The south-east Asian archipelago is pig paradise. This region hosts the highest wild pig diversity in the world.
That is probably because rising and falling seas have connected and disconnected this large scattering of islands for over 5 million years. When animals become isolated on islands, new species can evolve, and that seems to be what has happened here with pigs.
One of the products of evolution gone hog wild is the little-known Bawean warty pig. It is found only on the island of Bawean, the remnant of an extinct volcanic mountain in Indonesia's Java Sea.
Until now, scientists did not even know how many of these pigs there were.
The Bawean Island warty pig is so named because the typical male has "three pairs of enormous warts on each side of its face," says Johanna Rode-Margono, a researcher at Chester Zoo in the UK and a member of the IUCN/SSC Wild Pig Specialist Group.
Nobody knows why the males have these grapefruit-sized warts.
There are 172 to 377 pigs living on Bawean
"There are various different theories," says Rode-Margono's colleague Eric Meijaard, the founder of Borneo Futures and chair of the IUCN/SSC Wild Pig Specialist Group. "It could be a display function," meaning that nice-looking warts might be a means for males to impress the females.
Alternatively, the warts could play a role in fighting or defense. "Wild boars fight each other by slashing with their lower tusks, and the warts would protect the face, especially the eyes," says Colin Groves of the Australian National University.
In addition to their oversized warts, some male Bawean warty pigs have white beards and large tufts of golden hair fanning out from both sides of the head. Again, it is unclear why.
We do not even know if the Bawean warty pigs are really a distinct species. Some scientists argue that they are and should be called Sus blouchi, but others say they are a subspecies of the Javan warty pig (Sus verrucosus), in which case we should call them Sus verrucosus blouchi. However, "in conservation terms it doesn't really matter – it's a distinct taxon of an endangered pig species that requires conservation attention," says Meijaard.
To get a glimpse into the lives of these mysterious wild pigs, a team of six scientists including Rode-Margono and Meijaard placed video camera traps at 100 protected forest locations across Bawean island from November 2014 to January 2015.
Nobody knows why the males have these grapefruit-sized warts
Their findings have been published in the journal PLOS ONE.
Based on the camera trapping, the researchers calculated that there are 172 to 377 pigs living on Bawean, compared to about 90,000 human residents. For now there is no way to tell if the population is growing, stable or shrinking.
The low number is not terribly surprising. Bawean is no bigger than a small town, and the pigs are only found in a few isolated pockets of habitat.
Their small population means Bawean warty pigs qualify as a globally endangered species.
The local population is staunchly Muslim, so they do not eat pork
One issue is that the species is confined to a tiny island. "By definition, it always will have a small population, and small populations are just more likely to go extinct," says Meijaard. For example, they could be wiped out by a disease.
But the pigs' lifestyle also brings trouble.
The videos revealed that, like other wild swine, Bawean warty pigs are mainly nocturnal. They prefer to stay in the fringes of community forests. From there, they can launch forays into cultivated habitat, "where food is probably more abundant and rich in energy," says Rode-Margono.
This crop-raiding habit can bring them into conflict with farmers.
The main threat to the pigs is hunting, "because locals want to protect their crops from pigs," says Rode-Margono.
This is a very special and important population because it's the only population in the world
Luckily for the warty pigs, the local population is staunchly Muslim, so they do not eat pork, says Meijaard. They only hunt the pigs to protect their crops.
On other islands like Java, Hawaii and Monserrat, invasive pigs also pose a threat. Early European explorers introduced fast-breeding and highly adaptable wild boars to many islands, to ensure a ready food supply. These pigs can interbreed with the native species, destroying their distinctiveness.
Scientists were concerned that hybridization with wild boars might be a threat on Bawean, but it seems not to be. The camera trap data did not reveal any wild boars.
Rode-Margono is optimistic that, despite their crop-raiding reputation, she can get local people interested in conserving Bawean warty pigs.
When the researchers showed camera trap photographs to Bawean islanders, many were startled by the unusual animal. They thought their unique pig, with its charismatic warts, was "pretty cool," she says.
There are benefits to having them around. Wild pigs are "ecosystem engineers," she says: by uprooting soil in the forest, they play a role in fertilising and dispersing seeds.
They are also an "umbrella species," says Rode-Margono. Protecting them also provides protection for other rare mammals like the critically endangered Bawean deer.
It is possible their crop-raiding habits could be reined in by simple solutions, such as "chilli-pepper-sprinkled fences, which seem to have had some success with deterring elephants," says lead researcher Mark Rademaker of VHL University of Applied Sciences in The Netherlands.
"This is a very special and important population because it's the only population in the world," says Rode-Margono. With less than 250 mature animals on the island, and none in captivity, "it's quite important to keep an eye on them."
Lesley Evans Ogden is on Twitter @ljevanso