I've been working on an alien species for some time that combines several traits I find interesting into a single being. This species is for use in a fantasy story, but I want it to have a hard science anatomical structure so that I could use it in future sci fi stories as well. I will bold the areas where I need the most help in to make it at least slightly scientifically viable.

Basic Description: A semi-aquatic arboreal species of sexapods with digitigrade back legs and four arms. Look reptilian but are probably closer to a dinosaur biologically.

Habitat: They originated in a dense jungle dotted with swamps and rivers. The trees are very tall and thick, and have a wide canopy that covers the watery ground floor. There are large patches of dry land, but most of the area is wet.

Evolutionary Path: They evolved from a fresh water ambush predator much like a crocodile that's main method of attack was leaping out of the water and dragging it's prey down with it's arms. It eventually started climbing and ended up filling a different sort of predatory niche involving traveling through the trees and dropping down on unsuspecting prey, grappling them into the water.

Advanced Description: This species, on average, stand to be 6'5" to 7' tall on their hind legs and have six limbs. Certain members of the species grow much larger than that, but that will be covered below. They have a very slender, elongated build that minimizes bulk in favor of a lighter weight and more efficient strength. A set of arms coming from the torso and a pair of digitigrade legs, along with a long, prehensile tail that is roughly the length of the full body from head to toe. The upper set of arms are slightly longer than the legs in proportion while the lower set of arms are closer to what humans would consider proportionate to it's size. It's body is covered in a thin layer of scaled, leathery skin that is highly flexible and smooth, much like the skin of a snake. The color of the scales can change, largely in response to the creature's mood though they have some conscious control over the changing color as well. Each limb ends in three digits (Though I am debating on the number of fingers and toes) ended in slightly curved claws with dull edges and sharp points. The head has a slightly pronounced jaw-line designed to allow for superior bite force, but not so pronounced that it can't support lips for better vocalization. I am also debating on giving them small conal horns that aid in channeling sound waves so compensate for not having proper ears. For hair they have long, very thin feathers that grow from their cranium like a crest. It would appear like hair to anyone looking at them from a distance, and culturally is treated much like how we'd treat our hair. Ideally the feathers are water resistant in some way so that they do not become a burden to the creature when submerged or in the rain.

Breathing: I've done a lot of research into why a four armed creature can't breath, but I didn't find many work arounds in it's place so I started looking through the animal kingdom. The best answer I could find were turtles. Like turtles this creature would likely need sheets of muscle tissue around their lungs that constantly expand and contract around the lungs, allowing the creature to breath despite the chest being contained and unable to expand with it, similar to a turtle shell. It isn't as energy efficient as normal breathing of course, but giving the creature generally more efficient lungs and blood to make oxygenating the muscles more efficient can compensate a bit. This does still leave it with a loss of upper back mobility though, and a thought I had that I can't find much information about was that it's solar plexus is broken into flat vertibrae similar to a second spine. This would theoretically make the creautre able to bend it's upper back at least slightly at the cost of a weaker chest. I am unsure how scientifically valid this concept it though. Naturally as a semi-aquatic species it's muscles are very efficient at storing oxygen for later use, allowing it to stay submerged for about an hour at a time. It may also be able to absorb oxygen from the water through it's butt, much like the beloved turtles I referenced earlier.

Body Temperature: I was divided between warm blooded and cold blooded for this species, but came to a middle ground. The premise being that they are endotherms with the capacity to lower their metabolism and shut down the ability to produce body heat during periods of time when intense physical activity isn't necessary. So when it's warm out and there isn't a lot of hunting to be done they would convert to an ectothermic lifestyle, but when it gets colder or food is desperately needed they could convert back to endothermic. There should probably be some sort of trade off for this, but I can't think of one aside from it maybe using up a lot of energy?

