Warty pig (Sus cebifrons) nest digging

The piece of bark wouldn’t stay put. Why not?

Over a period of six weeks in late 2015, Meredith Root-Bernstein regularly stopped by the Visayan warty pig enclosure at Paris’ Ménagerie du Jardin des Plantes. The Ménagerie happens to be the world’s second oldest zoological garden, set up in the aftermath of the French Revolution as a place to bring previously privately-held exotic animals. It’s also a member of the European Endangered Species program, hence their four pigs, which are native not to Europe but to two islands in the Philippines.

In October of 2015, Root-Bernstein was watching the warty pigs as one of the females was digging a nest. Along with the usual dirt-moving via snout and feet, at one point the pig picked up a flat piece of bark in her mouth, and proceeded to dig, lift and push at the soil around her. It was over in a matter of seconds. But Root-Bernstein, an ecologist and conservation scientist, was intrigued. Was this an accident? A deliberate tactic? Just as we discussed for puffin stick scratching, more evidence was needed.

Delving for data

In the month and a half following her opportunistic observation, Root-Bernstein tried and failed to catch another glimpse of the behaviour. The pig-keepers at the Ménagerie didn’t recall seeing tool-use before, and neither did warty pig handlers at other zoos. But there was one oddity that kept cropping up: that piece of bark.

Even though none of the pigs cooperated by using it while they were being watched, the bark had a habit of moving around the enclosure, always appearing next to a freshly-dug nest. In the image below, I’ve circled the suspect in the centre and right photos (it was marked with a red arrow by Root-Bernstein and her colleagues, but I still found it a little hard to see):

Some would consider this co-location of a supposed digging tool and dug nests merely circumstantial, but as an archaeologist I can sympathise. We typically never get to watch our subjects use any of the tools we find. And my experience of digging pigs has mainly been the havoc they can wreak on otherwise promising excavation sites. Nevertheless, the evidence was enough to prompt Root-Bernstein to set up a more systematic investigation.

Step one was to add leaves and sticks to the pig enclosure, to see whether enriching the environment would lead to tool use while the pigs were feeding. Despite the animals carrying these objects around, and using their typical rooting behaviour—in which they explore by pushing material away from them using their snout—there was no sign of tool use.

Step two returned to the original context of Root-Bernstein’s observations: nest construction. The warty pigs build ‘farrowing’ nests, generally oval-shaped depressions that the pigs may line with leaves or other vegetation. For that the research team needed to wait until October 2016, when the behaviour was most likely to happen, especially in the late afternoon. They were particularly interested in what the female adult, Priscilla, was up to.

This time it worked. Priscilla used sticks or bark to dig three times, and one of her daughters also used a digging tool. In each case, it happened as part of nest construction. In a typical sequence, the females dug a depression with their snouts, then brought material to carefully arrange within it, and finally kicked soil back towards the nest (a practice the scientists termed ‘moonwalking’). Tool use was only seen as part of that final, soil moving task, with dirt moved sideways or backwards using a ‘rowing’ movement. At one point the only male pig, an adult named Billie, picked up a previously used stick. However, he didn’t use it for nest building, and mostly just wandered around holding it in his mouth.

In a further step, the researchers left human tools—wooden or bamboo spatulas—in the pig enclosure. Priscilla used one of them a couple of times on the first day of observation, but their use didn’t spread within the group. As before, tool use was only seen in association with the final moonwalking stage of nest construction. It’s also worth noting that most of the time, the pigs were happy to make their nests without any tools, treating them instead as an optional accessory.

This video, from the team’s published report, gives a good idea of the actions involved in the pig tool use:

History-making pigs

There is no doubt that the pigs picked up objects in their mouth, and then dragged or dug one end of the object into the ground. Root-Bernstein and her colleagues consider this to be the first evidence of deliberate tool-use in the Suidae (pig) family, stating:

it involved the manipulation of an external object (bark, stick, or spatula), it occurred exclusively and regularly within a goal-oriented repeated action pattern (digging a nest pit), and as its end result it altered both the distribution of the soil (to make a pit) and the physical properties of the tool user (a physical disposition, digging action) and thus likely also included information transmission to the tool user in the form of proprioceptive feedback different to that without tool use.

What is also clear, however, is that pig tool use in general must be exceedingly rare. Humans have been in contact with pigs, first as wild animals, and later as domesticated companions, for thousands of years. If these animals were habitual tool users we would know, just as we would know if cats or dogs or sheep or cows regularly used tools. However, that doesn’t rule out the possibility of tool use coming and going with different individuals throughout history.

And history gives us a unique perspective on just how long humans have been watching pigs. Visayan warty pigs (Sus cebifrons) are critically endangered in their natural habitat, which consists mainly of the Philippine islands of Negros and Panay. Genetic studies show that the closest relatives to S. cebifrons are the Bornean bearded pig (S. barbatus), and the Sulawesi warty pig (S. celebensis). And in January 2021, archaeologist Adam Brumm and his Indonesian colleagues revealed some startling evidence about the Sulawesi pig.

They reported finding paintings of pigs with a distinctive warty profile in the limestone caves of Maros-Pangkep, South Sulawesi. The paintings had small nodules made of of carbonate growing on them, which allowed the scientists to date the age of the underlying paint. The result? These pig paintings are at least 45,500 years old, older than any animals in cave art in Europe. The warty pig is the oldest known painted animal in the world.

This panel from the Leang Tedongnge cave site shows three such pigs, although only parts of the original images remain for the two of them on the right (I’ve numbered them 1-3 to help). You can also see two hand stencils made by the ancient artists above the left-most pig, which to give a sense of scale is 1.36m long:

Looking closer (see the image below, from the published report by Brumm and his team), you can make out the tufted crest of hair behind the pig’s head—marked ‘HC’ for head crest—and the facial warts or ‘FW’ that give the animal its common name. There are also two unidentified bumps underneath the pig’s chin, pointed out with arrows:

This illustration is not a one-off event. A second cave in the same region contained a warty pig painted at least 32,000 years ago, and a third cave had yet another pig, this time at least 43,900 years old. Clearly, these were animals with which the local inhabitants were very familiar, drawn at a time when other parts of the world still had Neanderthals. Unfortunately, none of the paintings shows a pig using tools, or even an unambiguous bit of bark sitting nearby.

And so this is where we’ll leave the warty pigs for now. From a practical point of view, we have limited but reasonably strong evidence that some captive warty pigs can use plant tools to help with their nest construction. But from a more poetic point of view, we can link those Parisian pigs to their distant cousins on the other side of the world, carefully depicted in some of the very first attempts by humans—tool-users extraordinaire—to draw another species.

Sources: Root-Bernstein, M. et al. (2019) Context-specific tool use by Sus cebifrons. Mammalian Biology 98: 102–110. || Nuijten, R. et al. (2016) The Use of Genomics in Conservation Management of the Endangered Visayan Warty Pig (Sus cebifrons). International Journal of Genomics doi: 10.1155/2016/5613862 || Brumm, A. et al. (2021) Oldest cave art found in Sulawesi. Science Advances 7 : eabd4648. || Aubert, M. et al. (2019) Earliest hunting scene in prehistoric art. Nature 57:, 442–445.

Main and first image credit: Root-Bernstein et al. (2019) || Cave art panel: CNN, https://edition.cnn.com/style/article/cave-art-indonesia-oldest-figurative-art-animal-image-scn-trnd/index.html || Cave art close-up: Brumm et al. (2021).