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I'm looking for biological explanations for some traits I want to give to an alien creature.

Description of the Creature

It looks like a five foot tall praying mantis, or perhaps a centaur. It has six limbs: Two big hind legs bent like a grasshopper's, along with two cat-like legs, on its lower body; and two forelimbs with huge horn-like claws coming from its upper body. It is covered in armour plates, and it runs freakishly fast (probably in the pattern of a cheetah). The creature also has three eyes on its head (two major and one minor), and two or more light-sensitive spots or demi-eyes on its torso that give it great situational and physical awareness. It is a herbivore and primarily a flight creature. It has some intelligence and tool use.

Environment: The creature lives in a temperate area of its planet, where the vast majority of creatures use toxins for defence and hunting (stingers, spit, squirters, even breathers). The armour is to prevent creatures injecting it with poison, and the speed is outrun various other predators which sometimes use enveloping tactics.

The question:

There are various questions I have for it, but the one I'll focus on for now is: I was thinking it would have a very stable upper body when it is running.

Later on, people attach some guns to its foreclaws, and find the creature is very good at running and shooting with them. But I wasn't sure why and how is the best justification for this upper body stability while running.

Possible Explanation

My first thought: It has very strong muscles up its back, and possibly air pockets particularly where the lower and upper body meet. These act as a shock absorber, and the creature keeps itself stable even while sprinting wildly.

The question being, why so much trouble to keep itself upright? An idea I considered, was if the creature was able to throw rocks with its claws. The idea is, its claw is the shape of a crescent. The creature holds the stone at the base of its claw, then as it lets go of the rock, it swings its arm up and forward. The stone is accelerated along the crescent, coming off at the end and speeding towards the target (if done while the creature is sprinting at over 40mph, that rock is going to pack a lot of punch). The claw may need a shallow indentation along the inside of it, so that the rock doesn't slip out of the claw.

That motion seems really tricky to pull off, but it's possible the creature, having claws all its life and practicing it, may eventually pull it off consistently. I thought I recalled some weapon that worked like this, but I cannot find it. The creature could use this to ward off predators and other threats/rivals from a safe distance, without risking damaging its claws (which it needs to cut trees and drink their sap).

. . .

This could explain the need for a stable firing platform, and its skill with firearms when they are introduced to its world, perhaps?

I would be interested in correcting this explanation, or hearing an alternate one.

Additional Creature Information

Here is some optional information about the creature not directly relevant for the question I asked, as it may be helpful. I'm quoting my comments form a conversation from a kind member who gave a detailed reply to my creature concept.

It seems it was necessary to clarify. The creature looks like an insect, but it isn't actually an insect. The forelimbs aren't really that heavy. The large claws are probably about a 15 inches long, so they won't be heavier than a bull's horns. The arms themselves aren't that special. The upright body gives the creature some forward weight, which might be a concern to gait. The grasshopper legs were intended for a powerful, sudden start to the creatures run, with the legs being so nimble they could get back into position for another spring in time with the front legs.

Potentially more importantly, part of the reason for the grasshopper legs was so the creature could scuttle while lying prone, exposing only its weird knees in the process. This is important for tactical reasons at one point in the story. Now, reverse grasshopper front legs may work as you suggest, and may well be a capital idea. I thought it may not be necessary, but the creature is meant to run like a devil so it may be just as well.

The tool use isn't anything to write home about. They throw rocks, and use sticks, and perform various acts similar to monkeys and crows. They can do more than would be expected with the claws, but they are still highly limited in applying their already limited intelligence. Its claws are like horns, they're only sharp near the tips. In the creature's case, they are important to its survival, it will have to rely on others of its species (who won't want it hanging around) to eat if its claws are broken. Also, they just aren't that aggressive, throwing rocks from a distance suits them better.

I can add the information. Didn't know how deep to go into the creature's physiology. The creature only cuts through a thick layer of bark to get at the sap of the trees, it doesn't cut trees down. Also, on the rocky throwing thing, it's mostly my idea for how they have these traits that become useful to the story, other alternatives are welcome possibilities.

