Saturday, 24 March 2018

The Pig Family: Wild Pigs of Indonesia

Bearded pig
Thanks to their adaptability, wild boar are a remarkably widespread species, being found from Portugal to Japan. All of their close living relatives, however, are much more restricted, each being isolated on different islands or island chains off the south-east coast of Asia.

The largest such island, and, indeed, the third largest island in the world, is Borneo. This is home to the bearded pig (Sus barbatus), which is also found, not only on the neighbouring island of Sumatra, but also on the Malaysian mainland. The Bornean and Sumatran forms of the animal are widely regarded as different subspecies - the latter going by the rather brilliant name of S. b. oi - although quite which of these two the Malaysian pigs belong to has been a matter of mild controversy.

Bearded pigs are somewhat smaller than wild boar (which, incidentally, are also native to Sumatra, though not Borneo), but they have proportionately longer legs and, perhaps because of their tropical habitat, less body hair. Also unlike wild boar, they have warty lumps on their faces, but these are not particularly obvious because they are covered by the "beard". This is found on both males and females, although it is apparently larger on the former, and consists of both a coating of long bristles on the lower jaw and a much larger bush of forward-pointing hairs at the top of the snout.

Bearded pigs will eat virtually anything, although only consume the meat of large animals if they find it as carrion. In practice, their habitat is almost entirely covered in tropical jungle, and, at least in the lowlands, the dominant form of trees are dipterocarps, and these almost inevitably end up as the major component in their diet. However, many of the local dipterocarps synchronise their flowering and seeding at certain times of the year, so that food is much more readily available then than at other times.

And this brings us to one of the most interesting and unusual habits of bearded pigs: their large scale annual migrations. Now, this isn't true of all bearded pigs, since it really does depend where they happen to live. In peninsular Malaysia, for instance, the dominant vegetation flowers pretty much year round, so there really isn't any point in moving. But, in other places, especially northern Borneo, where the seasonal ripening of illipe nuts provide a particularly tempting food supply, migrations can be significant.

Even so, they don't migrate along the same path every year, descending on one particular location in one year, eating as many of the available seeds as possible, retreating to the hills for the off-season, and then trying somewhere else the next time. When they do migrate, however, they do so in large numbers, with over a hundred pigs trampling through the jungle and happily swimming across even wide rivers that get in their way. The rest of the time, or where they live in areas that don't have a particularly rich food supply at any time of year, they live in much smaller groups, consisting of a mother and her immediate children, with the adult males staying away.

If they're determined enough, it's not just rivers that they will swim across, but the sea. There are numerous reports of them being found, sometimes in moderately large groups, a mile or more offshore, apparently swimming from one island to another. Most of these reports, however, come from the nineteenth century, and even the more limited migrations are far less common now than they were just twenty years ago. This is presumably due to the loss of their habitat to logging industries and expanding agriculture. It's interesting to note, though, that bearded pigs are active during the day, rather than at night, because they're apparently more frightened of clouded leopards than they are of human hunters - get rid of the local leopards, and they become more nocturnal.

The ancestors of bearded pigs must have entered their native islands from Asia, and they, along with the ancestors of the wild boar that are there today, may well have done so during the Ice Ages, when low sea levels meant that the islands were still connected to the mainland. That ancient coastline is marked today by the Wallace Line, a biogeographic boundary running across Indonesia, with Asian animals to the west, and Australian animals to the east.

But that rather simplifies matters, because, even with the low sea levels, there were still some islands in the straits between the two continents. As a result, while there are no "Australian" animals (such as marsupials) west of the line, there are a fair number of "Asian" species to the east of it. For the most part, these are animals such as bats that could easily cross the straits, but there are also some large terrestrial animals that managed to make the crossing... and these include pigs.

Due to frequent interbreeding of these pigs during their
evolutionary history, genetic studies do not give a consistent
picture; this is just one possible interpretation
The largest island in this ancient channel would have been Sulawesi, and today, that is home to the Sulawesi warty pig (Sus celebensis). These are relatively small, short-legged pigs, usually with dark hair. Most obviously, however, adult males have six large warty growths on their faces, with one pair each above and below the eyes, and a third, much smaller, pair on the jaw.

