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So, I'm creating a fantasy setting, and because why not, I decided to have giants. Not giant giants, just roughly 2 times the height of a human. Now, because physics is dumb, a giant species would have to have real large legs in order to support its massively increased weight. But just giving my giants thick, juicy legs is boring(though tempting), so I've decided to just increase the number of legs. More legs logically means more support, so I don't have to make them massive. However, I'm not quite sure if that's the case. The legs would need to have enough space between them to avoid getting tangled up in each other, so that would mean at the very least a larger base, which means more weight. Also, the extra legs would also have some weight.

Essentially, my question is this: would the support offered by the extra limbs outweigh(no pun intended) the added weight?

(Also, I know extra limbs mean less coordination, but I'll solve that later. Hopefully. Maybe. Probably not.)

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    $\begingroup$ Elephants are a hundred times as heavy as a human, and they walk on four legs just fine. They can even run on their four legs. $\endgroup$
    – AlexP
    Apr 28, 2020 at 0:34
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    $\begingroup$ @AlexP Saying that elephants can run is not exactly true. Elephants can travel very fast if necessary, but from what I hear they have a very unique gait at high speed that is not exactly running. Anyway, they can outrun a human even if their gait is not exactly a running gait. $\endgroup$ Apr 28, 2020 at 17:25

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Absolutely

With enough resources, it's possible for creatures to become truly massive. According to the CDC, the average height of an American woman is 5'4" and the average man is 5'9". You can definitely have a four-legged creature with double that height. As @AlexP points out in comments, elephants are roughly double human height and have four legs. Despite their size and four legs, Asian elephants can move at 15 miles per hour.

Human and elephant sizes

Elephants are the largest surviving land animals. But there's no physical reason that four-legged animals can't get even bigger. Several species of dinosaur absolutely dwarfed modern elephants and survived for a long time.

Very large dinosaurs

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Short answer:

Two legs are enough for a bipedal intelligent being that is twice as tall as a human, if it has a correctly designed body plan.

If a centaur like body plan is desired, adding two arms to a giraffe like being should produce a giant of the desired height.

Long answer:

If an average adult human is about 5.5 to 6 feet tall, roughly twice the height of an average human would be about 11 to 12 feet tall.

Humans who were over eight feet tall were very rare and many had health problems and short lives and also difficulty in moving. It is possible that the tallest human who was strong and active was Angus MacAskill (1825-1863) at seven feet nine inches.

https://en.wikipedia.org/wiki/Angus_MacAskill1

However, it was stated - accurately or otherwise - that eastern Roman or "Byzantine" general George Maniakes (d. 1043) was eight feet tall and strong, and that Roman soldier, general, and Emperor Gaius Iulius Verus Maximinus Augustus, or Maximinus Thrax, (c.173-238) was eight and a half feet tall and very strong.

https://en.wikipedia.org/wiki/George_Maniakes2

https://en.wikipedia.org/wiki/Maximinus_Thrax3

The tallest man known was Robert Wadlow (1918-1940) who reached eight feet 11.1 inches.

https://en.wikipedia.org/wiki/Robert_Wadlow4

However, the prehistoric "Giant of Castelnau" was reportedly 11 feet 6 inches tall based on bone fragments. If that is correct he would have been healthy enough at eight feet, and at nine feet, and at ten feet, and at eleven feet, to not die and even to continue growing. I wonder if 11 feet was actually his height or his length - if he was bedridden he wouldn't have been the only giant who couldn't stand up.

https://en.wikipedia.org/wiki/Giant_of_Castelnau5

The extinct ape Gigantopithecus blacki was much larger than humans but probably walked on four limbs most of the time.

Total size estimates are highly speculative because only tooth and jaw elements are known, and molar size and total body weight do not always correlate, such as in the case of post-canine megadontia hominins with a small-bodied primate exhibiting comparatively massive molars and thick enamel.[15] In 1946, Weidenreich hypothesised that Gigantopithecus was twice the size of male gorillas.6 In 1957, Pei Wenzhong estimated a total height of about 3.7 m (12 ft). In 1970, American palaeontologists Elwyn Simons and Peter Ettel approximated a height of almost 2.7 m (9 ft) and a weight of up to 270 kg (600 lb), which is about 42% heavier than a male gorilla. In 1979, American anthropologist A. E. Johnson Jr. used the dimensions of gorillas to estimate a femur length of 54.4 cm (1.78 ft) and humerus length of 62.7 cm (2.06 ft) for Gigantopithecus, about 20–25% longer than those of gorillas.[16] In 2017, Chinese palaeoanthropologist Yingqi Zhang and American anthropologist Terry Harrison suggested a body mass of 200–300 kg (440–660 lb), though conceded this was likely an overestimate and it is impossible to obtain a reliable body mass estimate without more complete remains.7

https://en.wikipedia.org/wiki/Gigantopithecus8

So an average gigantopithecus might possibly have been about nine feet or even twelve feet tall when standing.

Bears walk on four legs but can stand up on their hind legs. Kodiak bears, a subspecies of Brown bears (Ursus arctos) can be very tall when standing on their hind legs:

When standing fully upright on its hind legs, a large male could reach a height of 3 m (9.8 ft).7...Also, an individual named Teddy, which portrayed a killer bear in the movie Grizzly, stood 3.4 metres (11 ft) tall on hind legs and was the largest bear in captivity at the time.[citation needed]

https://en.wikipedia.org/wiki/Kodiak_bear7

A large male Asian elephant was trained to walk a considerable distance on his hind legs as part of the circus act, with a woman standing on his tusks. If this elephant was about 9 feet tall at the shoulders he would have been about 12 feet tall standing on his hind legs. Large male African elephants stand on their hind legs to reach tree branches, and might reach heights over twenty feet with their trunks.