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    $\begingroup$ "Two big hind legs bent like a grasshopper's ... and it runs freakishly fast (probably in the pattern of a cheetah)." How fast have you seen a grasshopper run? And how many five foot tall creatures have you seen with grasshopper-like legs? I think you're going to have some inherent problems here... $\endgroup$ – Azuaron Oct 20 '16 at 1:11
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    $\begingroup$ @Azuaron I would cut them some slack, we've never seen an alien but half of the posts on this site are based on the idea of extraterrestrial life; just because you haven't seen something, doesn't mean it cannot exist, and doesn't mean this person deserves a downvote. $\endgroup$ – Zxyrra Oct 20 '16 at 1:27
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    $\begingroup$ @Zxyrra Alien existence is probable. Based on things we have seen on Earth, and things we can speculate based on chemistry (silicon-based life), we can make guesses about those aliens. Now, think about something with grasshopper legs sprinting like a cheetah. Think about how its legs would have to move, versus how its legs could actually move. Then think about the amount of force that would put on the knee (the knee would snap). You can't just ignore basic biomechanics because we're talking about an alien and call it good. $\endgroup$ – Azuaron Oct 20 '16 at 1:41
  • $\begingroup$ @Azuaron I am just saying that it is plausible, not that it will occur. See my answer for an explanation of possible movement $\endgroup$ – Zxyrra Oct 20 '16 at 2:02
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    $\begingroup$ The grasshopper-style leg evolved for jumping, not running. The musculature and joint position is suited to quickly kicking backwards, not stepping forward. $\endgroup$ – rek Oct 20 '16 at 4:25
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Let's dissect what you've written. Even though this will stray outside the "question", several other problems could arise from a creature like this. You wanted a trait explanation, so I'll give you a full one :)

Two big hind legs bent like a grasshopper's, along with two cat-like legs, on its lower body; and two forelimbs with huge horn-like claws coming from its upper body.

  • The grasshopper legs seem plausible, and they could evolve with relative ease. Sounds good.

  • Your organism may be prone to falling forward if it has huge legs in the back and smaller legs in the middle; it would help when it started running, but it would hinder the running motion itself to always fall forward

  • This could be solved by swapping the cat legs for multiple sets of legs. Some pictures might give you ideas

It is covered in armour plates, and it runs freakishly fast (probably in the pattern of a cheetah)

  • Armour plates seem realistic for an insect-like organism. Sounds good

  • The cheetah pattern with over four legs, as I suggested above for weight distribution, would not work. There are two solutions to this: A) The organism scuttles very quickly with its six to eight limbs, which may be opposed to the hind limbs for locomotion. This is not a cheetah pattern, but it can still be fast, and it uses the most realistic body arrangement. B) Scrap the extra limbs, and replace your original catlike limbs with limbs of equal power and size to the back ones. Make them opposed to the back so it can run similarly to a cheetah.

  • Either way, while your creature will be able to run at decent speeds, hopping may also be plausible. Grasshopper legs may still be acceptable for leaping large distances.

The creature also has three eyes on its head ... It is a(n) herbivore and primarily a flight creature. It has some intelligence and tool use.

  • This all sounds fine, and the grasshopper legs could help it escape predators.

Environment sounds fine

Possible Explanation has room for discussion

  • Strong muscles would help, but shock-absorbers would only matter for the cheetah style of locomotion. If you opt for adding more limbs and have your creatures scuttle, absorbers will not be needed

  • You mentioned that your organism prefers to flee, and it's built to flee pretty well, so the whole rock-throwing idea is probably not necessary for its evolution. Still, it's possible.

  • Added: Additionally, rock throwing at a high speed may not be easy with a claw designed to puncture trees, i.e. serrated edges would inhibit that motion. You could consider two claws of different sizes and functions, one specifically designed for projectiles and the other for breaking plant matter. Some crabs in real life have evolved non-symmetrical claws which would support this evolution.

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    $\begingroup$ My answer has been thoroughly edited and should work with this new information now. $\endgroup$ – Zxyrra Oct 20 '16 at 4:03
  • $\begingroup$ One thought I'm having. Shouldn't the length of the legs equal out, with the front and middle legs? The hind legs are bent like a grasshoppers, so the creature's body should be level even though the back legs are longer. Still, the idea of two such pairs of legs is pretty interesting to me, so it may be better to continue with that. $\endgroup$ – J. Doe Oct 20 '16 at 4:06
  • $\begingroup$ @J.Doe The height from the ground should equal while standing but while moving there would be a big difference in height between the back legs extended and the front legs extended $\endgroup$ – Zxyrra Oct 20 '16 at 4:15
  • $\begingroup$ I had thought that when running, the legs would extend out at a 45 degree angle, so that the height of the legs would remain constant? $\endgroup$ – J. Doe Oct 20 '16 at 4:40
  • $\begingroup$ @J.Doe Possible but complicated / tricky for the organism $\endgroup$ – Zxyrra Oct 20 '16 at 6:09
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As always, please read On Being the Right Size by J. B. S. Haldane.

Here's a cheetah.

Here's a pronghorn, the third fastest land animal.

Here's a springbok, the fourth fastest land animal, running from a cheetah. Also includes a spectacular skeletal view of the running cheetah.

Number two is the ostirich, but we're talking about running on four limbs, not two.