Like many pig species, this has not been the subject of much study. We know that they are mainly active during the day, and that they live in family groups, typically of no more than around six individuals, although slightly larger groups may also exist. Although omnivorous, they seem to favour plants more than animal matter, and they have been reported from both lowland and highland jungles and from the edges of agricultural land - a set of options that covers just about the whole of non-urban Sulawesi. Judging from the ones in zoos, they have litters that average five piglets each, which are born with the typical striped coat seen in most pig species.

However, there is one remarkable fact about Sulawesi warty pigs, although it is one that was only discovered in the 1980s. The discovery arose from the fact that wild boar seem to be found not only in many parts of Indonesia east of the Wallace Line, but even in New Guinea, far enough to the east that it shouldn't have any "Asian" species at all. The assumption was that these were feral animals, descendants of domestic stock brought across to the islands in Neolithic times.

And, indeed, this does seem to be so. In fact, domestic pigs seem to have been introduced to the islands at least twice from the Asian mainland. But, in checking these feral animals against the local domestic stock, a surprising fact emerged: some of the domestic pigs weren't actually domestic pigs. Or at least, they weren't domesticated wild boar, but domesticated forms of the Sulawesi warty pig.

It turns out that these animals were locally domesticated even before the regular sort of domestic pig arrived. As early as 7,000 BC, Neolithic farmers transported these pigs from Sulawesi to Flores and Timor, while some later movements brought them to islands of the Moluccas, and even a couple of islands far to the west, on the other side of Sumatra. This other species of domesticated pig is very much in a minority, even in Indonesia, and, since the two species can interbreed, it's likely that a number of both domestic and feral animals in this part of the world are actually hybrids.

Wild Sulawesi warty pigs are reasonably common, although they tend to be more so in areas with a high Muslim population than a Christian one, because the former don't want to hunt and eat them. There are, however, very similar pigs on the other side of the Wallace Line, and they are faring much worse.

Javan warty pig
The island of Java is home both to wild boar and to the Javan warty pig (Sus verrucosus). The latter have unusually long heads, and the males have warts similar to those of the Sulawesi species, but even larger, making them easy to distinguish from the local wild boar. The females of the two species are harder to tell apart, but the warty pig sows are only half the size of their males, which are slightly smaller than wild boar to start with, so even they can be readily distinguished if you know you have an adult.

Probably because of their broad omnivory, the two species seem to inhabit very similar habitats, something that poses problems for conservation, if you need to favour the rarer one. However, the warty pigs do avoid the higher hills, and seem to be most common in lowland teak forests, from which they launch nighttime raids on the surrounding farmland - something that naturally doesn't endear them to the farmers.

Since Java is so densely populated, and teak forests are subject to significant illegal logging, it's unsurprising that the survival of the Javan warty pig is under threat. In fact, there has been a massive decline in their numbers in recent years, with half of all known populations having been wiped out between 1982 and 2006. It was formally declared an endangered species in 1996, being found only in small patches of isolated forest, each population unable to reach and mix with its neighbours. That the species hybridises with wild boar, and seem to have unusually small litters, only add to the threat. It may well be because of these rapidly declining numbers that the surviving animals are reported to be mostly nocturnal, and to travel only in small groups.

I should also mention two other possible species before leaving Indonesia. A small population of  pigs on Bawean Island, in the Java Sea, is usually considered to represent a unique subspecies of the Javan warty pig, but it has recently been proposed that it might be worthy of promotion to full species status as Sus blouchi. Given the small size of the island, it's unsurprising that it, too, must be endangered, and the best population estimate is that less than 250 adults survive.

Finally, some sources list a species named Sus bucculentis, from mainland Asia, as another close relative, often under the name "Vietnamese warty pig". This was described on the basis of two skulls discovered in 1892, and was not seen again until 1997. More recent analysis of the remains however, has shown it's probably just a wild boar, and not a true species. Whatever it is, every indication is that it's extinct, and has been for some time.

However, there are other warty pig species that definitely do exist, but to find them, we have to travel north, to the Philippines...

[Photos by "Rufus46", from Wikimedia Commons, and C.J. Cornish, in the public domain. Cladogram adapted from Frantz et al. 2013 and Gui-sheng et al. 2005.]

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