The prehistoric ground sloths of the genus Megatherium reach elephant sizes, and could stand and walk on two legs.

Megatherium is one of the largest land mammals known to have existed, weighing up to 4 tonnes[13] and measuring up to 6 m (20 ft) in length from head to tail.[14][15] It is the largest-known ground sloth, as big as modern elephants, and would have only been exceeded in its time by a few species of mammoth. The group is known primarily from its largest species, M. americanum. Megatherium species were members of the abundant Pleistocene megafauna, large mammals that lived during the Pleistocene epoch.

It had a robust skeleton with a large pelvic girdle and a broad muscular tail. Its large size enabled it to feed at heights unreachable by other contemporary herbivores. Rising on its powerful hind legs and using its tail to form a tripod, Megatherium could support its massive body weight while using the curved claws on its long forelegs to pull down branches with the choicest leaves. This sloth, like a modern anteater, walked on the sides of its feet because its claws prevented it from putting them flat on the ground. Although it was primarily a quadruped, its trackways show that it was capable of bipedal locomotion. Biomechanical analysis also suggests it had adaptations to bipedalism.[16]

Megaheriums might have been able to walk on two legs (and a tail) while standing ten to fifteen feet tall.

These examples show that it is possible for a nonhuman species to stand (more or less) on two legs for longer or shorter periods and reach heights of twelve feet or more. Although probably none of those examples were primarily bipedal.

The common Ostrich (Struthio camelus) is bipedal and can grow very tall.

Common ostriches usually weigh from 63 to 145 kilograms (139–320 lb), or as much as two adult humans.9 The Masai ostriches of East Africa (S. c. massaicus) averaged 115 kg (254 lb) in males and 100 kg (220 lb) in females, while the nominate subspecies, the North African ostrich (S. c. camelus), was found to average 111 kg (245 lb) in unsexed adults. Exceptional male ostriches (in the nominate subspecies) can weigh up to 156.8 kg (346 lb). At sexual maturity (two to four years), male common ostriches can be from 2.1 to 2.8 m (6 ft 11 in to 9 ft 2 in) in height, while female common ostriches range from 1.7 to 2.0 m (5 ft 7 in to 6 ft 7 in) tall.6

https://en.wikipedia.org/wiki/Common_ostrich9

The largest bird in the fossil record may be the extinct elephant birds (Aepyornis) of Madagascar, whose closest living relative is the kiwi. They exceeded 3 m (9.8 ft) in height and 500 kg (1,100 lb).9 The last of the elephant birds became extinct about 300 years ago. Of almost exactly the same upper proportions as the largest elephant birds was Dromornis stirtoni of Australia, part of a 26,000-year-old group called mihirungs of the family Dromornithidae.10 The largest carnivorous bird was Brontornis, an extinct flightless bird from South America which reached a weight of 350 to 400 kg (770 to 880 lb) and a height of about 2.8 m (9 ft 2 in).11 The tallest bird ever however was the giant moa (Dinornis maximus), part of the moa family of New Zealand that went extinct around 1500 CE. This particular species of moa stood up to 3.7 m (12 ft) tall,7 but weighed about half as much as a large elephant bird or mihirung due to its comparatively slender frame.9

https://en.wikipedia.org/wiki/List_of_largest_birds10

So those tall birds could be modified into intelligent beings by turning their wings, useless for flight, into arms and hands, and making their brains large enough for intelligence. Of course a large part of the height of these living and extinct tall flightless birds is long slender necks supporting tiny heads. If the heads were heavy enough to contain human sized brains, the necks would have to be much shorter and/or thicker to support their heads.

Of course modern birds including flightless birds are descended from small theropod dinosaurs. And there were large bipedal carnivorous theropod dinosaurs which were up to 50 feet long. However, the bodies of large carnivorous dinosaurs were carried in a horizontal position, and their total heights would have been about ten, fifteen, or twenty feet.

So even though humans cannot be simply scaled up to nine feet, twelve feet, fifteen feet, eighteen feet, etc. without severe problems, it seems clear that quadrupedal animals can stand on their rear legs and be bipedal and achieve such statures for longer or shorter periods, and also that truly bipedal animals can function at such statures for all their lives.

There is no need to abandon a bipedal body plan to achieve such modest statures as are desired for your giants, so long as they are not giant versions of humans, but rather beings designed to function at those high statures.

If you go with a centaur like body plan with four legs and two arms, giraffes (Giraffa camelopardalis) may be a good model. One could add another set of shoulders, supporting arms in this case, above and/or in front of the existing front shoulders of giraffes.

Fully grown giraffes stand 4.3–5.7 m (14.1–18.7 ft) tall, with males taller than females.[44][45][46] The tallest recorded male was 5.88 m (19.3 ft) and the tallest recorded female was 5.17 m (17.0 ft) tall.[44][47] The average weight is 1,192 kg (2,628 lb) for an adult male and 828 kg (1,825 lb) for an adult female[48] with maximum weights of 1,930 kg (4,250 lb) and 1,180 kg (2,600 lb) having been recorded for males and females, respectively.[45][46]

https://en.wikipedia.org/wiki/Giraffe#Appearance_and_anatomy6

The front shoulders of a giraffe are more than half the total height, so should be at least seven to nine feet above the ground. Adding another pair of shoulders to support the arms, and a neck, and a head with a large brain, and the centaur like giant should be about eleven to twelve feet tall, even if the length of the neck is a few feet shorter than a giraffe's neck.

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