Examine all of their gaits. Despite the pronghorn and springbok being hooved with narrow legs and the cheetah being a large cat with thicker legs and powerful paws, they all have the same running gait: front legs move mostly together, back legs move mostly together. Stretch everything way out, pull the ground with the front legs while bringing the back legs forward, then push off with the back legs while stretching the front legs forward. The whole idea is to provide continuous running force so that they never slow down despite the incredible air resistance they experience. In the springbok-cheetah video, pay particular attention to how the cheetah's spine moves; it's not running with just its legs, it's running with the force of its entire body.

Here's some grasshopper legs in glorious slow motion action.

Swapping out the front legs for cat-like legs is certainly a move in the right direction, but only if the second move is to swap out those back legs as well. You can't run like a cheetah with jumping-designed back legs. The back legs would have these gigantic pushes which would make the front legs either:

  1. Take many steps to the back legs' single push, which would ruin your ability to have full-body running and thus slow you down.
  2. Take one step with the back legs' single push, essentially making it a jumping-then-landing motion, which would make the front legs bear the brunt of the landing, which goes completely against a fast forward velocity. Additionally, you lose the continuous running force and lose a lot of velocity to air resistance while your legs are off the ground.

And this is your smallest problem. Let's talk about knees.

Grasshopper legs move in kind of a special way (bottom left picture). This is entirely due to their shape driving their muscular requirements. If you tried to make a human-shaped knee withstand walking pressure while bent like a grasshoppers, it would put all the pressure on the tendons. As anyone with a torn ACL can tell you, you don't want to put all the pressure on the tendons. So, you're stuck with a pin joint knee.

The problem with pin joint knees are that they put all the pressure on the pin. And pins are pretty small relative to the rest of the creature. This matters less in insects (which are full of pin joints) because of the square-cube law. Basically, as you increase in size, your pin is going to have its cross-sectional surface area squared, but the mass acting on it is going to be cubed. This kind of exponential imbalance is going to start shearing your pin before you get above house cat size.

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  • $\begingroup$ Indeed, I have noticed that in many fast animals, but not in the horse. I figured the creature would need to have a gait similar to this, to make it truly fast. You bring up some real challenges in the design, thank you for that. The obvious part is that the you would need enough muscle in the legs to pull them back in time to maintain the gait. I was considering you could do some of this with the torso muscles, the way the cheetah curls its body as the back legs come in. If this creature can exaggerate that motion, it may be able to have more muscle for the kickoff in the legs. $\endgroup$ – J. Doe Oct 20 '16 at 17:35
  • $\begingroup$ Something I should mention, is I was considering using ostrich legs as an example, that it may be a bit like a quadruped ostrich with a cheetah's torso. If you imagine large ostrich legs bent tight, their "knees?" sticking up in the air, it looks like a grasshopper's legs (to me, at least). And those legs, similarly, are very powerful. Some have pointed out it may be better to have the front legs be similar, however, which may be wise in this case. Either way, as you point out, giving the creature joints something more like an ostrich than a cricket seems wise. A pin is too little for this. $\endgroup$ – J. Doe Oct 20 '16 at 17:40
  • $\begingroup$ Thank you again for the well thought out post on the creature's running. I wish I could give you the green tick as well. $\endgroup$ – J. Doe Oct 20 '16 at 17:41
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Surprising that this creature would be a herbivore. Large front claws are typically a predatory adaptation (this creature's design overall seems like a cross between a cat and a praying mantis, both of which are predators) The ability to run fast as well as having long claws that it can use independently of running seems like it would be an excellent design for a predator that chases down prey while slashing at it. If it was a herbivore, I would expect it to be one that fights its predators head-on using its hands, rather than running away.

It is possible that these creatures use their front claws for fighting among each other for social dominance. Lots of four-legged animals rise up on their back legs to give them additional weapons when fighting (bears, horses, gorillas) and having an upright body already can make this easier.

There are other uses for long claws though. Some prehistoric semi-bipedal herbivorous mammals had claws that they probably used to break branches or strip bark off of trees. This is the inspiration I would go with: your creatures are (or were) tree-browsers. The reason why they have an upright torso is to grab the best branches from the top of the trees.

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    $\begingroup$ Certainly, it wouldn't be a bad trait for a predator, except they would risk breaking their claws every hunt. Similarly, if it defends itself, it risks injury (compromising its fast speed), and injury can mean death when so many predators use venoms and poisons, and it risks breaking its claws necessary for eating in defence. As you say though, I expect there is some grappling and competition with those claws in the breeding season. I could add fruit to their diet, they could probably crush that up with their claws and drink it like the sap. Good answer, thanks! $\endgroup$ – J. Doe Oct 20 '16 at 17:26

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