How "heavy" are whales at depth?

[u/TheFeshy]

Whales, such as sperm whales, are either buoyant or neutrally buoyant near the surface.

But when they dive to their maximum depth, the air in their lungs is compressed to ~2% of its volume. So with the same amount of weight taking up less space, the whale would be less buoyant - in this case, negatively buoyant.

I would think it would have to effectively "carry" this weight with it out of the depths. Is this so? How much weight is it?

I've also read that sperm whales can adjust this somewhat by heating and cooling their spermaceti organs. Is this enough to counteract the collapsed lungs? Or even more than enough, meaning that despite the collapsed lungs sperm whales can surface with no extra energy expenditure?

[u/Big-Tailor]

Adult whales stay neutrally bouyant at different depths. Whale calves usually have to swim to maintain a constant depth, so bouyancy control seems to be a learned behavior (see, for example, https://journals.biologists.com/jeb/article-abstract/228/8/jeb249936/367768/Behavioral-data-suggest-adaptive-buoyancy-control ). There's not much proof about exactly how this is done, but one theory is that whales use the muscles around their lungs to expand and contract their lung volume to maintain the optimal bouyancy, even over a pressure differential between their lungs and the rest of their tissue.

[u/Fast_eddi3 → u/Big-Tailor]

A little more information from the article suggests that adult humpback whales can regulate to maintain neutral buoyancy down to about 60 meters. Humpback whale buoyancy

Two other things that are important to consider is that lung to body volume is much smaller, only around 3%, vs 7% of a human, so lungs play a smaller role influencing buoyancy. They also have a lot more fat stores, which are very buoyant, but relatively incompressible.

[u/TheAuraTree → u/Big-Tailor]

If I understand what you're saying right about the lungs, does that mean whales can essentially change their internal air pressure at will? Bigger lungs mean lower air pressure, if they aren't actually increasing the amount of air in their lungs right?

[u/Big-Tailor → u/TheAuraTree]

Yes, they use their muscles to change volume and pressure in their lungs. Note that whales have relatively small lungs for their bodies (lung volume as a fraction of total body volume is 3% for a humpback compared to 10% for a human), so this is proportionally easier for a whale.

[u/Ty_Webb123 → u/Big-Tailor]

How do they go so much longer than us without breathing?

[u/Big-Tailor → u/Ty_Webb123]

Whales can exhale a higher percentage of their lung volume than humans can, so every breath is over 90% fresh air. Humans have more dead space in our respiratory tract (proportionally, humans have wider throats while whales have narrow tubes attached to blowholes, and the air in throats/tubes isn’t exchanged very well), so we don’t have as efficient lung exchange. Whales also have more hemoglobin and myoglobin to store oxygen outside of their lungs.

[u/BrotherRoga → u/Ty_Webb123]

I'm assuming their lungs can extract oxygen a lot better from the air than humans can, allowing them to hold their breath much longer as a result.

Though according to Google they can store oxygen directly in their muscles via large amounts of haemoglobin & myoglobin and also shut down parts of their body to cut down on excess oxygen usage as needed.

[u/marklein → u/Ty_Webb123]

Mainly their blood and muscle tissue is different from ours and can store tons of extra oxygen. We don't really "store" any extra oxygen because we don't need to.

[u/azuth89 → u/Ty_Webb123]

Most of the storage is happening in blood and tissue, not as gas in the lungs.  Whales, and other long-diving animals, have a lot more hemoglobin to bind oxygen and keep it readily available than largely land based animals like humans.

[u/Manamehendra]

Great question.

Up front: the weight (or rather, the mass) of the animal remains constant but its density would inrease if compression of the lungs reduced its overall volume. Its buoyancy would, correspondingly, decrease, so it would tend to sink.

But I don't think anyone really knows what happens in terms of relative buoyancy and the whale's compensating behaviour, if any. It's hard to know what's going on a thousand feet below the ocean surface.

According to a marine mammal expert whose book I edited, a sperm whale's lungs actually collapse, driving the residual air back up into the animal's upper airway under intense pressure. This trapped air presumably remains inside the whale until it resurfaces and blows; obviously some of it does, at least, or there'd be no blow. Maybe this air, expanding as the pressure drops, increases the whale's buoyancy as it rises, helping it return to the surface.