Substantial Growth: A unique attribute of the generally larger male half of the species is the capacity to grow indefinitely. Females may have this ability too but just don't for cultural reasons. Basically if one of these creatures is consistently overfed it will continue to grow larger like a snake in a large tank. In this way the normally 7' tall male could grow upward 10-12 feet tall, though at that point it wouldn't be able to stand bipedally anymore and would likely not be able to effectively climb, sacrificing it's ability to hunt for itself in favor of being large and in charge. Females and beta males would do the hunting while this giant alpha fought off other large predators. Fights between tribes would usually be settled by a fight between alphas, and in cases of all out war the smaller female could ride her mate into battle in the form of a cavalry archer. They spend most of their time in the water when they can so as to minimize energy usage between fights. Becoming this large takes years and is a long term goal for any male leader of a tribe. Being so large does shorten their lifespan however, much as it does in abnormally large humans. Males that don't want to become giant need only not over eat, as they can only get bigger by overfeeding. With a stable intake of food they won't grow far beyond the 7' tall limit. Alternatively I had considered frequent mating with multiple female partners to be what triggered the capacity to grow. A sort of hormonal shift. If this idea is even possible I'd like some input on which method of beginning the transformation makes the most sense.

Green Blood: Their blood is actually white, which I read somewhere is the most efficient in terms of oxygen transportation. However it also has a high concentration of biliverdin that gives it a green pigment, much like a specific group of skinks on Earth. It doesn't have to be biliverdin though, and if someone could tell me a better way to get green blood that would be very nice.

Can use a Bow and Arrow: I kind of fell in love with the idea of a four armed archer. Not to hold and fire two bows at once or anything, but simply for the ease at which it could reload and fire rapidly. I know the claws would be a hindrance, but perhaps an arrow grip where the claws are used to hold and notch the arrow, then pull back the string? If need be they could cover their string claws with some sort of padding. Or just a pinch draw.

So there it is. There's more to this species than what I've listed but I'm already worried this is too long. So is the described creature scientifically viable?

  • $\begingroup$ Just from what you have writen, I would say yes it is viable. Also the reason a strong chest is better in vertebrates it that it protects vital organs (Heart, Lungs etc) so you would want a stronger chest over a more flexable back $\endgroup$
    – Mr.Burns
    Jul 5, 2016 at 8:55
  • $\begingroup$ Just noticed, Welcome to the site, Great first quest as well $\endgroup$
    – Mr.Burns
    Jul 5, 2016 at 8:56
  • $\begingroup$ I'm not sure why you think it can't breathe and have you considered the skeletal structure? You've not mentioned it. $\endgroup$
    – Separatrix
    Jul 5, 2016 at 9:03
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    $\begingroup$ @MetalJimmor, if you stop and think while you breathe you'll find you probably do most of your breathing with your diaphragm not your chest. You could also consider just fixing the skeleton so it can breathe, the middle limbs below the chest on something similar to a second pelvis for example. $\endgroup$
    – Separatrix
    Jul 5, 2016 at 9:17
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    $\begingroup$ @MetalJimmor, it sounds like someone was hooked on them being "arms" and therefore had to link to the upper chest. Make them independent on a pelvis like structure, larger hole, slightly rotated to allow better upward and lateral movement, you end up with two "abdomens" to arrange the internal organs in but that probably gives you space for a larger more effective digestive system and a longer spine relative to the human body. Most importantly, it can breathe. $\endgroup$
    – Separatrix
    Jul 5, 2016 at 9:35

5 Answers 5



You are overthinking it becasue you were misinformed. Four armed creatures would be able to breathe without issue, Only a mammalian breathing system needs a flexible chest. Dinosaurs had an absolutely rigid torso and could breathe just fine, one group used air sacs the other pulled on the lungs directly via tendons attached to the liver similar to modern crocodiles. Crocodiles are the closest to your body plan with a very rigid chest and a more flexible lumbar region, they have something in between, lungs partitioned to work like a pseudo-air-sac and muscles pulling directly on the rear of the lungs, again with a liver intermediary. The lung is stretched like a piston chamber, in fact a fairly rigid chest makes it work better. There is quite bit over variation in this across archosaurs, with some turning the pubis into lever to give even more pull. Quite frankly the mammalian breathing system is awful there is no need to copy it if you don't have to.

enter image description here


Don't bother with external ears, Unless you give them mammal like inner ears they won't get any benefit for it. it makes even less sense in a semiaquatic species. even semi-aquatic mammals drastically reduce or loose external ears. Most animals have really poor hearing compared to mammals, or course mammals have very poor color vison. so if you are basing them on dinosaurs they have simple hearing and very good vision.


