The Pig Family: Wild Boar and the Domestication of Pigs

European wild boar

Generally speaking, the wild ancestors of agricultural animals have not fared particularly well. Wild goats are a threatened species, wild horses are an endangered one, camels are doing even worse, and wild cattle have been extinct for centuries. Wild sheep haven't done quite so badly, although they're hardly widespread, but, in general, one of the main problems facing such animals is that the sort of places they like to live are exactly the ones we want to turn into agricultural land to raise their domesticated kin.

Indeed, only two wild ancestors of widely domesticated herbivore are doing well: chickens and pigs. (This may, of course, depend on your definition of "widely"; I'm ignoring, say, rabbits). Indeed, the wild ancestor of the domestic pig is not only reasonably common, it's the single most widespread of any species of wild pig.

Wild boar (Sus scrofa) are found through virtually the whole of southern and central Europe and Asia, being absent only from the colder parts of the north, such as Siberia and Lapland, and from the harshest of deserts and highest of mountain ranges. They are also found on a number of neighbouring islands, from Java and Japan to Corsica and Sardinia, and along the African coast of the Mediterranean Sea. In many places, the population density of wild boar has even been increasing in recent decades, most likely due to milder winters.

These broad statements do, however, hide a considerable degree of complexity, both in terms of the actual distribution and of what, exactly, a wild boar is. Firstly, there are certainly places where wild boar have not had it all their own way. They have been extinct in Britain since the 17th century, in most of Scandinavia since the 19th, and in Libya, Egypt, and northern Japan since the 20th. On the other hand, they have not only been reintroduced into some of those places after going extinct, they have also been brought to the Americas and Australia. And that's not even counting feral, formerly domesticated, pigs ("razorbacks" in American parlance), which can be numerous in some parts of the world, and will, unsurprisingly, breed with actual wild boar if they can find any.

Then there's the fact that wild boar are quite variable in their appearance across this vast range. For instance, the wild boar of India have a long mane running down the middle of their back, which is entirely missing in their European and Chinese counterparts. In the nineteenth century, this led to the naming of a number of different species of the animal, but eventually it became clear that they blur into one another so much that they are, at best, subspecies of the same thing.

The exact number of these subspecies is a matter of some debate, with anything from four to twenty-five being accepted by different authors. Sixteen seems to be the most widely used number today, but this is far from a settled question, and it's not entirely out of the question that some of them might turn out to be distinct species after all - the banded pig (S. s. vittatus) of southern Malaysia and western Indonesia is the most commonly cited possibility here.

The reason for this success is, as you might expect, considerable adaptability. The natural habitat of wild boar is woodland, but they'll live pretty much anywhere that has reasonable cover, from scrubland to the edges of fields with tall crops. In addition to this, they will eat basically anything that isn't going to put up much of a fight, something that helps them when they are introduced to foreign climes to which they are not normally native.

The bulk of their diet is vegetable matter, to be sure, especially seeds and roots, but also including foliage. They do try to eat at least one high-energy food, but this can be acorns and chestnuts in Europe, or the fruit of tropical trees in the rain-forests of Southeast Asia. This naturally means that, where crops are available, they'll eat them, and they particularly like maize, despite this not being something they'd ever encounter in the wild. As a result, they can be a considerable pest, especially in places such as America where they aren't a natural part of the ecosystem. In this respect, it doesn't help that they are good at swimming, and have reached islands a few miles offshore without apparent human assistance.

Indian wild boar (note the mane)

Nonetheless, around 10% of their diet consists of animal matter, mostly insects and earthworms, but including at least some small vertebrates. Even here, the details vary depending on where they are, and they will rapidly adapt to new circumstances. Their relative intelligence may also play a part, and, for example, when acorns are difficult in find in the spring, they will actively seek out the hoards secretly hidden by small mammals to get them through the winter, and raid those.

Wild boar seem to be mostly nocturnal, although this may be an adaptation forced on them by a desire to avoid humans, and they are more active in the daytime when humans aren't around. They live in maternal groups, commonly called "sounders", of around twenty individuals, while adult males tend to live alone outside of the breeding season. Members of a sounder cluster together to sleep, re-using the same sites for several days at a time before moving on. They line these sites with branches and leaves, scraping a trough in the ground to rest in, and they are often close to water or mud in which they can wallow. They will also spend some time rubbing up against nearby trees and bushes, using scent to mark out their territory.

In tropical regions, wild boar breed throughout the year, but, in much of their range, they do so mainly in winter, although there can be a second season later in the year where conditions allow. At these times, adult males seek to rejoin the sounders, violently driving off potential rivals to mate with as many sows as possible. They do so by charging and slashing with their tusks, and even biting one another on the penis, which you'd think would put anyone off mating.

About four months after mating, sows leave the sounder to give birth, or "farrow" in a specially prepared and secluded den. Five or six piglets per litter is typical, although this can be much lower in some places, especially where sows tends to be younger due to hunting of their older, and larger, kin. The litter remains in the farrowing den for about four to six days, before rejoining the sounder, and they are fully weaned by four months of age. As with most other pig species, the young are born with striped coats, but these start to fade after around six months, and are entirely gone by the end of their first year. Even though they lack the visible tusks of males, sows are aggressive in protection of their young, charging and biting other animals that approach too close.

Pigs have been domesticated since the Neolithic, but the exact date is unclear, possibly because there was a long period of limited management of wild boar populations for hunting purposes before domestication proper began. 7,000 BC is a conservative estimate, which would place them broadly in line with the first domestic horses, but figures of 9,500 BC or even earlier have been proposed, dating back pretty much to the dawn of agriculture.

This probably first occurred somewhere around present-day Turkey or Iraq, some of the first places to develop actual farming. Evidence is strong however, that the domestication of pigs was not a one-off event, and occurred in a number of different places at different times. Sometimes, this was likely due to imitation. Judging from analysis of the DNA in ancient bones, the first domestic pigs in Europe were introduced from the Near East, but early modern European pigs show almost no hint of this, indicating that local wild boar were domesticated separately, after the original stock had been introduced, and eventually replacing them.

American razorback (feral pig)

At the other end of the continent, domestic pigs have been present in China since around 6,000 BC. Genetic analysis of modern Chinese pigs shows that they are not even the same subspecies as the European and West Asian sort, and so must necessarily be the result of an entirely separate domestication event. Likely the story is more complicated still, with pigs have been domesticated multiple times in different locations across Asia, with the Mekong valley of Southeast Asia being a particularly strong contender for another origin point, and others, including India, also having at least some evidence behind them.

This suggests that the domestication of pigs was, relatively speaking, not all that difficult, possibly because the wild animals are so adaptable. They have, however, found a more limited range of uses than cattle or sheep, typically being bred only for their pork, with leather a useful side product. While a small number of pigs are used to find truffles or for garbage disposal, they're not typically much use to farmers while they are still alive, save for the ability to make more pigs.

To complicate matters further, one breed of "domestic pig" isn't descended from wild boar at all, but from an entirely different species. It's one of many such species that became separated from the mainland stock of wild boars some time around the Ice Ages, as new islands became cut off to the south and east of Asia. Next time, I'll be looking at that, and some of the wild boar's other closest relatives...

[Photos of wild boar by Bert de Tilley and Soumitra Ghosh, from Wikimedia Commons. Photo of razorback from NASA archives, in the public domain.]