Scientists Found a Butterfly That’s Been Isolated for 40,000 Years

Scientists have officially named a new butterfly species after one of its strangest qualities: its eerie isolation.

In the remote glacial valleys of Canada’s Waterton Lakes National Park, researchers discovered the “curiously isolated hairstreak” (Satyrium curiosolus), a small butterfly that’s been living cut off from the rest of its kind for an estimated 40,000 years.

Unlike its closest known relatives, the hairstreak wasn’t wiped out by the last ice age. Instead, it clung to survival in a small patch of land that never froze, a so-called “glacial refuge.” Genomic studies revealed that this population is about 400 kilometers away from its nearest cousins, making its survival even more extraordinary.

“Understanding exactly how this population was isolated is a difficult question,” said Dr. Julian Dupuis, co-author of the study published in ZooKeys.

New Butterfly Species Discovered

While it resembles the half-moon hairstreak on the surface, the curiously isolated hairstreak lives a very different life. It depends on the silvery lupine plant to raise its young and has developed a rare partnership with the ant species Lasius ponderosae.

The caterpillars feed the ants a sugary substance called honeydew in exchange for protection from predators. Sometimes, the caterpillars even retreat into the ants’ underground nests during heatwaves—a survival trick almost unheard of among North American butterflies.

But this isolated evolution hasn’t come without cost. With only around 500 individuals, the hairstreak has extremely low genetic diversity. Usually, inbreeding spells doom for small populations. Here, however, something called “genetic purging” seems to have worked in the butterfly’s favor, allowing it to shed harmful mutations over thousands of years.

“You would assume that inbreeding depression is going to drive this thing to extinction,” said Dr. Zachary MacDonald, co-first author of the study. “But slow inbreeding has basically allowed it to get rid of its bad genes.”

That doesn’t mean the species is safe. With its razor-thin genetic margins and heavy dependence on specific plants and ants, any shift—be it from invasive species like spotted knapweed or the growing threat of wildfires—could tip the population into collapse. Conservationists are already monitoring the hairstreak closely, considering options like captive breeding but holding off for now.

“This is just the tip of the iceberg,” MacDonald said. “What’s most interesting to us is what do we do about the conservation of this highly endangered species now?”

The curiously isolated hairstreak may have survived ice ages, but whether it can survive the next few decades is far less certain.