When Luigi Boitani, Italy’s leading wolf expert, captured a hybrid in 1975, he says he “was met with everything from gentle opposition to [people who] said, ‘this is bullshit’.”

Time has proven Boitani right. Today, a growing number of studies point to the presence of hybrids in nearly every European country with wolves, and in some areas their numbers are growing steadily. In Boitani’s native Tuscany, and other regions, they have become endemic, accounting for as much as 70% of the wolf population. The rise has been driven by the increasing destruction of wolf habitats and the expansion of human settlements, which bring people, their pets, and packs of stray dogs into more frequent contact with wolf packs.

In some regions “they are basically all hybrids,” Boitani says. “In this case, there is nothing you can do. You cannot send the army and kill everything.”

Hybrids trouble conservationists partly for their unpredictability. They may increase conflict with humans, crowd pure-blood wolves out of their habitat, or reduce the viability of future offspring, hampering efforts to revive Europe’s wolf population.