What would be those defenses?
This critter takes fast-moving prey by diving at impressive speeds (timed at 243 mph). This one takes agile prey in the deep woods. Ten meters of altitude gets you away from terrestrial predators, but they are (as you note) hardly the only game in town.
Evolution favors designs that help the animal become more successful… either by avoiding predation better, finding and obtaining food better, or by reproducing in larger numbers. There is really nothing about a lighter-than-air balloon size that holds any advantage here. Most of the available non-oceanic food is on or about the ground or trees, and most predators happen to be in that area also. A lighter-than-air animal would not be able to take off quickly and would be easy prey for jumping or gliding animals. It would not be able to prey on airborne food any more threatening than an insect, and the agility of insects would make them very difficult for the inagile floater. And as mentioned above, a raptor would easily make mincemeat of a floating animal.
Comparing zeppelins to airplanes is really an inapt comparison. In animals, flying is basically a modification of gliding which is a modification of jumping which is a modification of walking, etc. This is not the case with flying machines, where humanity needed a way of getting great altitude for long-distance travelling as quickly as possible, and no internal-combustion engine was available. The two are really different problems.
Please. Their “prey” are sparrows and chipmunks.
Um… first off, ever tried to catch a squirrel lately? They’re wiley little bastards. Second, per the wiki link: “The Peregrine Falcon feeds almost exclusively on medium sized birds such as doves, waterfowl and songbirds.” If songbirds are food for a perigrine, the balloon bird stands zero chance.
I’m trying to imagine the defenses a balloon animal might have. Spines? Adds a tremendous amount of weight, and a predator just needs one little pinprick and the balloonist is toast.
The trouble is that any balloon animal is going to be very light and fragile compared to their size. But really small animals have other ways of floating…spiders can just release silk and be carried away. Insects can fly with tiny little wings. When you’re talking about really small creatures, the physics of flying is really different. It’s hard for me to imagine an evolutionary pathway from a small sluggy thing to a floating sluggy thing. And again, what does the critter eat? If it eats plants it’s better off attached to the substrate where the plants are, so it won’t be blown away from it’s food source. It it’s a predator, how does it catch its prey? With webs, like a spider?
Make it a couple of cell layers thick, with a distributed neural network rather than a localized central nervous system. The outer layer is normally very sticky, but can turn off its stickiness if need be. Maybe it’s mechanical, like the hairs on a gecko’s feet, or chemical, like spider silk glue. The inner layer secretes digestive enzymes. The middle layer has some limited contractile ability. As a whole, the thing is about as tough as a dry cleaning bag, and probably transparent.
It catches insects because they stick to its surface, which then puckers in and forms a blister where digestive enzymes start to break down the bug. Pitcher plants and venus fly traps capture bugs just fine without being able to chase them. The bug slurry is passed to the inside of the creature, where it runs down to pool in a nutrient-rich slime at the lowest point. Cilia on the inner face beat at this slime, gradually rolling the creature relative to the slime pool so as to expose all of the surface to the nutrients. The larger the bag, the larger the organism that can be captured. Maybe as they get bigger they develop tendrils to increase their surface area and the chance of capturing food. Perhaps a big bag can capture a small bird or a bat.
Bouyancy gas is produced as a byproduct of digestion. If the bag bumps into a non-food surface, the stickiness is reversed so as to release the creature. Tears or punctures of the gasbag are sealed rapidly both by muscular contraction and adhesion. If the bag is totally disrupted, the fragments can seal up and form new, smaller bubbles - although most fragments will probably die. Heck, throw in a life cycle where the thing can act like a slime mold; small fragments just ooze along and digest leaf litter until they are large enough to plump up and float. Floating may allow them to avoid predators, or expose them to a richer food source, or serve as a more effective way to disperse gametes, or whatever.