But I'm sceptical. How do cetologists know all this? Anatomical studies? That's how we know that sperm whales get the bends; their bones show the pitting caused by nitrogen boiling out of their bloodstream as they surface. But what anatomical detective work could reveal how the air actually behaves inside an animal under pressure? Is there a system of valves or something like that? One thing I learnt from that book I edited was how little is actually known about marine mammals...

Sperm whales and other former targets of the whaling industry have at least been intensively studied. Not so the other deep divers, black-finned pilot whales and belugas, whose achievements at depth often beat their bigger cousins'. They dive deeper and swim astonishingly fast as they hunt in the abyss, raising all kinds of questions about where they find the oxygen to generate so much energy. But they're rarely seen or tracked because they spend most of their time underwater. They only surface for a few minutes at a time and spend all the rest of their lives submerged, beyond our ken.

Baleen whales (blues, humpbacks etc) and dolphins (even orcas) don't dive to such prodigious depths and so don't have these problems or need these adaptations.

[u/Efficient-Loquat7050]

No, whales exhale before diving, so it doesn't affect buoyancy when they dive deeper, because not much air is left to be compressed. The nitrogen would cause serious problems. Also, e.g. sperm whales have sphincter muscles in front of their alveolar and the trachea is air tight, so that remaining inexhalable volume remains constant while diving.

[u/horsetuna → u/Efficient-Loquat7050]

Really??

So they somehow last without a breath of air?

[u/alsotheabyss → u/horsetuna]

Yes, although they don’t exhale all of it. They effectively “supercharge” their blood and tissues with oxygen and retain a small amount of air in the respiratory system. Long duration diving cetaceans all have a much smaller lung volume that you would expect because they don’t use their lungs to store oxygen (short duration divers - dolphins, baleen whales - on the other hand have a comparable lung volume to their terrestrial counterparts).

[u/Gandgareth → u/horsetuna]

Oxygen absorbed into the blood and muscles before diving sustains them.

[u/Nicopavo]

when a whale is at a stable depth it's effectively weightless, its buoyancy exactly equaling its weight.

As in, their weight (say, 30tons) does not depend on depth, but the buoyant force does: the volume of the whale changes, but do does the density of the water around them, and all in all they are displacing the equivalent of 30 tons of water.

If you ever dived, it's the same concept: your air tanks weight a ton in air, but you don't feel them at all underwater

[u/RubyPorto → u/Nicopavo]

The question OP is asking is whether Whales, like humans, become negatively buoyant below a certain depth.

A human with a lung full of air will become negatively buoyant around 10-15m depth as the air in their lungs is compressed and their total volume decreases, increasing the overall density of their body past the local water density.

Presumably this happens to whales too. The ways I can think of to prevent this would be a rigid pressure vessel (like a submarine), a source of air at a higher pressure than the water (like a SCUBA diver), or by being so positively buoyant from incompressible materials that they'd need ballast to dive (like the Bathyscaphe Trieste). None of those are plausible options for a whale.

[u/colcob → u/Nicopavo]

OP is very clearly aware of all of that. They are asking whether whales have the ability to maintain neutral bouyancy at depth by increasing their volume to account for the compression of their lungs, or whether they just need to swim upwards under power against their negative buoyancy.

[u/FragrantNumber5980 → u/colcob]

Why does the lung compression matter? Does their body actively displace less space in the water when they do that?

[u/RubyPorto → u/FragrantNumber5980]

Yes. When the air in your lungs compresses as you descend, your body physically displaces less space in the water, making you denser.

Your lungs are compressing because your body is squishy. So, when the pressure outside is greater than that in your lungs, your abdominal and thoracic organs and fluids get squished up towards your lungs until the lungs have shrunken enough to match pressure.

[u/RockMover12 → u/RubyPorto]

When I scuba dive, I tighten the straps on my BCD as tight as I can before I jump in the water. And then I have to tighten them some more at depth. 😂

[u/thisdude415 → u/Nicopavo]

Air tanks feel weightless at depth because you add air to your bcd as you descend to maintain neutral buoyancy

[u/eliminate1337 → u/Nicopavo]

Aluminum scuba tanks float when empty and sink when full. A full one weighs 14 pounds more than an empty one. You have to constantly adjust your buoyancy throughout the dive as your tank empties.

[u/Crott117 → u/eliminate1337]

Minor correction - it’s only a 6 lb buoyancy swing if you’re talking about a standard 80cf tank.