Collserola Natural Park looms over Barcelona, rising to about 500 metres at the Tibidabo peak. This forested ridge effectively walls off the city’s growth. Collserola is rich with wildlife, home to more than 190 animal species. Overlooking a city of more than 1.5 million residents, which welcomes tens of millions of tourists each year, it has become a battlefront between humans and nature. On many a hot Catalan night, wild boar from Collserola, alone or in gangs, descend on the city and mingle with the human population carousing after hours.

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The encounters between Barcelonan and beast are numerous, peaking in 2016 when police logged 1,187 phone calls about nuisance boars on the loose – wild hogs rooting up turf, munching trash, attacking dogs, plundering cat-feeders, holding up traffic and running into cars. For the past decade, Barcelona has been desperately searching for a way to keep the boar from colonising the leafy neighbourhoods – some home to footballers, bankers and celebrities – that back up against Collserola. The low point came in 2013 when a policeman shot at a boar with his service revolver, but hit and maimed his partner instead.

Listed on the World Conservation Union’s most invasive species list, the wild boar does well in just about any environment, from semi-arid plains to alpine forests and marshy grasslands. But more and more, they are drawn to city life. In Barcelona and Berlin, Houston and Hong Kong, groups of wild boar have been seen roaming around town at all hours. In Rome, where I live, boars rooting through uncollected piles of trash have come to symbolise the decline of the city.

The arrival of wild boar in town squares and city parks is forcing us to confront a new reality: we are bumping up against the limits of urbanisation. This is a crisis we have largely inflicted on ourselves. City sprawl is driving the species out of its dwindling natural habitats and forcing it to live alongside us. At the same time, we entice it with the tides of garbage and wasted food that wash around our cities. For years, boar have been fattening up on our crops. And now they follow us into our dirty, sprawling cities. Although their numbers are increasing as they migrate to the cities, the move is making them – and us – sick. Boars carry a host of diseases, including tuberculosis, hepatitis E and influenza A, that can make the jump to humans.

In addition to spreading disease, wild boar each year cause thousands of road accidents. In January, a group of wild boar crossed a highway south of Milan, leading to a three-car pile-up which killed one driver and injured several more. The boar destroy property, devour ground-nesting animals – including endangered turtles’ eggs – and crops, such as fragile vine roots and shoots. Italian farmers estimate the boar inflict €100m (£90m) worth of crop damage annually. As the animals’ toll on public health and the economy climbs, communities from Texas to New South Wales have begun to wage war on the species – a campaign fought in public parks, on golf courses, on farmland and on street corners at dusk.

It was in 2014, when this species seriously threatened the global pork industry, that the boar’s presence went from nuisance to existential threat. Boars can carry African swine fever (ASF), an incurable and highly contagious virus. Known as “pig ebola”, it kills wild and domestic pigs, creating an animal health crisis that is rapidly becoming a geopolitical one. To save the bacon from ASF, countries have been erecting physical borders with neighbours, threatening embargos, incinerating millions of farmyard pigs and offering bounties for the culling of wild boar.

A European consortium of wildlife experts, conservationists and healthcare experts, Enetwild, has, since 2017, been tasked with leading research into the link between wild boar and ASF. “The wild boar problem has been progressing for decades,” says Joaquín Vicente Baños, a Spanish scientist and coordinator of Enetwild. “It’s just that now we are seeing the consequences.”

Wild boar now number more than 10m in the EU, the group says. “Conflicts between humans and wild boar will increase,” says Baños. The numbers are putting more pressure on cities to manage the population of a pest that’s bigger than a rat, with behaviours more complex than a pigeon or stray cat.

Boar eradication strategies have been trialled, including contraception, poison and selective culling. In Berlin, the city pays a team of stadtjäger, or trained street hunters, to pick off nuisance wild boar within city limits. They have shot thousands, but there are still roughly 3,000 in the German capital, populating the city’s green outlying enclaves and parks and venturing on to streets at night, according to the German hunting lobby.

In rural Texas, they use helicopters to flush the wild hogs into the open. A marksman, flying shotgun, picks them off one by one. “It’s expensive on a per-hour basis,” says Michael J Bodenchuk, a wildlife biologist and director of Texas Wildlife Services, a division of the US Department of Agriculture, who often does the shooting. “But cheap on a per-pig basis. Because we’ve got so many pigs!” The only thing to put a crimp in his kill rate was the recent federal government shutdown. “We lost a month of flying,” said Bodenchuk, which put them behind their kill targets.

