Which animals of these 4 can sleep with one eye open?

Ducks have developed an amazing ability to let half their brain sleep while keeping the other half awake. Researchers report that ducks manage this trick to stay alert for predators while still getting some shut-eye. The findings suggest that different parts of the vertebrate brain can sleep independently of each other. More than that researches noticed that ducks that were next to each other slept with the outside eye open. Sleep researcher Niels Rattenborg and animal behavior experts Steven Lima and Charles Amlaner, placed four ducks in a row of clear tanks and waited for them to doze off. The ducks in the middle tanks would almost always sleep with both eyes shut, while those on the ends kept one eye open for about a third of the night. When the researchers rotated the ducks to different spots, the bird that had slept with its left eye open on one end, for example, would sleep with both eyes closed in an interior tank and with the right eye open in the tank at the other end.