In a remote corner of the Scottish Highlands, researchers are about to see what happens when cats go wild.
In June, the first captive-bred European wildcats (Felis silvestris) fitted with GPS-tracking collars were released in Cairngorms National Park, UK. If things go according to plan, they will form a sustainable population of an animal that once roamed forests across Britain, but is now all but extinct.
“The last hope for Scottish wildcats is the captive wildcat population,” says Jo Howard-McCombe, a conservation geneticist at the Royal Zoological Society of Scotland (RZSS) in Edinburgh, UK, which is leading the wildcat breeding effort.
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There’s just one problem: house cats (Felis catus) or, more precisely, their DNA. All of the captively bred wildcats — and the handful that still live in the wild — bear varying levels of domestic-cat DNA from decades of interbreeding. Researchers worry that interbreeding with roaming pets will further dilute the genome of the wildcat — also known as the Highland tiger — until there’s nothing ‘wild’ left in it.
But now, insights from ancient genomics are giving captive breeding efforts a boost1,2. An ancient-genomics study published today found that wildcats have started breeding with their domestic counterparts only relatively recently, in the late twentieth century2. For at least 2,000 years before that — when Romans, Vikings and other ancient cat-loving societies took domesticated felines everywhere they went — the two species rarely mixed, even when living side by side.
“That changes the thinking about the prospects for preserving the Scottish wildcat in its ancestral form,” says Jonathan Losos, an evolutionary biologist at Washington University in St. Louis, Missouri, who was not involved in the studies. “It’s not a lost cause.”
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Researchers are still debating the details, but it’s clear that domestic cats mostly descend from a wildcat species (Felis lybica) that is distinct from European wildcats and that lives in what is now North and sub-Saharan Africa and the Middle East. Scientists think that F. lybica cats formed mutually beneficial relationships with early farming communities in the Middle East thousands of years ago. But it might have been ancient Egyptians who transformed these partly domesticated creatures into something more like today’s house cats, says Leslie Lyons, a feline geneticist at the University of Missouri in Columbia. “They mummified thousands of them, tens of thousands. I would say they were the first cat breeders.”
In 2017, an ancient-DNA study found that some ancient cat remains in Europe and the Middle East, which dated to around 800 bc and later, had the same mitochondrial DNA markers as Egyptian mummy cats3. Then, 1,000 years later, the same lineage turns up in feline remains from a Viking port near the Baltic Sea.
It makes sense that early Europeans imported their cats, rather than attempting to domesticate the wildcats that roamed their forests. Modern African wildcats are much more tolerant of humans than are their “hellaciously mean” European counterparts, says Losos. “No matter how nicely you treat them, they grow up into being nasty animals that are terrible pets.”
Despite their darker dispositions, it can be hard to distinguish European wildcats from their domestic counterparts. Wildcats — which still thrive in larger, less hybridized populations in some parts of Europe and west Asia — tend to be slightly bigger and have bushier tails than house cats. As a result of interbreeding with their domestic counterparts, the wildcats in Scotland now constitute what biologists call a ‘hybrid swarm’ of mixed interbreeding individuals, and wildcat ancestry is at risk of vanishing.
But all is not lost for truly wild Scottish wildcats. A captive population of around 150 animals lives in more than 35 zoos, wildlife parks and other sites across the United Kingdom. These animals carry varying amounts of domestic-cat DNA — but less than the current ‘wild’ population does, says Helen Senn, head of conservation at RZSS and its Saving Wildcats programme.
In 2019, Saving Wildcats won European Union funding for a captive breeding programme, with the aim of revitalizing the wildcat population. Animals are bred in a wildlife park in the Scottish Highlands and raised in large, naturalistic enclosures away from visitors.
So far, 19 wildcats have been released into a tract of land of more than 600 square kilometres called the Cairngorms Connect landscape. The goal is to release around 60 wildcats by 2026 to give the animals a decent shot at forming a self-sustaining wild population.
