Note: this is not a duplicate of Anatomically correct Giants or What would be the tallest possible height for humanlike creatures in earthlike conditions? because my giants are far from humanoid. Anyway, the main question here is why they would evolve rather than how they would evolve.

This question is one about my ongoing worldbuilding project about viable mythical creatures and how they fit into the modern ecosystem. The last three were: Is petrifying vision plausible in an animal?, Hydras as parasitic-mating, polyandrous amphibians? and Are wing-walking griffins viable?

The premise:

This question will be about another important creature in the project, the giant. Most fantasy depictions of giants show them as massive humans, but I'd rather shake things up a bit. In my project, giants belong to the clade Tethytheria, with their closest relatives being the rhinoceros-like Embrithopods.

Here is a rough dendrogram of giant phylogeny I made (The taxon at the end of each lineage represents a Linnaean order):

enter image description here

The giants are the order shown in bold, Gigantoidea. Edit: I forgot to include the clade Tethytheria when annotating the dendrogram, it should be above the line that leads to elephants, manatees, embrithopods and giants.

The problem:

But why would very large quadrupedal browsers evolve bipedalism?

The ancestors of my giants would have looked something like this, but not necessarily horned:

enter image description here

Image source: https://alchetron.com/Embrithopoda

And now, they are bipedal and 10-18 metres tall. What evolutionary reason would cause this to happen? Why would heavy, quadrupedal herbivores end up as bipeds?

The giants:

At the moment, that question is a bit broad, so I'll give you some information about the giants I have in mind.

Giants spend most of their adult life dormant, hibernating half-submerged in the earth. During this time, plants will often grow on their backs. The giants absorb nutrients from these and the surrounding soil to sustain them in their sleep. However, they require nutrient-rich soil to survive. Thus, they rise every 200 years to predate the hydras (Their mass births occur at the same time, and they emanate poisonous gases that, in large quantities, have a very negative effect on the soil's nutrient content).

They also reproduce at this time, and baby giants spend about 20 years roaming and growing before their first 200-year hibernation. Their longevity is because of their size and inactive stages.

So, with that information in mind, consider this: What would cause quadrupedal Embrithopods to take on a bipedal posture? I suggest reading up a bit on Embrithopod ecology, anatomy and physiology should you require further information.

If you find this question broad, missing details or having errors, please let me know and I will amend the question as soon as possible.

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    $\begingroup$ I'd suggest you look at this Wikipedia page on the Timeline of Human Evolution. Note that we evolved from quadrupeds and also note that the size has evolved as well. I am, however, skeptical that a 10-18 meter tall entity is viable. My impression is that evolution grows large things which then evolve into more practical mid range sizes. There are far more smaller sized organisms than large sized organisms. Bigger may not be better from an evolutionary standpoint. $\endgroup$ – StephenG Jun 6 '18 at 15:42
  • $\begingroup$ @StephenG Coincidentally, I was actually looking at that page earlier, but for a different reason. I guess the main problems for tethytherian bipedalism are a) size and b) not many reasons, at least as far as I can see. But, then again, that's why I'm here isn't it? $\endgroup$ – SealBoi Jun 6 '18 at 16:58

Most of the answers currently focus on what the Rhino-giant might want or need that could cause selection favoring large bipeds, so I'll go the other way and focus on what they don't want.

Your hydras are already a threat to the giants. I'm not sure if you've explained their poison yet, but it sounds like a great way to kill off some nearby giants for a quick calorie boost during mating season, either for energy to mate or to provide easy food for the young. The giants, when they were quadrupeds, were pretty helpless against the hydras. Charging isn't a great tactic against serpentine, multi-headed creatures. Hands, on the other hand (pun intended) are quite useful for grabbing those snaky necks. They also make it much easier to "un-dig" oneself after being awoken from hibernation by a hydra bite. Larger size makes for thicker skin, which means the first bite is a lot less likely to maim or kill.

Side note - I know I said I was focusing on threats, but large size also means more surface area for absorbing nutrients, especially if they have long limbs and digits that can act like roots.

I know your world is filled with other myth-inspired beasts, so I'm guessing there's probably at least 1 aerial predator to deal with, too. A bipedal strategy allows a lot more options for fighting off an attack from above, and large size makes the giants much harder to carry off. The adults would have been plenty large already, but perhaps the young were small enough to be carried away? As the species grew larger, even the young were too heavy to be taken. The thicker skin is helpful here, too, for surviving talons and bites.