Barcelona takes a different approach. Shortly after the calamitous 2013 police shooting, the city hired a team of veterinary scientists from the Autonomous University of Barcelona (UAB). The vets practise a form of wildlife management on the streets of one of Europe’s most densely-populated cities. Their duties involve pre-planned kills – targeting females in their prime reproductive years and their young, rather than adult males – they also accompany police on late-night calls in case they are needed to euthanise a boar. During the day, they conduct citizen outreach efforts and supply data and reports to city officials about waste management and where the city is falling behind on trimming vegetation along roads, parks and squares. The effect of this partnership is that boar-human clashes in Barcelona have fallen by more than half, results that are gaining attention across Europe. “They’re doing great stuff,” Sebastian Vetter at the Research Institute of Wildlife Ecology in Vienna, told me.

But while scientists and conservationists see real promise in the Barcelona programme, politics and public opinion might just sink it. Brussels last year rejected a UAB-led funding request that, the team hoped, would lead to an EU-wide plan to manage the urban boar problem along the lines of the Barcelona model. Jorge Ramón López Olvera, the UAB vet scientist managing the programme, told me his contract with Barcelona, which expires in 18 months, hinges on the whims of city hall. Urban boar are a new urban issue, Olvera says, one that is confounding and dividing city dwellers. Homeowners want them off their street. Animal rights activists want them relocated in a humane way. Hunters prefer the status quo, while politicians just want the problem to go away. And they don’t all agree on whether Olvera’s methods are best for Barcelona. After six years on the job, Olvera has learned that what to do about the boar has become an emotionally charged question. “It’s a human-to-human conflict as much as a human-to-wildlife one,” he says.

One evening in late May, Olvera picked me up in a beat-up Dacia Logan station wagon with a blowgun in the back and enough drugs to knock out a charging elephant. We drove to Llars Mundet on the periphery of Collserola. Within its 14 wooded hectares there are public-housing projects, a sports complex, a senior-care home, a primary school and the Universitat de Barcelona campus, where residents and staff endure frequent unwelcome visits from families of wild boar. City hall, the local council and Olvera’s team had scheduled for that evening a “proactive capture”, one of eight planned for hotspots throughout the summer months.

We came across a white van parked in a clearing. It belonged to Estrateko, a local animal control firm that works with Olvera’s team. A few metres beyond, suspended above the ground, was a drop-net trap that Estrateko had set up, and a circle of corn feed strategically placed below the netting. They had rigged the trap with a wifi-enabled trigger. All the boar action – if there were any – could be viewed on a private app from a smartphone or tablet. Swipe right on the app and a signal would trigger the release of the trap’s central spring, dropping the 10-by-10-metre net on the ground, ensnaring any animals below. The plan was to catch two boar families after dark.

I took a seat in the front of the van, with Enric Vila from Estrateko and a scientist, the Catalan naturalist Jordi Baucells Colomer. These catch-and-kill stakeouts are scheduled at night when there are more boar than people about. I had never been on a trapping expedition before, so I didn’t know what to expect. But when I spied a cyclist, then a jogger and then dog-walkers from my vantage point in the van, the whole hunting vibe vanished. Then I noticed we were parked right next to the trap. I figured this would be a long night.

Facebook Twitter Pinterest The net trap, ready for wild boar. Photograph: SEFaS/Ajutament de Barcelona

Just then Vila pointed. “Senglar!” (Catalan for boar), he whispered excitedly. Two sturdy females and eight striped piglets were directly in front of us. A lone female tentatively approached the bait. The others hung back. Vila predicted they wouldn’t go for it. Sure enough, they all trotted off. A few minutes later, however, an even larger group arrived. Vila and Colomer had expected them, but not quite so soon. And then something surprising happened: a third group, unknown to my fellow van passengers, showed up. A confrontation between the two groups – more than 20 boars in all – broke out. The original group protected its turf, chasing off the newcomers. This was not going to plan. “It’s getting too hot,” Colomer said. We high-tailed it out of there, joining Olvera and the others a safe distance from the trap.