You may want to go with sea snake like or crocodilian scales with a more pebbled texture, overlapping scales reduce flexibility, snakes have them for added traction.

Body temperature.

You basically described endotherms that can torpor, like squirrels. In the short term it is pointless, the cost of heating the body back up out weights the savings, it is only useful for long periods of inactivity like winter.


Being fast swimmers will make them slow climbers, fast climbing will make for slow swimmers, they will end put being bad at hunting in one of the terrains. They are optimizing two very different forms locomoting with very different needs. climbing favors light weight, long highly flexible limbs with fine motor control, strong grip with long thin fingers, good depth perception, long flexible tails, ect. Swimming favors high density, short stiff wide limbs, Flat stiff digits, powerful stiff flattened tails, ect. If they hunt like crocodiles they also need eyes high on the body and the ability to sit still for long long periods without becoming lethargic, while climbing hunters need to be constantly on the lookout for food, and either high speed or extremely good stealth.


Again it sounds like crocodiles, who also continue ot grow throughout their life. Note however their growth does slow down however once they reach maturity. A hormonal change is possible but you will likely see drastic differences between males and the shift happens before mating, often triggered by nutrition, something like elephant seals with single dominant male that is huge and a few sneaky small males. This however will make for poor cooperation because female have little choice in partner, so your first idea may be better.


Animals with chlorine based blood like annelid worms have green blood. Alternatively combine copper, makes blue blood, used for oxygen transport (crustaceans and octopi) and vanadium makes blood yellow,(beetles and sea cucumbers) weirdly the vanadium compounds are likely not used for oxygen transport, and we really don't know what it is used for, so you can justify them having both and no one can say otherwise. Our best guess is it is part of their immune system but it really is just guess work at this point. Blue plus yellow gives you green blood.


Climbing claws could be a bonus or a hinderance, it really depends on how curved they are, the more curved the more they hamper by hanging up the bowstring. You may want to use a Japanese style/Mongolian draw where the thump is used not the fingers, this can allow even fairly curved claw to work, although they will almost certainly put some kind of protection on the bowstring to keep the sharp point of a claw from fraying the bowstring. Something like a string loop or thumb ring will protect the bowstring, Both are common historically and easy to make.

enter image description here


Green blood: you'll probably have to invent a respiratory pigment that remains green in colour both when it is carrying tons of oxygen (in the arteries) and when it is depleted of oxygen (in the veins on the way back to the lungs). Probably something copper based. If there is someone here who knows enough chemistry to tell you the colour of various copper compounds in different oxidation states, they'll be able to help

There is a real world 'green blood' but it turns red when carrying oxygen: chlorocruorin

Substantial growth: this is the bit I have problems with. Not the growth potential itself - you describe that well. The problem is how this might have evolved in the situation you describe. For instance, to get big the male must eat lots. And after it IS big it will need more food than a small male. Yet it is the small males and females doing the hunting and then - presumably - sharing their kills with the big males. Why are the small males doing this? They could just keep the meat for themselves and grow bigger. How does the big male enforce sharing of the meat? In lions, it is because the male travels to the kill site and chases the lionesses away from the meat, but your big males spend their time resting in the water.

Also because of the shortened life span, there has to be an ENORMOUS advantage to getting big. In evolutionary terms, this means fathering lots and lots and lots more offspring than long-lived small males. In pretty much all the species where males are bigger than females (apart from humans - we're complicated), that increased male size and strength is for the sole purpose of beating the living crap out of other males of their own species to prevent them getting laid. It is not for defence against predators. It might be useful for that, the same way fingers are useful for pointing at things, but they didn't evolve for that purpose.

So your males mate with lots of females because they are big. Rather than your suggestion of becoming big because they mate with lots of females.

Also, if your big males hang around in water, the only predators they can fend off are water ones, or ones that come to visit them. If policemen stay in in the police station the only criminals they catch are those who break into that police station! :-) Predators are going to be all over the tribe's territory, and will be picking off small males and females when they are out hunting or gathering firewood or whatever.

So if you want this to be evolutionary rather than cultural I think your big males need to be more active. They need to be bullying food out of smaller males, patrolling their patch to drive off other big males who are after their women, kicking small males who are getting to medium size out of the tribe as a potential threat (this includes their own sons), and generally behaving like a bad-tempered boss stallion, rutting stag or beachmaster bull elephant seal.

