Sunday 12 August 2018

The Pig Family: The Strange Case of the Sulawesi Pig-Deer

Sulawesi babirusa
Warthogs are fairly odd-looking animals, as are forest hogs, among others. But, at least to my mind, when it comes to wild pigs, nothing quite beats the babirusas.

These strange looking pigs inhabit the island of Sulawesi in Indonesia. Unlike most of the others in the group, this has been an island for far longer than pigs have been in existence - when sea levels were lower, most islands to the west were joined with Asia, and those to the east with Australia, but the waters around Sulawesi are so deep that it remained isolated. I've mentioned this before, in the context of warty pigs, so it's interesting to note that those channels must have been crossed by wild pigs on no less than three occasions (the third sort of Sulawesian pig, Celebochoerus, died out in the Ice Ages).

Sulawesi babirusas (Babyrousa celebensis) once lived across the entire island, although they have been absent from the south for thousands of years. While the whole island is covered in tropical rainforest, babirusas seem to prefer lowland rivers, and perhaps even marshy habitat, but, in modern times, are more commonly found in the interior highlands, where humans are less likely to bother them.

Babirusas are about the size of warthogs, and therefore slightly smaller than wild boar. Their nose-disc is smaller than in other pigs, and seems to lack some of the muscular and skeletal adaptations that ensure the high mobility of the discs in those species, suggesting that they are not as effective at rooting around in the soil. More significantly, the animal appears to be almost entirely bald. Indeed, they are sometimes said to be hairless, although this isn't really true: if nothing else, there is a scraggly tuft on the tip of the tail, and there are usually some short and sparse hairs across the rest of the body. Even so, they are hairless enough that the piglets are not striped, as is more typical, but simply have a drab brown colour.

But what's truly weird about babirusas is their tusks of the adult males. The tusks of the lower jaw are relatively normal, if unusually long, projecting out and to the sides, rather as they do in warthogs. The teeth in the upper jaw, however, are rather different. In piglets, these point forwards and slightly downward, but as the animal ages they rotate upwards and inward. These initial milk teeth are soon shed, to be replaced by the permanent tusks, which continue the rotation as well as becoming much longer.

By the time the animal is fully grown, the tusks are pointing upwards; the tooth cavities stick out slightly from the sides of the skull, and point in precisely the opposite direction to that which you would expect in any normal animal. The teeth then grow straight upwards, actually penetrating the upper lip and the flesh of the snout until they stick up through the skin behind the nose. They then continue to grow in a long curve, spiralling backwards and in towards one another so far that they regularly rub against and erode the bones of the forehead, snout, or other parts of the skull.

In males, they can reach 25 cm (10 inches) in length, but the exact shape and form seems to vary considerably between individuals, with 'aberrant' arrangements being surprisingly common. In fact, if they don't keep them properly worn down by rubbing them against trees and the like, they can grow so long that they stab into the skull, potentially causing serious injury or death. The upper tusks in females are far less spectacular, and typically don't penetrate up through the flesh of the snout at all.

These odd tusks don't seem to have anything to do with gathering or eating food, and, in fact, it's quite hard to see how they could do. Unfortunately, however, we don't know much about the babirusa's natural diet. They do seem to be omnivores, and, in captivity, will chase down and eat small birds and mammals, although they prefer vegetable matter, especially fruit. Indeed, it has been suggested that they need a significant number of fruit trees in their habitat in order to prosper. Certainly, they become less common where wild fruit trees have been cut down, although, of course, that might just be because such areas are more disturbed for other reasons as well.

The word 'babirusa' is Malay, and literally translates as "pig-deer" (some local species of deer are known in English as "rusa deer", from the same word root). This name obviously relates to the tusks, which somewhat resemble horns or non-branching antlers, but it was once thought that the animals resembled deer in another way, too: they were believed to be cud-chewing ruminants with a multi-chambered stomach.

They aren't, but the stomach does have a second chamber, albeit one not as distinct as those in ruminants. And, like ruminants, they do seem to ferment their food in the stomach, even if they don't regurgitate as cud. In 2004, microscopic studies of the stomach lining showed that that the upper part of the main chamber was covered in a bizarre honeycomb-like structure formed of thin cellular tubes covered in a thick layer of bacteria. This arrangement is unique, so far as we know, and presumably has something to do with fermentation of fruit and other foodstuffs in order to make it more digestible.

