Published online 13 August 1998 | Nature | doi:10.1038/news980813-5

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The sleep of the platypus

The platypus, apparently, is a surprisingly deep sleeper. What’s more, it spends more of its time in so-called ‘REM’ sleep than any other mammal.

These are the conclusions of a study on sleep in the platypus by Jerry M. Siegel of the Sepulveda Veterans’ Affairs Medical Center, North Hills, California and colleagues. Their report appears in a special number of Philosophical Transactions of the Royal Society devoted to the biology of the platypus (Ornithorhynchus anatinus), celebrating the bicentenary of the discovery, in Australia, of this remarkable animal.

‘REM’ stands for ‘rapid eye-movement’ and is the kind of sleep in which the brain can be more active than in it is while awake, the animal twitches, and the eyelids flicker - hence the name. In humans, REM sleep is associated with dreaming.

But does the platypus have an extraordinarily rich dream life? Possibly not, say the researchers: “cats, opossums, armadillos and other mammals not known for their intellectual achievements have far more REM sleep, whether calculated in hours per day or as a percentage of total sleep time, than humans.”

And why study sleep in the platypus anyway? After all, the platypus is an obscure and extremely primitive creature, distantly related to humans. The answer lies in that primitive state: studying the physiology of the platypus could yield clues about the life and behaviour of the very earliest mammals.

The platypus belongs to a group of mammals with very ancient roots. Apart from the platypus itself, the group - the monotremes - includes two species of echidna, or ‘spiny anteater’. All three species are confined to Australasia. Monotremes lay eggs, like birds and reptiles, but unlike all other mammals. They also have a range of other reptile-like anatomical features, features that have been lost in more ‘advanced’ mammals. Researchers think that monotremes have been distinct as a group for at least 80 million years, long before the dinosaurs became extinct.

Monotremes have taken a cameo role in studies on the evolution of mammalian brain function. A study in 1972 suggested that the echidna Tachyglossus had no REM sleep. This was important, because it implied that REM sleep must have evolved in higher mammals. Subsequent research made this result look anomalous, as REM-like sleep phenomena have since been observed in birds and some reptiles: in which case, the echidna may have lost the capacity somewhere in its evolution.

This is the conundrum that Siegel and colleagues have been investigating. First, it turns out that the term ‘REM’ is a misnomer: animals may show REM sleep even though their eyes don’t move, and their bodies don’t twitch. REM is properly defined as a characteristic pattern of activity in the brain, generated by specific neuronal pathways in the brainstem - whether or not this activity is carried forwards into the ‘higher’ centres of the brain (where it is manifested as dreaming). Recordings from discreetly implanted electrodes show that the echidna does, after all, show a kind of REM sleep generated by the brainstem, even though it is rather muted and the animal shows no outward signs. Young animals show more REM sleep than older ones, and it could be that very young echidnas have a more active sleeping life (including twitching) than older ones.

The platypus, though, shows all the classic outward signs of REM sleep. Indeed, an account from as long ago as 1860, before REM sleep was discovered, reported that young platypus showed ‘swimming’ movements of their forepaws while asleep.

Despite these differences, the REM sleep of the platypus and the echidna is confined to the brainstem: the forebrain shows the regular, steady patterns of neuronal activity associated with deep, dreamless sleep. This suggests that for all their REM sleep, monotremes do not dream.

These findings set our understanding of the evolution of sleep on a firmer footing. It now seems that the ‘core’ brainstem activity manifested as REM sleep has extremely ancient roots, going back to the reptilian acnestors of mammals as well as birds. The elaboration of REM sleep into the forebrain is a later innovation: but whether it evolved once and monotremes have since lost it, or if it evolved more than once, is something that only more work on birds and reptiles can establish.

From Nature 11 August 1898.