For the programme to be successful, preventing further interbreeding with domestic cats will be crucial. The Saving Wildcats team has implored people living near the site to get their house cats spayed and neutered and to take other steps to protect them from wildcats. Senn is hopeful that hybridization can be limited.
Researchers say that it’s important to figure out when and why domestic cats and wildcats started mixing in the first place. “We need to understand the factors that led to this to prevent it happening again,” says Howard-McCombe.
To investigate when cats descended from African wildcats started mixing with European wildcats, an international team of researchers analysed the genomes of dozens of ancient and modern cats, which mostly lived in Europe and included an 8,000-year-old animal from Scotland that is thought to have lived before the arrival of domestic cats there1.
The ancient domestic-cat genomes, including that of a 2,000-year-old feline found at a Roman palace in West Sussex, UK, contained barely any European wildcat DNA. The 600-year old remains of a wildcat — which might have been tamed — found at a ruined castle in the north of England had no trace of domestic cats in its genome, although other cat remains at the site had mitochondrial DNA commonly found among domestic cats. “They’ve been living side by side for 2,000 years, so it is surprising that they haven’t interbred,” says study co-author Mark Beaumont, a population geneticist at the University of Bristol, UK.
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One explanation could be the stark differences in the two species’ behaviour, says Laurent Frantz, a population geneticist at Ludwig Maximilians University in Munich, Germany, who jointly led the study with Greger Larson, an ancient-DNA researcher at the University of Oxford, UK. Domestic cats are happy living with humans, other cats and even dogs, whereas wildcats prefer a solitary life. “If you go into any town today you can see domestic cats hanging around. In wildcats that doesn’t happen,” Frantz says.
In Scotland, this icy coexistence seems to have lasted well into the twentieth century, finds a study that analysed the genomes of living and ancient cats as well as museum specimens of potential wildcats predominantly from the early twentieth century2. The team, led by Howard-McCombe, Beaumont, Senn and population geneticist Daniel Lawson, who is also at the University of Bristol, estimate that domestic cats and wildcats probably didn’t start mixing significantly until the 1970s. “That was a huge surprise,” says Beaumont.
The current wildcat breeding programme, although it excludes animals with particularly high levels of domestic-cat DNA, chooses breeding pairs with the aim of preserving genetic diversity. Senn’s team, however, is hoping to go further, through an audacious breeding strategy that could see house-cat DNA slowly whittled out of the genomes of wildcats.
The process — called de-introgression — involves sequencing the genomes of captive wildcats and pairing up those animals with domestic DNA that mostly occurs in non-overlapping regions of their genomes. By chance, some kittens in the litter would, hopefully, inherit more wild versions of genes than either parent has, and those individuals would be included in future rounds of breeding to further reduce the levels of domestic-cat DNA (see ‘Rewilding a genome’).
De-introgression has never been tried before, says Lawson. In a preprint published in July, he, Senn, Howard-McCombe and Beaumont estimated that it would take 10–20 generations to recover the complete Scottish wildcat genome4, and the team has now applied for funding to put this theory into practice.
Yet getting rid of domestic-cat DNA from the wildcat genome is not without uncertainties. “We don’t know what genes make a wildcat a wildcat,” says Senn. Some house-cat DNA could be helpful: certain gene variants might help populations to resist diseases, for example. The team’s analysis of wildcat genomes found signs of this: domestic-cat genes involved in immunity seem to be under natural selection in Scottish wildcats’ genomes, potentially protecting them from diseases circulating in house cats. “It’s probably the case that wildcats would already be extinct if it wasn’t for hybridization,” says Senn.
Early results from the breeding programme are positive. In a 13 October update, RZSS reported that 18 of the released wildcats were still alive, had occupied a suitable habitat and were hunting successfully. But the animals have a hard winter ahead and the success of this year’s release won’t be clear until the breeding season. Senn’s dream is to see wildcats roaming large tracts of Scotland — an outcome that could decades, she concedes. “In conservation, we’re in the business of hope.”