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ONE of the most primitive mammals has thrown theories about the evolution of
rapid eye movement sleep into disarray. A controversial new study suggests that
egg-laying echidnas do have REM sleep鈥攂ut only when the temperature is
just right.

REM sleep remains a mystery. Researchers speculate that it may do anything
from helping consolidate memories during dreaming to mixing up fluid in the
eyeballs to ensure oxygen gets to the cornea.

All birds and placental mammals that have been tested have REM sleep, but
according to several studies, echidnas, whose ancestors split from other mammals
around 120 million years ago, don鈥檛 experience it. That would suggest that REM
sleep evolved at least twice: once in the mammalian line, and once in birds.

Stewart Nicol of the University of Tasmania in Hobart and his colleagues
wanted to know if temperature made any difference. They placed needle electrodes
under the skin of six short-beaked echidnas to monitor brain and muscle
activity, eye movement, respiration and heart rate. When the animals slept at 25
掳C, they had typical mammalian REM sleep. 鈥淚t was unmistakable as far as we were
concerned,鈥� says Nicol. But at 28 掳C or 15 掳C, REM activity decreased or
disappeared entirely.

But Jerry Siegel of the University of California in Los Angeles, who did one
of the earlier studies, is not convinced. 鈥淵ou get echidnas in the desert, in
the snow, everywhere in Australia,鈥� he says, so it is unlikely that their REM
sleep is so sensitive to temperature.