Olvera and his colleagues are not exterminators. There is no “Boar Busters” logo on the side of the Dacia. The car is property of UAB where, as a professor of veterinary science, Olvera teaches undergraduates and is part of the department’s wildlife ecology and health group. The unit outsources its expertise to municipalities and local firms. Olvera’s other research projects take him out of town, to study mainly chamois in the Pyrenees and Iberian ibex – ungulates, like the wild boar. His boar work is more complex, if only because there are so many humans to contend with. Planning meetings with alarmed business and property owners can get animated, as can gatherings with hunters who don’t always care for hunting tips from “the university guys”. Animal rights activists would like nothing more than to shut the boar-catching operation down. Olvera’s philosophy is to keep a low profile: “We don’t want to call attention to ourselves.”

But it is clear Olvera, a native Barcelonan with a cyclist’s build and a near endless reserve of energy, likes to talk about boar. He shares with me tales of the more than 300 emergency calls they have responded to in the past six years. There was the time a frisky male descended into the heart of the city, appearing in Plaza Catalunya just as the clubs shut for the night. Carles Conejero Fuentes, the youngest member of Olvera’s team at 28, took care of that situation. (The clubbers, mostly inebriated, failed to notice the 49kg animal in their midst, Fuentes said.) Gregorio Mentaberre, Olvera’s friend and colleague for 20 years, had a close call when one charged him in an alley. He leapt out of its path and felt the animal graze his leg, before recovering enough to take aim with his blow-dart.

Olvera, 43, seems to get the trickiest calls, including when police, in the small hours of the morning, had to shut down a major roadway outside the CosmoCaixa science museum because a group repeatedly zipped across a grassy central reservation into east- and west-bound traffic. Olvera had trouble with one particular boar that he couldn’t get out of the road. He and an officer approached it in the Dacia at low speed. The officer grabbed hold of the steering wheel as Olvera leaned out the window, steadied the blow pipe and forced from his lungs a burst of air. The tranquilliser dart sank into the boar’s fleshy backside.

Once the boars are incapacitated, blood tests are taken. They are then euthanised on the spot. Autopsies are performed the next day in a lab at UAB. Blood tests indicate the overall health of the boar population and whether they pose a risk to the public.

Olvera views the city as a giant laboratory, and the boar an indicator of Barcelona’s ecological health. He is collaborating with Jordi Serra Cobo, a Spanish biologist famous for his work on how viruses make the leap from animals to humans. The two were introduced by an official in city hall. They want to know how many urban boar carry hepatitis E, and what that means to humans. In the blood tests of city-caught boar, they see worrying seasonal up-ticks; 44% carried hepatitis E this spring. Olvera calls the work a potential early warning system for the city. “Wild boars are pissing and defecating in city parks where children play. If children can get sick, we want to know before it becomes a problem,” he says.

The boar management plan relies heavily on the data they collect on population size and migration patterns. A major objective is to identify the “boar corridors” – feeder roads and paths into town – and halt their movements from Collserola, their natural habitat, to the bustling city streets below. They can now predict where the boar are likely to appear next during the May-to-September high season. They build heat maps by gauging numerous factors including rainfall, temperature levels (hot, dry conditions tend to send adults on a quest into the city for shade, food and water) and matching that with police phone logs of citizen boar sightings. “If we know where the wild boar hot spots are,” Olvera explained. “we can plan our captures where we believe it will have the biggest impact”.

It is not enough to track and trap boar where the city meets nature. Since 2013, the UAB team has been on the street almost daily. They are on call round the clock. They also work closely with city officials to make rubbish bins and park entrances boar-tight. They run an education campaign, visiting school children and talking to park-goers to explain the need to keep food waste contained, and to keep a safe distance should they encounter, say, an unpredictable sow with her cute piglets in tow (no selfies!). Last year, boar pest calls fell to 480 – a 60% drop from 2016. Olvera is cautiously optimistic the situation is coming under control. “We are effectively chasing them back into the forest now,” he says.

Across much of the world, the wild boar population has exploded since the 1980s, coinciding with the arrival of warmer winters, the improved crop yields of industrialised farming and the declining number of predators, including hunters. (Hunters grumble that millennials and property developers are killing their sport.) The boar’s adaptability and high intelligence make them one of the most prolific large mammals on Earth. “The wild hog,” observed The New Yorker writer Ian Frazier, “is an infestation machine.”

Young sows can produce as many as two litters per year, averaging 5-6 piglets – and as many as 14 – per brood. Boar become sexually mature when they reach about 35kg. If food supplies are plentiful early in life, females reach breeding age well before their first birthday. Meanwhile, their taste for junk food or crops such as corn and cereals boosts fertility: wild boars that feed on such a high-calorie diet, scientists observe, produce larger litters, and more often. Olvera observes something else. “Urban boars,” he says, “live large and fast. They die younger than the boars of the forest.”

