What do platypuses dream of?

During REM sleep, named after the rapid eye movement observed during this phase of slumber, people dream and, scientists suspect, memories consolidate. In a surprising development that may challenge theories of why REM sleep arose, researchers have found that the platypus, considered one of the world's most primitive mammals, spends up to 8 hours a day in REM sleep, more than six times the amount that people experience. "They're REM sleep champions," says Jerome M. Siegel of the University of California, Los Angeles and Sepulveda Veterans Affairs Medical Center.

Siegel and his colleagues at the University of Queensland in Brisbane, Australia, made this discovery by recording eye movement, muscle activity, and brain wave activity in four captive platypuses. Videos of the sleeping animals clearly showed their closed eyes moving rapidly.

Platypuses; belong to a rare branch of mammals, the egg-laying monotremes. Studies of the only other two monotreme species had suggested that the animals don't have REM sleep, implying that it evolved after monotremes diverged from the other two mammalian branches, marsupials and placentals.

The platypus sleep study questions that idea. "REM sleep didn't evolve relatively recently in the mammalian line," says Siegel. Since birds also experience REM sleep, the phenomenon may date back 250 million years, to when the last common ancestor of birds and mammals lived. Previously, says Siegel, researchers thought that birds and mammals evolved REM sleep independently Moreover, since he believes that REM sleep is an old evolutionary development, Siegel suspects that it arose to aid very basic brain stem functions rather than for dreaming or helping memory.