A while back, I asked this question about the plausibility of giant floating whales and received an excellent and high-scoring answer from Dubukay demonstrating the unfeasibility of the idea. However, more recently I discovered some flawed assumption in the answer and wrote my own answer, demonstrating that, if we assumed a more reasonable gasbag skin thickness than Dubukay had, the idea became much more plausible. In my answer, I calculated that a creature with a mass of 500 kg (not counting the mass of the hydrogen) needed just over 400 kg for its gasbag. This leaves us with 100 kg for everything else.

Now, this flying gasbag will have to do more than just float around to survive. To find food, it will probably have to have some way to move around in the air. Blimps typically use propellers to do this, but propellers are unlikely to be viable for a biological creature for a variety of reasons. Aquatic creatures typically use some kind of flipper to move through the water, but because of the low density of air, the flippers would probably have to be impractically large. So the question is, what would be the best propulsion system for this type of flying creature?

The same way a nautilus swims around in water: jet propulsion. Nautiluses move using a hyponome, which expands to pull in water from the sides of the nautilus, and contracts to expel a jet of water.

The bio-blimps can majestically wheeze across the land using what is essentially an organic bellows, just like the nautilus. Whenever it wants to move, it expands a bladder to pull in air from broad vents on the side of its body, and then compresses the bladder to expel the air through a much smaller vent to propel itself.

Presumably they already have various valves and sphincters and what not for maintaining their main gasbag, so adding one smaller gasbag for locomotion should a breeze.

One alternative method of propulsion that could be considered is: Basically having none at all...

Consider a symbiotic system where your 'bio-blimps' are more 'giant floating green houses' - Main creature feeds off light algae/lichen-like secondary lifeforms, which the main creature effectively cultivates as a means to gather energy from the sun by funneling rainwater to them.

Further input could come from birds who nest in/on the bio-blimp, depositing the left overs from meals they collect from elsewhere, which in turns feeds the secondary bio-mass.

Bio-blimp's survival is no longer tied to it being able to navigate toward food, as its food source naturally grows or comes to it, and it is free to drift on the winds while adjusting its lifting bladders to maintain a comfortable altitude.

Hydrogen Rockets

Consider that your gasbag is full of hydrogen. It would be entirely possible for it to take some of its excess hydrogen and allow it to escape in a direction of its choosing. It could then generate a spark and move because of the explosion (the valve would have to be stronger than the surrounding skin, or course).

I don't think this is a practical solution, but it seems negligent to not mention it.

Don't Fly, Fall with Style!

Ok, so it's going to involve flying too, but...

Your creature is going to have some way to control its buoyancy. Presumably muscles that change the shape slightly, thus changing the amount of air displaced, thus changing the buoyancy. So use this ability to your advantage.

The creature grows small wings or flippers. Not nearly enough to lift the creature, but enough to generate some lift. Now the creature can control it's flight by trading height for speed.

Your creature moves by:

1. Expanding to increase buoyancy and increase altitude


2. Contracting significantly, triggering a "fall"


3. Using its "wings" to control the fall


4. Expanding again before it hits the ground

Effectively, your creature copies the locomotion of birds of prey, only instead of using thermals to gain height, it uses buoyancy. The lift from the wings will have both horizontal and vertical components, so some of the energy will be converted into making the dive longer, and some of it will go into moving your creature towards its objective.

So your sky whales gracefully float upwards before suddenly diving in whatever direction they want to go.

For obvious reasons, wings are by far the best propulsion system as far as animals go.

Since that seems to not be possible due to that whale's anatomy however, and since we've already excluded flippers, I can think of two possible means of locomotion :

• Jet propulsion : Such as the one squids use underwater by quickly ejecting water out of a valve, allowing them to reach great speeds. However due to the low density of air, jet propulsion would likely be very weak in terms of acceleration unless unrealistic amounts of pressure are applied. If you are thinking of a slower type of animal though that would be an idea.




• No locomotion at all ! Some animals simply travel by drifting, such as jellyfish. Of course in the air that is not really an option, since food would probably be scarce thus depriving you of the luxury to drift aimlessly just waiting for food to land into your mouth.

In conclusion, the best option then would be a combination of both proposals, that is aerial whales that mainly navigate the wind currents by letting themselves drift along the currents, with the ability to (slightly) steer using jet propulsion. That way no enormous amounts of jet pressure would be needed, thus staying in the realm of plausibility.

I think an animal like that would have the best chance if it mostly relied on:

• Following winds at different altitudes like a hot air balloon


• Having an omnivorous diet & not chasing any quick moving prey. Their diet could consist of nectar & tree top fruits, along w/ insects & maybe occasionally birds or squirrels


• Instead of chasing prey, relying entirely on passive mechanisms. One possibility would be an organ that charges in the sunlight to glow as an insect lure at night.