Let sleeping ducks lie

An excursion into sleep, prompted by learning by chance that mallards often sleep with one eye open, first noticed at reference 1. After which I turned up references 2, 3 and 4, among others.

I have been reminded that sleep is a rather more complicated business that shutting down at dusk and opening up again at dawn. For example, one might go through half a dozen sleep cycles during the night, with a period of what is called REM sleep (of reference 5) in the middle of most of those cycles, periods in which the arousal threshold is high, muscles are slack and thermal regulation is suspended. Thus accounting for the need to sleep lying down in secure, warm places. And sleep is very necessary: deprive someone of sleep for any length of time and death will eventually follow.

More particularly, I learn that while humans put the whole of their brain to sleep, some animals, particular some birds and most sea animals  - whales, seals and such-like – are able to put one hemisphere of their brain to sleep at a time. Generally speaking this seems to mean that the contralateral eye is turned off – that is to say, if the left side of the brain (for example) is asleep, then the right eye is turned off. Sometimes this is called unihemispheric-monocular sleep  or Un-Mo sleep. A compromise between being properly asleep and being properly awake, a compromise driven by some fact of life or other.

In the case of ground roosting birds like ducks, this compromise is mainly about keeping an eye out for predators, and in reference 2 we are told that ducks can manage their Un-Mo sleep according to circumstances – not doing so much of it, for example, if they are in the middle of a flock. Let the ones on the outside of the flock be the sentries and do the looking out.

In the case of mammals living entirely in the water, like whales, this compromise is mainly about coming up for air from time to time and maintaining thermal regulation, tasks which can be managed with just half the brain online. While seals can do Un-Mo sleep while swimming on the surface, on their sides, paddling with just the one flipper.

From which I branched off briefly to animals like horses, who, it seems, can sleep, at least after a fashion, at least some of the time, while standing up. While BH pointed out that when our children were young, she often used to half sleep, in some sense or other, with some part of her brain keeping an ear out for untoward childish activity.

PS: along the way, I came across the fact that the mouths of whales are not connected to their lungs, as is the case with most land animals. Their lungs are connected to their blowholes on the tops of their heads, their version of the nose in evolution speak. I wonder what that Irish creationist and politician, Mr Edwin Poots, would make of all this? While I associate from his name to the splendid Mr. Pooter.

References

Reference 1: http://psmv4.blogspot.com/2021/05/buckfast-abbey-with-inn.html. First notice of this matter. With a visit to Wikipedia prompted by confusion over the name ‘mallard’.

Reference 2: Half-awake to the risk of predation – Niels C. Rattenborg, Steven L. Lima and Charles J. Amlaner – 1999.

Reference 3: Avian sleep – Amlaner, C. J. & Ball, N. J. – 1994.

Reference 4: Unihemispheric sleep and asymmetrical sleep: behavioral, neurophysiological, and functional perspectives – Gian Gastone Mascetti – 2016.

Reference 5: https://en.wikipedia.org/wiki/Rapid_eye_movement_sleep.