In the heart of Canada’s Rocky Mountains, an unassuming yet remarkable butterfly has been quietly flying under our scientific radar for years. With a wingspan of an inch to an inch and a half, and wings that are brown on top and greyish brown with black spots below, this population was long thought to belong to the Half-moon Hairstreak (Satyrium semiluna). However, the isolated hairstreak butterflies of Blakiston Fan in Waterton Lakes National Park, Alberta, have now been recognized as a distinct species: Satyrium curiosolus, or the Curiously Isolated Hairstreak.

A recent study by an international collaborative team, published in ZooKeys, uncovered the unique evolutionary history of this population. The results were striking: Satyrium curiosolus has been completely isolated from its closest relatives for quite a while—possibly up to 40,000 years— becoming more and more genetically and ecologically unique along the way.
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The science behind the discovery

“Our whole-genome sequencing of S. curiosolus revealed strikingly low genetic diversity and exceptionally high levels of historical inbreeding compared to the geographically nearest S. semiluna populations in British Columbia and Montana, more than 400 km distant,” says co-first author Zac MacDonald, a La Kretz postdoctoral researcher at University of California Los Angeles Institute of the Environment and Sustainability. Despite its small population size, genetic data suggest that S. curiosolus has likely maintained itself as a stable, independent lineage for tens of thousands of years. “Like the Channel Island Fox, S. curiosolus may have purged some of its harmful recessive genetic variation through a long gradual history of inbreeding, allowing it to persist as a small and completely isolated population today,” MacDonald adds.

Satyrium curiosolus is found in a distinct habitat unlike any other population of S. semiluna that we know of. While its relatives thrive in sagebrush steppe, S. curiosolus occupies a single alluvial fan that is more accurately described as prairie-grassland, where it associates with different plants and ant species. Satyrium curiosolus relies exclusively on silvery lupine (Lupinus argenteus) for larval development, a plant not known to be used by S. semiluna populations in British Columbia.

“Furthermore, we recently discovered that S. curiosolus larvae have mutualistic relationships with a particular species of ant (Lasius ponderosae), which has not been observed in other S. semiluna populations,” says James Glasier of the Wilder Institute/Calgary Zoo, who was also part of the study. Satyrium curiosolus caterpillars provide the Lasius ants with a sugary excretion called honeydew to eat, while in return the ants protect the caterpillar from parasites and predators. Caterpillars also retreat into ant galleries when disturbed, or when it gets too hot out, and adult females have been observed laying eggs right near the entrances to Lasius colonies under Silvery Lupines. This content is taken from Pensoft Publishers

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"Curious isolation: new butterfly species discovered", MachPrinciple, July 02, 2025, https://machprinciple.com/post/Curious-isolation-new-butterfly-species-discovered