far back on the head, some distance away from the long snout. Look at horses and antelopes like hartebeest. Warthogs are an extreme version of this, and certainly the most extreme among pigs. Just look how far back, and how high up, those orbits are. They’re virtually at the top of the skull. The orbits are made even higher up than they might otherwise be due to the topography of the surrounding bones: the face anterior to the orbits is concave, and the space between the orbits is concave too (Groves 1981).

Male Common warthog skull in profile: the position of the orbits is just nuts. Photo by Didier Descouens, CC BY-SA 4.0.

Where are the incisors? Incisor teeth are a normal feature of pigs. Desert warthogs lack them completely, both in the upper and lower jaws, whereas Common warthogs ordinarily have two upper and six lower incisors (D’Huart & Grubb 2005, Grubb & d’Huart 2010). Seeing as warthogs are specialised for a life of grazing in savannah, semi-deserts and deserts, and seeing as the Desert warthog is more specialised for life in arid environments than the Common warthog, it seems that incisor reduction and loss, and incisor replacement by a keratinised pad, are advantageous in arid-land grazers.

Detail of Desert warthog rostral region: where are the incisors?
And what's that extra bone at the tip of the snout? Photo by
Darren Naish.

Is that an extra bone at the tip of the nose? If you just had a good hard look at the front of the snout while looking for those missing incisors, you might have noticed the irregularly shaped, single bone positioned right at the tip of the snout, wedged between the bones that form the margins of the bony nostril. This element – called the prenasal bone – isn’t a unique feature of warthogs, it’s a normal feature of the pig family that helps support the nasal disc. It also forms an area of attachment for unique muscles used in elevating and moving the disc during rooting. However, the prenasal of warthogs is more firmly connected to the surrounding bones that it is in other pigs, and the muscles that attach to it are smaller and less able to move the nasal disc than they are in other pigs too (Herring 1972). This makes sense in an animal that rarely roots in soft ground and is, instead, a grazer that mostly interacts with dry, often hard, sediment.

Male Common warthog displaying the characterstic 'warts',
'kneeling' in characteristic fashion. Image by Charlesjsharp,
CC BY-SA 3.0

And... where are the warts? There are three sets of lumps on the warthog face that we call ‘warts’*: a preorbital pair (positioned between the eyes and tusks), an infraorbital pair (located beneath and slightly behind the eyes), and a mandibular pair (situated on the sides of the jaw). These are somewhat sexually dimorphic, since they tend to be larger in males, and females lack the mandibular pair. The giant, flaring upper canines of the skeleton and skull shown above reveal both animals to be males. So... where are the warts? The answer is that they’re not visible in the skeleton because they have no osteological correlates. That is, there are no underlying bony features linked to their presence (Groves 1981).

* We really shouldn't. Better names include callosities or caruncles.

If we only knew of warthogs as fossils, we’d have no idea that they have warts. Consequently, we wouldn’t have called them ‘warthogs’. Well, they’d be extinct, so we wouldn’t have given them a common name at all, but you know what I mean. We’d have called them grasspigs, or savannahpigs, or tuskpigs, or... vlakvarks. This is the Afrikaans word for these pigs and means something like ‘plain-dwelling pigs’. I like it.

Pigs of several kinds have been covered on Tet Zoo before. See...

Refs - -

D’Huart, J. P. & Grubb, P. 2005. A photographic guide to the differences between the Common warthog (Phacochoerus africanus) and the Desert warthog (Ph. aethiopicus). Suiform Soundings 5 (2), 4-8.

Ewer, R. F. 1956. The fossil suids of the Transvaal Caves. Proceedings of the Zoological Society of London 127, 527-544.

Groves, C. 1981. Ancestors for the Pigs: Taxonomy and Phylogeny of the Genus Sus. Technical Bulletin 3, Department of Prehistory, Research School of Pacific Studies, Australian National University, pp. 96.

Grubb, P. & d’Huart, J. P. 2010. Rediscovery of the Cape warthog Phacochoerus aethiopicus: a review. Journal of East African Natural History 99, 77-102.

Grubb, P. & Oliver, W. L. R. 1991. A forgotten warthog. Species 17, 61.

Herring, S. W. 1972. The facial musculature of the suoidea. Journal of Morphology 137, 49-62.

Randi, E., d’Huart, J. P., Lucchini, V. & Aman, R. 2002. Evidence of two genetically deeply divergent species of warthog Phacochoerus africanus and P. aethiopicus (Artiodactyla: Suiformes) in East Africa. Mammalian Biology (Z. Säugetierk.) 67, 91-96.