If it is cultural, then it is a whole other kettle of fish. Then you can invent traditions and laws that say only the heir to the throne gets to eat swan meat or bloopbloop berries. And it is something in those foods which triggers the size increase. Or simply that royalty eats better than the peasants!

  • $\begingroup$ Unfortunately the site made me cut my post rather short so I wasn't able to get into the social aspects of the bigger males, but during the animal phase of the species' evolution what you described would be pretty accurate. I imagined it like a cross between our world's lions and walruses. Beta males have a different breeding scheme that involves sneaking around behind the alpha's back, which has it's own risks involved. I might make a separate post concerning the social development of this race into sapience. $\endgroup$ Jul 5, 2016 at 11:13

I'm wondering if you have any justification as to why this species has become intelligent, or rather more intelligent than leopards or lions? Are they supposed to be around that level of intelligence?

Why and how would this species evolve the manual dexterity necessary for tool creation and utilization (eg. bows and arrows)? What would distinguish it's arms from it's legs in terms of function?

Why would a water or tree dwelling ambush reptile have slender, and therefore fragile, legs? In general, reptiles have very stocky, thick limbs for their size, especially Alligators and Crocodiles, the supposed ancestors of this animal.

Why would the animal's legs be different lengths? Animals that walk on four legs generally have either similarly length limbs or longer hind limbs. The only animals I can think of with longer forelimbs are tree-dwelling primates who move through a swinging motion. This supports an arboreal lifestyle, but the swinging-through-trees, horizontal lifestyle instead of the up and down a single tree, drop bear lifestyle of the animal.

These are all just thought questions, and I do not by any means want to discourage you from going forward in using these creatures. I just want to understand how and why you chose your creature. It seems like a being with a set of interesting, almost cobbled-together set of traits that don't really make sense from an evolutionary standpoint. It could be that I am being overly pessimistic, but it seems hard for me to divorce the idea of intelligent life too far from our only data point, humans and great apes. I have read and enjoyed many stories that don't really justify the existence and traits of their chosen creatures, but if you want to have an explanation, you might as well consider my questions.

  • $\begingroup$ I actually had a whole section on different modes of locomotion the body plan I chose can accomplish, and it all seems pretty natural to me. I just didn't have space as the site was telling me my post looked like spam. But in general, yes. I took the basic design for arboreal primates and applied it here, which is why the forearms are longer. This species spends most of it's life in the trees, using the water more as a means of escape or to drown potential prey before hauling it back up to eat. The slender frame is to make them less heavy and more mobile in the trees. $\endgroup$ Jul 5, 2016 at 22:51
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    $\begingroup$ @MetalJimmor Related to the locomotion and body plan, that makes much more sense to me. I think I was just picturing it in my head much differently, more along the lines of an ant or a alligator with six thin limbs (lol). Do you plan to have the creature have similarly jointed limbs to an arboreal primate, like a gibbon? Also, it should be possible for the creature have opposable thumbs, as amphibians and dinosaurs have had them in the past. If I were you, I would somehow limit claws so as to prevent them from getting in the way, perhaps by making them retractable. $\endgroup$
    – Dent7777
    Jul 6, 2016 at 12:57
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    $\begingroup$ @MetalJimmor As far as evolution goes, if you can explain the gap between a water-based ambush hunter and a semi-arboreal ambush hunter, then you should be good beyond that point. I would guess that, without tools and advanced sapience, they would be limited to creatures smaller or similarly sized for prey. This would encourage both social development through pack hunting and tool development for hunting, as both would allow them to hunt larger prey. $\endgroup$
    – Dent7777
    Jul 6, 2016 at 13:11
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    $\begingroup$ @MetalJimmor For attack tactics, I would have your creatures drop down upon prey, using the non-middle limbs for grappling and the middle limbs for slashing with claws. Once your creature has basic stone hand-tools, I would have your creatures use small hand-daggers in their middle limbs for stabbing the prey while grappling. This aught to be one of the best attacks for grappling for your creatures, and should be used alongside poisons whenever possible, all throughout their evolutionary path. $\endgroup$
    – Dent7777
    Jul 6, 2016 at 13:18
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    $\begingroup$ @MetalJimmor Cont: Then, you get spears, allowing the creature to both attack from range and use all its weight to stab when dropping on to creatures. From spears to throwing spears to bows is a classic evolution of tools. Fully formed attack tactics should be based on luring in a larger prey item, limiting their ability to flee using traps or terrain, peppering them with arrows, and then dropping on the animal with heavy spears (utilizing gravity) and finishing it off if necessary with grapple-shanking. If any of this is helpful, I would appreciate an upvote on the comments and answer. $\endgroup$
    – Dent7777
    Jul 6, 2016 at 13:23

