The Platypus Essay, Research Paper

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. 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.