The hoary marmot is well known to hikers in the western mountains of North America, where a high-pitched whistle often welcomes visitors to the alpine country. The whistle is actually a warning to other members of the colony of approaching danger. They are sometimes called 'whistlers' because of it, but they are more commonly identified by the name inspired by the "hoary" mantle of white fur that covers their shoulders and backs.

Hoary marmots can weigh up to 13.5 kg (30 lb.), but are usually 5 to 9 kg (11 to 20 lb.). In summer the hoary marmot will gain body-fat on its diet of lush alpine plants. The stored energy allows the hoary marmot to spend the seven to eight winter months in hibernation far beneath the snow.

In early summer, young marmots spend hours engaged in playful wrestling matches. Though the pushing and grappling can be quite vigorous, marmots have only to give a sharp yelp to end the bout. Hoary marmots grow to adult size by late summer.