Advanced Description: There seems to be no need to weaken the jaw to permit lips, as almost all vertebrates, including several such as T. rex and O. megalodon, seemed to have had lips. Waterproof feathers need some sort of oil covering to not get wet, but as they are intelligent, this shouldn't be a problem

Breathing: Why not use the dinosaurian breathing method of a rigid lung with air-sacs surrounding it? This works well with a rigid ribcage, as can be seen in birds, and should work fine in this creature



With the others I'm confused why a diaphragm won't work, but let's forget about that. The biggest problem with your system as described is that muscles don't "expand" Birds have a system of air sacs, described in a previous question here so I'll recycle the image:

enter image description here

Now what's apparent from this is that a different evolutionary path could fairly readily have surrounded each sac with the sort of musculature you describe (let's call it a detrusor muscle), though this one is skeletal muscle and much faster-acting). The creature still needs some sort of bellows that can expand to suck in air, but one option is that since you say the chest doesn't expand, you could have some sacs pulled open when others contract. Conceivably such a system could have many small orifices, each specialized only for inhalation or exhalation, with different breathing circuits operating in parallel. Perhaps on your world there are underwater plants that fill bladders with pure oxygen from photosynthesis, and your creatures thrust sharp scales near one opening into the cavity and drain it of its oxygen, retaining it in their body as a storage for some hours while breathing ordinary air, so that at a moment of exceptional stress they can supercharge their metabolism. Or perhaps they store metabolic waste products, such as exhaled ammonia, in a bladder as a signal or a means to deter predators.


Explaining the evolution of extra limbs is tricky. I'm going to go with ... fly swatting. Your critter had powerful webbed feet (front legs I think??) for rapid bursts of swimming underwater. The swamps and jungles gave it an opportunity to leap from the water and grasp a tree branch high above, from which it hung with its arms (I think) until prey arrived to be caught with its powerful claws. All very good except that the flies and mosquitoes on your world are merciless. If we suppose that your "tetrapods" are more segmented and less canalized than ours and occasionally have mutants with an extra segment with extra arms or legs to go with it (digitigrade hind legs??), perhaps some of those mutants were simply able to swat (and perhaps eat) these intimidating pests.

Green blood.

Evolving a green pigment isn't especially hard, though you do need to absorb both red and blue light. Here though, let's do ... structural color. Like the scales of a butterfly wing (see this blog for an abundance of lovely images), the blood cells have periodically spaced ridges of protein. Perhaps they have a means of regulating oxygen affinity by changing the distance between layers of protein. Whatever the reason, their blood is usually green, but the blood cells can move the layers to a different distance (unlike butterfly wings) resulting in different colors. Blood passing through the scales can do this for color signalling - there is simply a rapid paracrine signal, maybe nitric oxide, that communicates to the blood cell as it enters. But the blood will not reset its color until its oxygen is depleted, so if it is shed, it remains whatever color it was. The result of this is that the organism could potentially shoot its blood (like a horned lizard) and the blood would remain whatever color it had been until it dried out. So it could actually paint multicolored messages and images biologically (decoys?), before developing tools to do so with less cost.

Large males.

Make them queens. The females lay their eggs externally, before they are fertilized. But they do it inside the male, which can then store the eggs internally, keep them safe, fertilize them, incubate and nurture them internally, and lay them at the appropriate time. This is a more extreme notion of the midwife toad. The males attract females by being mighty, sturdy protectors capable of defending their nests, bulked with massive stores of nutrients that guarantee the offspring will successfully complete an internal phase of development.


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