Dolphin Address 06
March 26th 2006
I would like to close-up on a phenomenon I have reported before concerning the dolphin. In my experience figuring out something apparently magic does not only result in the satisfaction that you gain insight, it also halts your curiosity. You managed to open a door, delighted in your ingenuity, patted yourself on the shoulder and turned elsewhere.
A number of times I was floating tête-à-tête with the dolphin on the surface, when suddenly, without moving a fin, she sank straight down for some two meter. I wondered how she does this and figured out the following with Archimedes in mind: A body submersed in water will gain buoyancy equal to the weight of the water displaced. I conjectured she must diminish her body volume, roughly comparable to holding in your belly. Then she displaces less water, reduces her buoyancy, and, her total body weight remaining equal, she sinks. I was happy to reach this conclusion, indulged a bit in my dolphin cleverness and left it at that. On occasion I checked my insight with the vet of the Duisburg Zoo and he refined my discovery: dolphins are able to muscularly compress the air in their lungs, thus reducing their body volume. Good, nice to be corroborated.
Recently I was occasioned to explain this phenomenon to a receptive mind. I drew a parallel to the way in which divers use their in- and deflatable stabjack. When you descend in water, the pressure will increase in depth. This will result on your total body volume, including your diving gear, being decreased, particularly the air bubble packed neoprene of your wetsuit. Here too the overall weight will remain equal, so buoyancy is reduced. To compensate for volume loss the diver can inflate his stabjack by letting in air from his tank. Now this is a bit tricky, because if you let in too much air, buoyancy will push you up. Not only that, when you go up, the water pressure will diminish, allowing your stabjack to push out a larger volume, making your ascend even faster. If you do not deflate some air from your stabjack you are going to have a ‘balloon ascend’. This is basically dangerous, as it can very well cause decompression sickness, depending on what depth you ascended from and how long you have been there. Many people even died from the sudden release of nitrogen, causing bubbles in the blood circulation and inflicting ‘bends, staggers and chokes.’
To wrap it up you can say that the dolphin has an in-built stabjack as she can manage her body volume with her lungs. This seems very clever, but mostly because we are proud to be able to understand this. To the dolphin this must be a routine that she applies when she needs it.
At the end of a dive, when a diver has used up most of the air in his tank, his overall weight has diminished by the weight of the air spent. For a 12 litre bottle this would be about 3 kilo. This weight loss adds to his buoyancy. If he has released all the air from his stabjack he can only diminish his body volume by breathing out his lung content and keep it this way as long as he can or breathe in and out very quickly to get his oxygen and lose his volume again. Especially in shallow water, where water pressure has its major diminutive effect it is pretty hard to remain submersed in this stage. Picking up a stone from the bottom can counter balance buoyancy, depending on its weight.
The dolphins overall specific weight, like ours, varies on either side of that of seawater, the difference between floating and sinking is made by her lung content and her ability to compress it. This you can’t really observe, since only a relative small volume of displacement does the job. Every litre of seawater weighs a little bit over a kilo, so if 3 kilo does it for a 100 kg diver, 6 would do it for a 200 kilo dolphin.
When you let this insight settle you realise that there is more than meets the eye in a dolphin swimming. Apart from her visible ‘handling’ of the water she must routinely incorporate adjustments to her buoyancy that you don’t see, but now you know.
I cannot diminish my body volume by compressing the air in my lungs, but I can by tucking in my belly. This I will try in the water, and if it works I do have a surprise for the dolphin next time we meet.
Jan Ploeg, WWR&DC, Fanore, March 26th 2006
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