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Whales are majestic creatures of the sea, as they glide more gracefully through the water than their size should seemingly allow.
What kind of evolutionary path would it take for whales to glide gracefully though the skies?
Would it be possible on our planet, or would other conditions need to exist?
I have my own thoughts, but I'd love to hear others.
Edit:
It's worth noting that whales come in all sizes, so it doesn't have to be blue whale sized.
You can have a whale shaped creature that floats in the air through lighter-than-air gas in their body and less internal heavy structure that the traditional whale has.
The square-cube law limits the size of creatures in the air and on the ground. The buoyancy of water allows whales and other creatures to be much larger, since the musculoskeletal structures of the animals don't have to fight as much gravity.
When you're moving from the ocean to the air, you lose that benefit.
The answer about flying plants here states that you could use hydrogen as opposed to helium in order to provide lift to the animal in question. There isn't enough helium in the air on our planet, but you could imagine a planet with other mixtures of gas.
For a whale, you need to lose a ton of mass. What you're left with essentially is no longer a "whale," it simply has the external shape and surface area of a whale but internally it's an entirely different animal. You need a reason for the "whale" to be in the air instead of the water.
Breeding would take place in the same way where the creatures mate belly to belly and ejaculate in the air, since there isn't really any physics against that. Your creature's floating ability does need to be able to support the weight of its sperm, eggs, and organs.
For food, you have to give the creatures reason to be up there to begin with. Perhaps the planet has massive amounts of biomass or tiny creatures (perhaps other living gasbags!) in the air that the entity ingests in some form. It would be the skies' version of krill.
Addendum, with an idea thanks to ckersch:
With enough flora and incredibly tall trees, the floaters could easily be herbivores. There's a limit imposed by physics and biology on how tall flora can actually grow, but for all observers down below, "in the skies" as a metaphor would be a sufficient description of the life above.
Dragon Giraffes.
Stick with me on this, I'll explain.
Picture a herbivore, similar to giraffes, with a long head and neck designed to eat the leaves from trees. As a defensive mechanism it develops bladders filled with hydrogen that it uses to breathe fire at anything that attacks it.
Over time the bladders expand and it starts using them to support some of its weight, allowing it to reach higher and higher into the trees without needing such long legs, they actually walk on their hind legs and use their front legs to stabilize themselves while hydrogen bladders extending from their back and neck support much of their weight.
They evolve over time to reduce their weight and can control the lift from their hydrogen bladders by contracting or expanding them. Eventually they complete the transition to becoming airborne, and in the process avoiding all predators that might attack them. They drift on the winds much of the time using modified tails and fore-legs as wings to maneuver and grazing from the tops of trees, their long necks now being used to reach down rather than up. They can land and do so if the wind becomes too strong but they spend most of their time drifting at fairly low altitude and grazing from the tree-tops beneath.
If it were me, I wouldn't start with whales, but with arboreal creatures like flying squirrels. They can actually stay aloft quite a while, depending on the winds.
If there were some kind of selective advantage to staying aloft longer ... well that's probably how birds initially evolved. But say instead of developing wings to keep a loft a bit longer, they instead hit on producing and saving lighter-than air stomach gasses (most likely hydrogen).
Now if there's some good reason to be aloft, and you don't have to be terribly fast to catch your food (like raptors do), then there's no reason why a body set up this way couldn't be sized up drastically. There's a certain amount of "overhead" weight for internal organs, but as you size the body up you'd have a higher and higher percentage of volume devoted to holding the lighter-than-air gasses, and could also scoop up greater amounts of food in one pass.
So what I'd do at this point is also design a prey for these creatures; some kind of airborne equivalent to krill and/or plankton. If they stick to hovering just off the ground then pollen and/or insects might foot the bill. For higher up, you'd have to invent those creatures too.
While there would have to be some pretty massive musculoskeletal changes needed to make something like that feasible on Earth, such changes actually become much easier whenever you alter the density of the air in a world. For example, Titan (one of Saturn's moons) contains air about 4 times the density of Earth. On such a planet, even a human could achieve flight with some sort of wing-like apparatus. Pedal powered aircraft would even be possible.
XKCD did a great "What If" on the topic (posed with respect to the ability to fly on various planets).
So this is my thoughts on it, but I really wanted to see what other people came up with.
I will not be choosing this as the answer.
Ok, So an adult Dwarf Sperm Whale weighs 550lb. Hydrogen has a lifting capacity of 68lb per 1000 cubic feet. So you'd need a bit over 8000 cubic feet of hydrogen to lift a Dwarf Sperm Whale.
Now if a whale could evolve to a life in the air... Hollow bones. No blubber reserves... maybe replace them with gas bags. Symbiotic algae bio-reactor to make the hydrogen. Larger fins to maneuver in the air.
Maybe thicken the atmosphere as Colt McCormack suggested.
You could get to where you don't need as much hydrogen to lift it.
Regarding the evolutionary path of these creatures (since that was part of the question), it could be that they evolved in watery regions with many small rocky islands. The water itself may be filled with predators, which would encourage creatures to jump from rock to rock (or at least minimise the time in the water) rather than swim or wade.
Being able to make yourself lighter by first filling yourself with small gas bags (giving yourself a little bit of levity) would give an advantage to doing this - that would promote the beginning of the gas-filled creatures, and of course the more they evolved, the more efficient the gas bags became until they were eventually able to float entirely.
The creature probably uses electrolysis to get hydrogen from water. Ascending and descending could be a light muscular system which allows the creature to compress the gasbags, ironically making it more work to descend than to ascend.
The only problem is punctures (and the inflammability of hydrogen).