Wild boar are smaller than the farmyard pig, but they are growing more prodigious. Some are so accustomed to human foods (they love factory-made cat food, too), they are becoming obese. In the autopsy lab on UAB campus, Olvera’s colleague Fuentes cut back a piece of an exterminated urban boar’s midsection revealing a thick layer of spongy tummy fat. He measured up the girth between his thumb and index finger. “In boars, it should be half that,” he told me. (UAB autopsies routinely reveal in the stomach traces of rubbish, including pieces of plastic, and undigested human-prepared foods such as chicken and sandwich meat.)

Facebook Twitter Pinterest Aiming a tranquilliser dart at a family of wild boar in Barcelona. Photograph: SEFaS/Ajutament de Barcelona

Swine, or sus scrofa, have confounded humans for millennia, since pigs were domesticated 9,000 years ago. Keeping pigs penned in is not always easy, and the ones that escape adapt to the wild in a matter of months. They don’t just change their habitat, but also behaviour and appearance in subsequent generations. They grow a coat of coarse hair. Their tails straighten. Tusks lengthen. They become super-adapters, shape-shifting problem solvers with speed and agility. “Wild pigs can run up to 30 mph. They can jump over fences less than 3ft high and have ‘climbed’ out of pig traps with walls 5 to 6ft high,” writes Billy Higginbotham, a wildlife conservation expert at Texas A&M AgriLife Research.

If the wild boar senses it is being hunted, it becomes more nocturnal. “In the forest, a wild boar can smell you well before you can see it,” says Torsten Reinwald, a German hunter and press officer for Deutscher Jagdverband, the German hunting lobby. In the city, the boar’s innate “flight distance” threshold – the distance in which it perceives a human as a threat – drops to five metres, Reinwald says. The traditional hunting tactic, known as drive hunting is massively inefficient for conservation. It typically involves more than a dozen hunters and their hounds, working in concert for hours, to bag a few boar. The objective is to startle the prey in the direction of the line of fire, a tactic that occasionally puts hunters in the line of fire too.

Hunters and scientists attest to the animal’s intelligence. Sebastian Vetter, of the Research Institute of Wildlife Ecology in Vienna, tells me he finds it increasingly difficult to round up new specimens for behavioural studies. Despite abundant numbers, wild boar become scarce precisely at the moment the researchers seek to lure them into a wood-fenced enclosure for observation. Vetter wonders if changing something – his entrapment strategy, his appearance, his car, anything – might improve the odds. “They seem to know all my tricks,” he marvels. “It’s like they’re smarter than me.”

A native to Eurasia and north Africa, the wild boar, thanks to its adaptability, can be found on every continent outside of Antarctica. In almost every case of humans introducing swine to a new region, a vast number escape and quickly find the new surroundings to their liking. And then they multiply.

British hunters drove the wild boar to the brink of extinction centuries ago. At the time of George Orwell’s Animal Farm, there were virtually none in the UK. In recent decades, though, UK boar numbers have swelled. Their reintroduction, according to the Department for Environment, Food and Rural Affairs (Defra), can be explained by “escapes and deliberate releases from wild boar farms”. Defra estimates wild boar numbers in the past 10 years jumped from a few hundred to more than 4,000 in Wales, Scotland and England, including the group that turned parts of the Stourhead National Trust estate in Wiltshire into a no-go zone last year, and another that ran amok in Kent, shutting down the A21. Bodenchuk, who frequently speaks at international conferences on wild boar management practices, says he sees the tell-tale signs in the UK – rapid urbanisation squeezing an already dwindling habitat and no central plan to control the numbers – for what he calls “a pig bomb in about 60 years.”

African swine fever has been raging in parts of Europe for the past five years, putting pressure on health officials and scientists to contain it. For Barcelona, the closest active ASF problem area is in Belgium, 620 miles away. But the naturalist Jordi Baucells Colomer told me that, even at that distance, nobody assumes they are safe. Spanish pork production accounts for roughly 1% of GDP. And Catalonia, with a population of 3m pigs, is particularly vulnerable, he says. “It’s not a matter of if, but when,” he said. And then, using the same metaphor as Bodenchuk, he concluded: “It’s a bomb.”