To make the thing unpalatable to predators; consider whether you would actively seek to eat a dry cleaning bag covered with glue that stank of decomposition. A bird attacking a gasbag would get its feathers fouled and have trouble flying. Or the bacteria inside the thing could be toxic, like in the mouth of a Komodo dragon. Small predators and parasites might find it worthwhile to eat such a thing, but a hawk sure wouldn’t bother.
Just because there are creatures today that could kill a beholder doesn’t mean one hasn’t evolved in the past. They wouldn’t necessarily show up in the fossil records. And there are plenty of creatures that inhabited the Earth that are now extinct. Many of those creatures don’t make much sense on the face of things.
Snakes and worms – why no limbs?
If there were no flying creatures, people would be “proving” that Earth “obviously” couldn’t possibly support animal flight.
Why would a weak hairless mammal with an oversized cranium evolve?
If we were a society of sentient beholders, what would you be telling the nutjobs with a theory that no beholders, much less sentient beholders could naturally evolve?
What I’m imagining is some animal whose main body is fairly small - maybe a meter or so across - with a large gasbag - 20 meters or so across. You need a large ratio of gasbag to main body mass. But on the plus side, the gasbag is essentially free size - it might consitute 99% of the creature’s volume but it’s only 1% of its mass.
Assuming the lift is provided by a lighter-than-air gas, once the gasbag is inflated it’s fairly low maintenance. Unlike a bird, the creature doesn’t need to put a lot of energy into flying. It can essentially drift with the wind for most of the time. Think of it as a giant inflatable sloth.
Poison. If it’s skin is as lethal as an poison arrow frog very little is going to try to prey on it.
I suggested a frog style tongue, earlier. I like brossa’s idea too.
If it’s like a slime mold as one of his suggestions, combine it with my poison idea and it can hunt by deflation. When a good sized animal walks under it, it can vent all it’s stored gas and fall on the animal, enveloping it. The poison and it’s decentralized nature will keep the animal from being able to really damage it; something liket that could be torn to shreds and reassemble itself anyway. It absorbs the dead animal, reinflates and drifts off to seek more prey.
If you’re indigestible, there’s no real need to fly above the reach of predators.
So animals are at risk only when the wind drifts a large gasbag directly overhead (with no vegetation intervening)? And when plunging from as little as 16 feet, it will take more than a second to “pounce,” during which maneuvering will be limited at best? Sounds rather easy to avoid being prey.
How often do you look up? What if the bottom of the beholder was the same color as the sky?
Seldom, because I am fortunate enough to have no airborne predators. Not all animals are so lucky.
Then it would cast a shadow.
The problem with a balloon-of-prey is the descent rate in either pouncing on prey, or ascending out of reach of predators. It would be extremely difficult to change buoyancy fast enough to attack or flee effectively. Dumping the gas might be relatively easy, but re-gassing would probably take hours, during which time the animal would be helpless.
There are so many problems with this approach that maybe the most convincing argument is to look at all the non-aquatic life in nature and see that there is apparently no evolutionary branch that even started down this path. It’s not that it’s totally inconceivable, it’s just that active flight is a less costly and more competitive solution no matter which way you look at it.
Plenty often enough to be aware of sizeable balloon-like critters hovering low overhead. And I imagine my vigilance would increase if these creatures actually existed and had acquired a reputation for being lethal.
It would still easily be visible within a few hundred feet as it drifted in on the wind.
Large, unmaneuverable, moves no faster than the wind - it’s hard to imagine a predator that would be easier to avoid.
I’m imaging this creature as a herbivore/insectovore. They might be slow but anyone can sneak up on a tree. And there are numerous animals and plants that attract insects to them rather than pursue the insects.
How nimble would this critter be? I’m thinking it might be surprisingly nimble. Squeezing the gas bag would compress the gas and cause descent. Relaxation causes ascent. Releasing gas propels it in any direction.
But not rapidly, unless the gas is under meaningful pressure and a significant amount is released (it’s an action-reaction thing). The critter is then lacking buoyancy and thus immobilized and vulnerable.