Merriam-Webster's Word of the Day for December 21, 2025 is:

hibernaculum • \hye-ber-NAK-yuh-lum\  • noun

Hibernaculum (plural hibernacula) refers to a shelter occupied during the winter by a dormant animal, such as an insect, snake, bat, or marmot.

// Local scientists are studying the longevity of bats who use bridges and other aboveground hibernacula versus that of bats who roost all winter in subterranean caves.

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Examples:

“Adult female bees begin looking for a hibernation location, or hibernaculum, in the fall. If the gardener is planning to deadhead any spent flowers from the summer, aim to prune stems at varying heights (8" to 24") as a nesting site for these bees. Many perennial flowers and shrubs have pithy stems that will serve as a good location. A few common Oklahoma garden plants that are good candidates include roses, purple coneflower, salvia, bee balm, and sunflowers.” — Sherry Clark, The Shawnee (Oklahoma) News-Star, 8 Oct. 2025

Did you know?

If you’re afraid of snakes or bats, you probably won’t enjoy thinking about hibernacula, where hundreds, even thousands, of these creatures might be passing the wintry months. Other creatures also use hibernacula, though many of these tend to be less crowded. The word hibernaculum has been used for the burrow of a woodchuck, for instance, as well as for a cozy caterpillar cocoon attached to a wintry twig, and for the spot in which a frog has buried itself in mud. Hibernacula are all around us and have been around for a long, long time, but we have only called them such since the late 1700s, making hibernaculum only a few decades older than the more familiar verb hibernate. Both words come from the Latin verb hibernare, meaning “to pass the winter,” which in turn comes from hibernus, meaning “winter.”