I have one more rather unconventional idea for explaining size. Camouflage. Usually, smaller is better for this, but the giants are using plant life to hide themselves while buried, and a conventional size doesn't allow much to grow, and certainly nothing large could grow without growing roots into or around the giant, killing or trapping it. If that doesn't happen, it's hidden by the moss, but that large area where nothing but moss is growing is a bit suspicious... Once large enough, though, the giant could have a very thick hide that is quite effective at holding roots, allowing bushes and small trees to grow, which hide it a lot better. When out of the ground, it may even be able to pass for a tree at a distance, if it's gathered enough growth and is thin enough.

That's all I've got for now. I hope it helps!

  • $\begingroup$ This is great. Just the kind of answer I was hoping for. Also, on the subject of Hydra poison, poisonous gas radiates from their bodies. The eggs also do this, and the eventual buildup of gas emanating from an underground nesting site collapses the cave roof, creating a sinkhole and a means for the hatchlings to emerge. They carry this trait into later life as a means of protection. $\endgroup$ – SealBoi Jun 6 '18 at 18:11

Trees, in a world that goes from prairie to forest due to climatic changes the bipedal form becomes advantageous to herbivores of all sizes. Wide grasslands that shift through scrub and into climax closed canopy forest cover as the climate warms and rainfall increases over geological time. This would probably be because of continental drift rather than a post-glacial warming event, but either is technically possible. The grazers on those glasslands have to evolve new strategies to feed as the height of feed stock rises or they die out, the shifts are gradual but culminate in a bipedal giant cropping acorns right out of the tops of old growth oak forests in a similar way to Giraffes feeding on the top leaves of Acacias.

  • $\begingroup$ This answer looks the most promising so far, but it may just have to be a "lampshade", to put a semi-viable explanation for a problem whose solution isn't necessarily such. In other words, I think it would be more likely that the quadrupeds would simply evolve longer necks, as the antelopes did when the giraffe evolved. Nevertheless, evolution is highly unpredictable, so this answer may be the solution to my problem. $\endgroup$ – SealBoi Jun 6 '18 at 17:10
  • $\begingroup$ @SealBoi Sauropod dinosaurs had to do both to keep up with redwoods and the like. $\endgroup$ – Ash Jun 6 '18 at 17:12
  • $\begingroup$ Good point. Thanks for the good answer. $\endgroup$ – SealBoi Jun 6 '18 at 17:12
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    $\begingroup$ I was thinking of the sauropods too. This is a more realistic scenario than the absorbing nutrients from the dirt one. If you want your giants to eat hydras, they can be opportunistic carnivores like deer or hippos. $\endgroup$ – Willk Jun 6 '18 at 23:03

Well in nature a lot of excessive and often counter survival characteristics can emerge from sexual selection. A good example are birds, huge colorful feathers that makes you visible for all the predators are not exactly a good survival strategy, but when mating the most visible male birds have an edge over those who aren't that colourful. Because of this sexual dimorphism in those species its usually very noticeable. In your story it could be that the female giants preferred those who could stand higher in a mating dance, eventually this lead to bipedalism in males and that trait got passed on to the females of the species. In that case the females would usually be a lot smaller than the males and a lot more agile.

  • $\begingroup$ I've tried to edit this into more than one sentence, as it was a bit indigestible as it was, please check I haven't lost something along the way. $\endgroup$ – Ash Jun 6 '18 at 16:53
  • $\begingroup$ @Ash Yes, this came up in the review queue for first posts coincidentally, and I suggested an edit for it (As of yet un-peer-reviewed) so it has better grammar, spelling, formatting and general understandability. $\endgroup$ – SealBoi Jun 6 '18 at 16:56
  • $\begingroup$ @SealBoi Huh oh well as long as Eric is happy with one version, or the other, or both. $\endgroup$ – Ash Jun 6 '18 at 16:58
  • $\begingroup$ So THAT's what happened to Tolkien's Ent women. They got too small to successfully reproduce with the giant men they were so attracted to! $\endgroup$ – Josh Jun 6 '18 at 18:08

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