How many polar bears live in the Arctic? It’s a seemingly important question, say wildlife conservationists, as for most of the 20th century, polar bears were fair game for hunters. But getting a count proved more difficult than scientists originally expected.

Let’s start with the first, and most obvious problem: Polar bears live on ice and snow, and are white (kind of; see today’s bonus fact). They blend in, making it hard to get an accurate count. The solution? Infrared. Scientists can go out at night, armed with infrared cameras, and look for bear-shaped heat signatures in the area. Only one problem: Polar bears are almost entirely invisible in infrared vision. The polar bears’ layers of fur and blubber trap their body heat well below their skin, leaving the outer layer to be roughly the same temperature as the snow around them. The bears’ face and breath can be seen in infrared, but that isn’t enough to go on.

So how do scientists take a polar bear census? Ultraviolet light helps, but for other reasons — specifically, the fact that polar bears roam over exceptionally large areas — the best practice is tranquilize a bear when found, tag it with a tracking device and unique identifier, and move onto the next.

Bonus fact : Polar bears aren’t actually white. Their fur is a series of clear, hollow, translucent tubes which reflect light, making the bear appear white. It’s the same reason why snow — which is, at its core, crystallized water (and therefore clear) — also appears white.

From the Archives: The Undiscovered Island of the Polar Bear: An island whose only inhabitants were polar bears, which is in part why mankind overlooked it for centuries.

Related: This polar bear is both visible and adorable.