You may have seen axolotls — an amphibian in the salamander family with a permanent smile and pink, feathery gills — in a pet store or as a plushie in a window, but the endearing animal’s popularity seems to be rising just as it has become critically endangered in the wild.

James Monaghan, professor of biology at Northeastern University, specializes in the friendly looking critters, studying their amazing regenerative capabilities. “Axolotls have just exploded in [popularity] the past couple of years,” he says.

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Photo by Alyssa Stone/Northeastern University