Why do beavers have transparent eyelids?

Beavers have transparent eyelids to see underwater. These extra eyelids are called nictitating membranes, and they slide across the eyes like clear protective goggles when a beaver dives or swims.

The membrane helps shield the eye from water, mud, branches, and other debris while still allowing the animal to detect its surroundings. That is especially useful for a semi-aquatic animal that spends much of its time in ponds, streams, and underwater entrances to its lodge. The eyelid is not meant to improve night vision or attract mates; its main role is underwater protection and vision.

Beavers have several other adaptations for life in water. Their nostrils and ears can close, their dense fur provides insulation, and their lips can close behind their large front teeth while they gnaw underwater. A beaver can also remain submerged for roughly 15 minutes. The transparent membrane is therefore one part of a broader set of physical features that help beavers forage, escape predators, and build in wet habitats.