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. 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). 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. This is the conundrum that Siegel and colleagues have been investigating. 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 was important, because it implied that REM sleep must have evolved in higher mammals. A study in 1972 suggested that the echidna Tachyglossus had no REM sleep. Monotremes have taken a cameo role in studies on the evolution of mammalian brain function. Researchers think that monotremes have been distinct as a group for at least 80 million years, long before the dinosaurs became extinct. They also have a range of other reptile-like anatomical features, features that have been lost in more 'advanced' mammals. Monotremes lay eggs, like birds and reptiles, but unlike all other mammals. All three species are confined to Australasia. Apart from the platypus itself, the group – the monotremes – includes two species of echidna, or 'spiny anteater'. The platypus belongs to a group of mammals with very ancient roots. 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. 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. In humans, REM sleep is associated with dreaming. '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. 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. Siegel of the Sepulveda Veterans' Affairs Medical Center, North Hills, California and colleagues. These are the conclusions of a study on sleep in the platypus by Jerry M.
What's more, it spends more of its time in so-called 'REM' sleep than any other mammal. The platypus, apparently, is a surprisingly deep sleeper.
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