We first introduced Merino into our collection with the Harrier Long Sleeve, which we tested over the course of a humid New England summer by asking our team to run in one for as many days in a row as possible before a wash. We lost our nerve around two weeks, but the top still didn’t smell. Spring is one of our favorite times to run in Merino, so we wanted to break down what makes this fiber so special.

The Merino sheep is a rugged mountain-dwelling breed native to Spain and domesticated in Oceania. The Merino thrives in temperatures and conditions too severe for lowland breeds thanks to its unique wool. It's soft against the skin – not at all itchy. It’s also naturally breathable: wool fibers quickly absorb moisture vapor then move it away from the skin to evaporate into the air.

Merino wool’s performance properties are all-natural. Elasticity helps these tops stretch with you, yet return to their original shape. In contrast to synthetics, Merino is an active fiber that reacts to changes in body temperature over the run. It keeps you warm when it's cold, and cool when it heats up.

As it evolved over millions of years to protect sheep against the elements, Merino wool absorbs UV radiation and provides protection from the sun.

"Don’t be fooled by this top’s simple look. Yes, it has a flattering cut and comes in a host of understated colors. It’s also a workhorse, thanks to a blend of nylon and merino wool, which provides stink-free staying power even if you wear it a few runs in a row.”

— Outside, Best Women’s Running Gear

Wool has been used for clothing for centuries, from Grecian togas to Scottish kilts, but the Industrial Revolution brought wool production from individual homes to factories. With wider distribution, wool became a staple of 19th Century activewear wardrobes from henleys worn by rowers to early cross country singlets, adapted for its versatility and performance.