How would you like to sleep with one half your brain asleep and the other half awake? Dolphins sleep this way. Recently, scientists at Indiana State University have discovered that ducks sleep this way too. They found that ducks sleep half awake so they can rest and watch for danger at the same time. After putting their ducks in a row and videotaping them, some researchers found ducks on the end of each row spent more time asleep with one eye open, apparently looking for _  "The more the ducks felt threatened, the more they slept with one eye open," said lead author Niels C. Rattenborg, a graduate student at Indiana State University, Terre Haute. "The unique aspect is not that they do it, but that they control it. When they sleep at the edge of a group, they tend to realize greater danger, so they spend more time sleeping with one half of their brain." Ducks with one eye open were still awake enough to detect predators, said the authors of the study, which appears today in the journal Nature. The researchers studied four groups of four ducks held in plastic boxes, which were arranged in a row. Ducks on the end were found to sleep with one eye open 31.8 per cent of the time, compared to 12.4 percent of the time for ducks in the central position. Also, ducks in the central position did not open one eye more than the others, while ducks on each end kept the eye facing away from the group open 86.2 per cent of the time. Brain wave readings of the ducks showed that the half of the brain receiving signals from the closed eye indicated that half of the brain was sleeping. Signals from the half of the brain receiving signals from the open eye showed a state between fully awake and asleep. The under lined word "predator" is most likely to mean  _  .
an animal that is likely to be friends with ducks a human being who looks after and feeds ducks a scientist who does research work on animals an animal that hunts, kills and eats other animals