The skull of a male,
showing the oddly rotated tooth roots
Babirusas seem to be social animals, living in groups of up to a dozen or so individuals, although much larger aggregations can sometimes be seen near salt licks. Groups consist of up to three adult females, and a number of young. Most lack adult males altogether, and the few that do have them only have the one. This imbalance between the sexes means that adult males are more commonly seen on their own, doubtless hoping to find a herd with a suitable vacancy. Males have been seen digging their snouts into soft sand (as noted above, it has to be soft, because they cannot root in denser soil as most pigs can) and dribbling saliva into the resulting furrow. This is likely some sort of scent-marking behaviour, perhaps advertising their presence and availability to females or one another.

Despite this, males do not seem to be overly territorial, although they do compete with one another for access to females when the opportunity arises. Most such competitions consist of threatening from a distance, or making sudden rushes to unnerve the opponent, but, at times, they can break out into physical violence. Once again, this turns out to have nothing to do with those oddly shaped teeth. Instead, the animals actually stand up on their hind legs (they can also do this to eat leaves from trees), holding their snouts vertically up out of the way, before proceeding to ferociously wallop one another on the chest and shoulders with their front legs.

Normal pigs do not behave in this way.

Although they are anatomically capable of giving birth to, and suckling, litters of up to four piglets, even triplets are a rarity, and singleton births unusually common for pigs. This results in a greater investment of the mother in the offspring she does give birth to, and, for example, it takes a lot longer to wean a young babirusa than it does for a domestic pig. At least in captivity, breeding takes place throughout the year, with some sows giving birth twice in the same year; it's less likely that they would have enough food and opportunity to do so in the wild, however.

For much of the 20th century, it was thought that there was only one species of babirusa, but more recent data has shown that the previously presumed subspecies are all separate species. While the species on the Sulawesi mainland is by far the most common, it so happen that, when the animal was first scientifically described in the 18th century, it was on the basis of a specimen from the island of Buru, some distance off the east coast. Since we now know that the mainland form is a different species, the original scientific name now refers only to that isolated eastern population.

This animal (Babyrousa babyrussa) goes by a number of common names, including hairy babirusa, Buru babirusa, Moluccan babirusa, and golden babirusa. As the first of those names implies, it is much hairier than the more common sort, with a thick, if bristly, pelt and tail tuft. The tusks tend to project outwards and away from the skull, rather than converging inwards, and they also tend not to be so long as in the Sulawesi species.

Despite this being the "original" species, very little is really known of it, and most references to B. babyrussa from the 20th century will in fact be about what's now described as the Sulawesi species. They live, not just on Buru Island, but also on the smaller islands of Taliabu and Mangole to the north. Some evidence suggests that they may be less sociable than their Sulawesi counterpart, and are more likely to be encountered alone, but otherwise, what we do know suggests that they are likely very similar.

Even less is known of the Togian babirusa (Babyrousa togeanensis), which lives only on a cluster of relatively small islands in the Gulf of Tomini, between the northern and southern arms of Sulawesi proper. They are said to be larger, on average, than the other two species, and to have a thin coat of short hairs across their body, with a well-developed tuft on the tail. The most obvious difference, however, is that the tusks on the males project mostly forward, rather than upward, and are much shorter than in the other species. Because of the advance of local agriculture, and because the small size of the islands means that there weren't very many of them to start with, their already low population is probably shrinking, and they are officially listed as an endangered species.

A proposed fourth species is known only from skeletal remains found in the south of Sulawesi. If it was ever a distinct species at all, it's probably extinct, and may have been so for some time.

But with these, arguably the strangest of all pig species, I finally reach the end of my survey of living members of the pig family. However, those of you who live in Latin America, or the southwest US (Texas, Arizona, New Mexico) may be aware of what appears to be a completely different sort of wild pig found in those areas, and, in places quite common. It's called a peccary, or javelina, and there's a very good reason that I haven't mentioned it yet. I daresay many readers of this blog will already know why that is, but for those who don't, the next post in this series will explain all...

[Photos by Terence Ong and Didier Descouens, from Wikimedia Commons.]