I'd like to tip my hat to this week's fortnightly challenge with this question by revisiting an old one.

I asked a question about the Science behind a naturally invisible creature a while ago. At the time, I was handwaving the evolutionary concerns of my Sneaky Devils and looking only at the end result. From the answer to that question, I reengineered my Sneaky Devils so they constantly emitted a magnetic field specifically designed to interfere/disrupt the visual processing of nearby creatures (induced hemianopsia), but which has no effect on similarly developed creatures (they can't force each other to ignore the other).

Suppose that a predatory creature developed in this fashion. What would be the evolutionary concerns guiding its development?

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    $\begingroup$ I suppose the main evolutionary concern is why it's using that field to very precisely target an extremely tiny part of the target's brain in an extremely specific way instead of just targeting some other portion and possibly killing them outright. The described mechanism really has a bit too many moving parts which aren't useful on their own to be a likely candidate for evolution. $\endgroup$ – Saidoro Apr 20 '15 at 17:36
  • $\begingroup$ @Saidoro Feel free to make a case for artificial engineering as the creature's origin. I will weigh all answers with equal consideration. $\endgroup$ – Frostfyre Apr 20 '15 at 17:48

Assuming predator not parasite (ala vampire).

I would assume they would be similar to some animals that wait and ambush their prey when it gets close.

This could be like the big cats such as the Cougar or panther. spiders with their webs, some snakes.

Some of the big things that would be need to be known would be what type of prey is their normal meal? bugs, birds, small mammals, large mammals?

is it like a cat and kills it's prey outright or like a poisonous snake, bites/injects into its prey and waits for death?

I suspect since it is already working on being unobtrusive, that it would go with poison or a very fast lethal strike. Being invisible isn't the end all be all, there are still several things that the animal would have to contend with. Sound and smell being to large ones. Most predators have a strong musky scent, and with lack of vision, it's prey are going to start paying even more attention to smells.

Sound comes from stepping on things which would of course encourage the stealth mode of waiting until the prey comes within striking range.

Last, the prey might not be able to 'see' the predator, but the field it generates might be detectable or the prey might be able to recognize the 'blind spot' for what it is and be able to react to it.

As a waiting predator, it would be more likely to have a very large range of possible prey, since beggars can't be choosers and they would be more willing to kill and eat anything that came across it's path (including other predators). This would be good for it, since having only 1 or 2 species would encourage the prey to adapt to the predator faster, smelling it, hearing it, seeing where it is 'not' etc.

  • $\begingroup$ I had envisioned a variety of predators, with some being active hunters and others playing the waiting game. Good analysis. $\endgroup$ – Frostfyre Apr 20 '15 at 19:11
  • $\begingroup$ Invisible snakes...shudder. $\endgroup$ – Josiah Aug 12 '15 at 15:31

Considering that the predator is essentially imposing a form of mind control on its prey by forcing them to fail to see its presence, there is no reason that the predator cannot simply wipe all possible senses of the prey from detecting it (eg hearing, smell).

It is generally easier to shut down all independent volition entirely than to precisely target the sight of the animal. An example can be seen in the emerald cockroach wasp, which preys on cockroaches. The cockroach, once injected with the precision neurotoxin, is incapable of escaping or even moving independently, and has to be guided by the wasp into a burrow, where it will be devoured by the larvae of the wasp. In this case, the wasp only needs its prey to be alive, but not capable of independent action.

As already stated in the comments, it does not make sense that an animal with such an extreme degree of control over its prey would not be able to use the same abilities to directly target and kill/incapacitate its prey. Therefore, the only reason the predator is rendering itself invisible is that it needs its prey to be alive and conscious. Otherwise, there is no discernible evolutionary benefit to have such a form of mind control, when its mind control apparatus is powerful enough for direct killing or incapacitation.

Using mind control also solves the problem of the predators being unable to target conspecifics with their invisibility, as their mind control techniques cannot be used on species which also possess mind control.

An example comes to mind are the Thrintun from the Known Space series by Larry Niven.

The Thrintun evolved on a planet where most animal species possessed telepathic ability. Natural selection led to an environment wherein no individual species possessed great advantage over any other; defenseless off-world species, however, were easy prey. A single Thrint can manage a few dozen sentient beings simultaneously; with an amplifier helmet, the same Thrint can control an entire planet.

Thrintun referred to their telepathy as "The Power". In all their religions, The Power was self-evident of their manifest destiny to rule the galaxy. A loss or congenital lack of The Power is cause for exile, or more usually murder by the family, in their society. These unfortunates, called Ptavvs, are tattooed pink and are enslaved by other Thrintun.

Their success in spite of low intelligence and poor skill with tools was due to the great advantage of their telepathic abilities; they never suffered much selective pressure to improve what they had. A Thrint's slaves could be far more intelligent than their master, but intelligence provided no protection at all against enslavement.

In your case, the predators could be using their innate mind control capability to render themselves invisible to their prey, while at the same time enslaving their prey to do day-to-day tasks which the predators are themselves incapable of performing. The predators periodically cull the population of prey/slaves for food.

  • $\begingroup$ Wouldn't this require the predator in question to be intelligent enough to enslave a population? $\endgroup$ – Frostfyre Apr 20 '15 at 19:58
  • $\begingroup$ @Frostfyre Unless you can think of a better reason for them to keep the prey animals alive and conscious, there doesn't appear to be a scenario under which a predator would willingly keep its prey alive and conscious. $\endgroup$ – March Ho Apr 20 '15 at 20:00
  • $\begingroup$ Why don't bears kill all the salmon in the river? $\endgroup$ – Frostfyre Apr 20 '15 at 20:10
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    $\begingroup$ More significantly, there would be the question of why wasps don't kill caterpillars. $\endgroup$ – Saidoro Apr 20 '15 at 21:14
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    $\begingroup$ @Saidoro Parasitic wasps (on cockroaches and caterpillars) actually come very close to this. They inject their prey with neurotoxins which completely disable their prey, leaving them unable to move around of their own volition, but still alive, because they need their prey to be alive. The difference is that the predators which induce hemianopsia leave their prey completely conscious and capable of independent thought, which has no good explanation unless they need the prey to be capable of independent thought. $\endgroup$ – March Ho Apr 20 '15 at 21:19

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