Let's say I have an aquatic humanoid, vaguely like a selkie. As creatures that spend a lot of time in water, it would be great if they can stay under water for "a while".
The human breath-holding record is over 20 minutes, but this a) is exceptional, and b) such feats typically involve a lot of preparation.
What changes / differences from humans are needed to allow these humanoids to routinely hold their breath for, say, 5-15 minutes, and to be able to do so without significant preparation? In particular, what of these changes would be "noticeable", either to a human observing such a creature, or to a human that was somehow magically transformed into such a creature?
- These creatures are mammals. Not birds, reptiles, insects, or plants. Not cyborgs. Not something with exotic anatomy and physiology. Preferably no nano-machines or other technological augmentations are involved. (I'm slightly less adverse to outright magic, but would still strongly prefer to avoid it.)
- They look superficially human except for their noses, which resemble the nose of a seal or sea lion. (A longer "snout" is also okay, though I don't expect it to matter.)
- They should be able to remain underwater for repeated stretches of at least five minutes after not more than thirty seconds of breathing (shorter is better). Ideally, they should also be able to stay under for 2-5 minutes after a single somewhat-deep breath if they've been above water for at least a minute or so beforehand.
- They only need to be able to do this in "warm" (~25°C) water. If it works in colder water too, that's great, but not necessary. If changing this requirement to 30°C helps, that's acceptable.
- Along with the last point, while they may have a bit more fat on average than a "healthy" human, it's well within e.g. American averages.
- They don't generally dive very deeply (only ~5m), so if the target "several minutes" is unattainable at greater depths / pressure, that's fine.
- This all needs to happen during moderate physical exertion (swimming, but not strenuously). More extreme conditions (e.g. panic, strenuous exertion) are allowed to significantly reduce the time they can stay under.
- Absent such extreme conditions (or being stupid about trying to push how long they can stay under), this should be about as dangerous for these creatures as similar feats are for dolphins and pinnipeds; i.e. minimally. Doing this day after day should not be significantly hazardous or lead to long-term health problems.
Basically, I'm looking for a humanoid that can dive, swim, and generally loiter underwater (in terms of time, anyway; mobility-wise they'd be essentially human) like a pinniped or a dolphin.
...and no, this question doesn't really help.
Good answers should:
- Give at least an overview of the physiological differences.
- Explore (at least briefly) what such a creature needs to do to prepare for a dive. (The goal is something like "take a few deep breaths".)
- Explore what such a creature would experience during a dive, particularly how (or if) the experience would differ from a human swimming underwater.