Our younger daughter, Mara, loves to read things like Weird But True! – filled with tons of interesting facts about the natural world. Mealtime conversations are often peppered with such declarations as “elephants drink 800 glasses of water a day!”, to which I would typically respond with something like “I’m glad I don’t have to load the dishwasher in that family!” You never know where the discussions will start or end, and mealtimes are often protracted affairs.
The other morning at breakfast, Mara suddenly blurted out: “sharks don’t blink!”. Naturally, the follow-up questions came fast and furious – can they blink but don’t need to? Do they even have eyelids? What happens when they sleep? Do they sleep? After getting myself a second cup of coffee, we started to investigate and the next thing you know, we found ourselves staring at this incredible image:
We also ooh’d and aah’d over the amazing slideshow here from National Geographic, by photographer Stephane Granzotto.
What is happening?? No, these are not sharks, obviously. But our discussion of sharks and whether they sleep1 then led to other sea creatures. An important difference between sharks and marine mammals such as whales, dolphins, and seals is that the latter need to surface regularly to breathe, so resting needs to take that into account.
For many years, scientists have known that marine mammals in captivity sleep with one side of their brain shut down at a time – the fancy name for this is uni-hemispheric sleep. It allows them to swim, surface for air, maintain contact with their group, avoid dangers, but still get a little break. It is incredibly hard to study such creatures in the wild so, until recently, it was assumed that the same was true out in the deep blue sea.
However, in 2008, a team of Scottish and Japanese researchers made a stunning discovery when they literally bumped (gently, with their engines off) into a sperm whale who was part of a pod drifting vertically near the surface, seemingly fully asleep and unaware of their proximity!
(Two images here from https://doi.org/10.1016/j.cub.2007.11.003 Copyright © 2008 Elsevier Ltd. Published by Elsevier Inc.)
These researchers tracked the movements of 59 sperm whales worldwide using devices attached by suction cups, recording a total of 562.9 hours of data. They observed that in less than 10% of the time, whales performed shallow, vertical, ‘drift dives’ with very little activity. If this is their only “sleep” time, it is tiny – the least amount of sleep observed in any other mammal studied. A subsequent study of similar behaviour in wild harbour porpoises found a similarly small percentage of drift time, although this study noted that both species are known to spend a significant amount of time floating at the surface (or “logging”), which may also provide rest. But the porpoises do NOT drift in the vertical orientation …. so why do the sperm whales?
That’s where the physics comes in! Miller and his team observed that even when the whale started a drift dive facing downwards, the tail would slowly sink until the whale had passively reoriented into an upright position. This slow-mo flip is driven by buoyancy acting differently on different parts of the whale. The tissue, which is more dense than seawater, is located more towards the back end: this part sinks. The less-dense oils inside the large spermaceti organ and the air in the respiratory system tend to be more towards the front, so this part floats. Don’t believe me? You can do the experiment yourself at home! Behold, my homemade Play-doh whales!
No doubt your whales will look INFINITELY better than mine, but here’s all you need: some Play-doh, some small Styrofoam balls (or ping pong balls), and a tank of water. Whale A was shaped with a Styrofoam ball in its head, Whale B was shaped with a Styrofoam ball near its tail, and Whale C has no Styrofoam at all. I then placed each one in a tank of water, horizontally at the surface, and watch what happened (sound on please):
So, Whale A, with the less dense head, ends up vertically upright in the water, Whale B is vertically upside down in the water, and Whale C sinks since it doesn’t have enough buoyant forces to remain afloat at all, completely consistent with the sperm-whale buoyancy theory. The harbour porpoises presumably maintain their typical swimming orientation during their resting periods because they are more uniform in their density.
One more case to drive the buoyancy point home: northern elephant seals. A study published in 2010 by Yoko Mitani and collaborators observed that elephant seals begin their drift dives with a steep descent, propelling themselves quickly through the “danger zone” of 0 to ~150 m where their predators hunt. Once they get deep enough, they flatten out their dive, which increases their drag considerably. They also roll over, drifting slowly in the ‘belly up’ horizontal position for their rest, described in the paper as like a falling leaf. Mitani and team theorized that the thicker blubber layers on the belly compared to the back cause the passive roll to take place. This high-drag orientation prevents the seal from drifting downward too far, which would take more energy to return to the surface. (image from Mitani et al paper, Copyright ©2010 Royal Society Publishing)
So, belly fat for the win (at least if you’re an elephant seal)! The things we learn around our kitchen table!
1In case you’re wondering, sharks don’t sleep over extended periods like us, but they do have times of activity and times of rest.
Stereotypical resting behaviour of the sperm whale; P. J. O. Miller, K. Aoki, L. E. Rendell, M. Amano, Current Biology 2008 Vol 18 No 1, pgs R21-R23
Three-dimensional resting behaviour of northern elephant seals: drifting like a falling leaf; Y. Mitani, R. D. Andrews, K. Sato, A. Kato, Y. Naito, and D. P. Costa, Biology Letters 2010 Vol 6 pgs 163 to 166
Silent porpoise: potential sleeping behaviour identified in wild harbour porpoises; A. J. Wright, T. Akamatsu, K. N. Mouritsen, S. Sveegaard, R. Dietz, J. Teilmann, Animal Behaviour 2017 vol 133 pgs 211 to 222