There is no vaccine to combat African swine fever, one of agriculture’s most feared animal diseases. Pigs, feral or domesticated, pass it to other pigs. With a near 100% mortality rate, ASF kills swine within 10 days, putting whole farms and boar colonies at risk. (It poses no health risk to humans.) ASF causes high fever, weakness and vomiting in the stricken pig, its skin turning red or blue. Affected pregnant sows often abort spontaneously.

Vets first observed swine fever in Kenya in 1921. It spread through the pig population of European settlers there, most likely as the non-native swine came into contact with warthogs and other indigenous species. Because the virus persists for months in the carcass of the infected swine, the contagion is extremely difficult to halt if boars perish in the bush. ASF jumped around Africa for decades, appearing most notably wherever European colonials wanted to keep pigs. In the second half of the 20th century, following global trade routes, ASF spread from Angola to Portugal, then to Spain, the Caribbean and Brazil, ravaging parts of the pork industry for years.

Facebook Twitter Pinterest Wild boar bin-raiding in Spain. Photograph: SEFaS/Ajutament de Barcelona

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The most recent strain bedevilling the EU arrived via Georgia in 2007. It spread to Russia and Belarus and then bordering EU states in 2014, as well as into China, where it has had a devastating impact. The nature of the outbreak – ravaging some regions, but sparing neighbouring territories – leaves little doubt: people, not animals, are the main disease vector. The virus can latch on to clothing and persists in food scraps, leaving pigs or wild boar that eat, say, a discarded salami sandwich, or those who come into contact with contaminated hunters or farmers, especially vulnerable. To date, 10 EU countries have reported ASF outbreaks (it is active in nine), causing the quarantining of pig farms along the EU’s eastern border from Estonia to Romania.

According to European commission data from June this year, between quarantines and culls, ASF outbreaks on farms is beginning to come under control. ASF among wild boar, however, is as bad as ever. In 2015, as reports of the disease were streaming in, European Food Safety Authority officials convened an emergency panel of conservationists and wildlife disease experts. They laid out an unsparing containment strategy: if you drastically reduce the number of wild boar, you will eradicate ASF. In practice, that means cutting off their food supply and upping the kill rate – to 70% of the wild boar population within impacted regions – with the explicit targeting of young sows and piglets.

Beginning in 2015, many EU countries responded by extending boar hunting season year-round. Bounties were paid to hunters and record numbers were slaughtered. Those measures may have backfired in places. In Poland, a group of scientists wrote an open letter to prime minister Mateusz Morawiecki in January, urging an end to the eradication hunts. With so many hunters traipsing through the forest, and handling dead carcasses, they argued, it gave rise to the perfect conditions for spreading swine fever. Sure enough, nearly 1m wild boar were killed in Poland from 2015-2017, and the number of confirmed cases in Poland went from 44 in 2015 to, according to commission data, more than 2,500 a year ago.

The stakes are high. If a producer lands on an ASF quarantine list it can be ruinous – for them, and their region. In January, after Australian authorities discovered Chinese imports carrying the virus, officials raised the spectre of restricting the global pork trade from affected countries. “We’re talking about it in the US,” Bodenchuk told me. With a $22 billion pork industry to protect, the USDA announced in May it would begin testing Asian pork imports for ASF.

In Europe, as ASF spreads from east to west, fears of contagion are beginning to divide the bloc from within. Hog-tight barriers have already been erected along the French/Belgian, Danish/German and Bulgarian/Romanian borders in the past year. Boar barriers have become highly unpopular gestures in some countries, reopening old wounds. “Under the guise of biosecurity, it brings back the argument of erecting fences on Europe’s eastern border,” Marianna Szczygielska, a Polish researcher and postdoctoral fellow at the Max Planck Institute for the History of Science in Berlin, told me. In a sign of how political Europe’s boar problem has become, thousands of mainly urbanite Poles took to the streets of Warsaw in January to protest against the government’s boar crackdown measures. Or so it seemed. “I’m sure not everyone was out there in the name of animal rights,” Szczygielska added. “It became the perfect vehicle for anti-government protesting.”

In Italy, several hunters, farmers and urban wildlife management officials have told me the boar plaguing Italian towns and causing car accidents are not a local breed, but rather an overly aggressive invader from eastern Europe. Scientists say there is no evidence, but that has not stopped officials, including a southern Italian politician closely aligned with Matteo Salvini, the far-right interior minister, from using this argument to justify boar culls in Puglia. Before Italian politics turned overtly anti-immigrant, it was anti-boar.

Back in Llars Mundet on that May evening, darkness fell. On a gravel road near the park’s entrance, Olvera and the Estrateko team discussed trapping strategy, one eye on a tablet device broadcasting the live feed from the trap’s infrared camera. Footage showed several boar circling. Clearly, the racket we had made minutes earlier, racing away from the trap site, hadn’t fazed the animals.

One of those who joined the trapping operation as an observer that evening was Mercedes Vidal, a politician whose constituency includes this district. The day before, Barcelonans had gone to the polls in a tightly contested mayoral race. The boar problem never rose to the level of a talking point in the mayoral campaign. Barcelona has bigger issues. Mass tourism and the secessionist question loom over every discussion about the city’s future. Nonetheless, the boars are a frequent complaint in her district, Vidal told me. “A concern,” she added, “not a panic.”

Vila from Estrateko approached Vidal and me. He held out the tablet. We could see that a group of boars had settled in to feast on the bait. It was time. With a swipe, he set off the trap. Terrorised screams filled the night. We ran in the direction of the commotion.

Facebook Twitter Pinterest ‘Their taste for junk food or crops such as corn and cereals boosts fertility.’ Photograph: SEFaS/Ajutament de Barcelona

I arrived at the trap site somewhat winded. The squeals were deafening. Eleven boars were captured in the netting, the nylon strands wrapping tighter and tighter around their thighs the more they bucked and struggled. Olvera and Mentaberre pounced, injecting each boar in the backside with a tranquilliser.

Before long the boars were breathing easy, then snoozing, then snoring. Olvera and Mentaberre took blood samples. A few minutes later they administered the euthanising injection. Lifeless boars were stacked in the van, and the trap was set up for round 2. Just before midnight, more hellish squeals pierced the gloom. Again, the catch was knocked out, blood was drawn and they were killed, bringing the death toll for the evening to 21 – eight adult females and 13 piglets.

The following night, in Nou Barris, another part of the city, the trap was set up again – this time in a grassy clearing a few metres from a bus stop. Down the road was an asphalt football pitch, high-rise apartment buildings and a school. The team caught and killed eight more – seven females and one large aggressive male.

In mid-June, the preliminary lab tests on the 29 slain boars came back. I scrolled through the results on my mobile. These four-legged city dwellers were sick and contagious. One tested positive for salmonella. Three carried rickettsia-infected ticks, a pathogen that can move from animal to human. Fourteen had campylobacter, which the World Health Organization calls a top cause for diarrhoeal diseases in humans, and “the most common bacterial cause of human gastroenteritis in the world”. Seventeen tested positive for the antibodies against the hepatitis E virus, with six showing signs of ongoing infection. One consolation: no trace of ASF.

On my last day in Barcelona, Olvera and I met with Míriam Martínez, a vet who oversees wild and farm animal protection for FAADA, the Spanish animal rights group. It has the ear of many in city hall and has sparred with Olvera in the past. Martínez and Olvera agree on one thing: that for the safety and wellbeing of citizens and animals, the number of urban boar must be reduced. But she believes Olvera’s methods to achieve that end are morally wrong. The animals should be repatriated to reserves, or sterilised, she insists, not euthanised. When Olvera pointed out the science on sterilisation serums is still unproven and that the cost, legality and health risks of moving boar to a distant reserve make it near unfeasible, Martínez cut him off. “We are here to find alternative solutions, and we will work with government officials on it,” she said.The meeting ended with the two sides far apart.

This discord reveals the precariousness of Olvera’s position. If the local political winds were to shift, he acknowledges, their city contract may not be renewed at the end of 2020. Olvera plans to re-pitch the EU for funds, but unless they find the right European partners and backers in Brussels, it is a longshot.

On the drive back to the UAB campus, Olvera shared his professional journey. He had wanted to work with animals – “wildlife, not a dog-and-cat vet” – since he was a boy. During his studies, he became fascinated with ecology, and then grew concerned about the impact humans have on nature. Criticism of his current work, he says, doesn’t bother him. He is not sentimental about the urban boars he is contracted to kill, not even the piglets. He is proud of the science they are contributing to this new field of urban wildlife management. Despite the long odds, he hopes that science will restore the natural balance – the distance that once kept us safely apart. In short, he wants science to help save the boar from us.

“The urban boar has become a species unto itself,” he says, a species that is looking less like its wild forebears from the forest. “It’s become habituated to city life, and we are transforming it. It’s not healthy – for the boar, or for us.”

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• This article was amended on 30 July 2019 to clarify the figures given for Barcelona’s population.