How to make my Beast-Of-Burden work?

Question

Disclaimer: This question is the first of a new series of questions of mine about introducing hexapedae to the fauna of my conworld. There are/will be other questions addressing i.a.: ecosystems, evolutionary factors, taxonomy

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Setting: In my conworld the world is divided into two humongous continents, each taking up about half of the total landmass of the planet. Each located at the Northern and Southern poles respectively.

1 Equatorial Belt    | Saltwater
2                    | Saltwater
5 Northern Polar Sea | Saltwater
6                    | Sweetwater

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Creature: The Beast-of-Burden (further BOB) is quite versatile. Despite its size and weight it can move quite fast - not anywhere near horses, mind you; though it prefers a more lumbering pace and gait. It has roamed the northern regions of the continent for as far back as anyone can think and prove and although it is not the only hexapedal mammal, it certainly is the most prominent in our lifes.
Even though its got quite a temper and stubbornness to it, domestication proved easy enough according to historical texts. Since the olden days its role in our daily life has changed little. It still is used in agriculture and similar areas, harnessed in front of plows and carts; and it still is used as a pack-animal nearly anywhere the automotives with their heavy engines and tyres can't go. Even the military with their airships still make use of them.
Now besides their use in labour there's not much else they're good for. Their coat doesn't grow thick enough to be worth shearing and spinning into thread. Butchering them does not yield much meat compared to their size and their consumption of plant matter, alas the few bits are quite delicious. Their long gestation and the fact that they drop multiple younglings (similar to dogs) would make one think they'd be good for dairy; but their milk has this weird tang that just makes you want to pour it into the sink - though it's amazingly rich.

Overview:

• Form:
  • Hexapedal (6-legs)
  • Adults somewhat taller than oxen, caribous, bisons, etc.
  • Broad backs allowing for carrying things and young animals
  • Cannot swim, do not float (see the addendum at the end of the question)
  • Natural lifespan of some 30-40 years
• Habitat:
  • Northern part of the northern continent
  • Mountains & Valleys
• Reproduction:
  • Two sexes
  • Long gestation
  • Multiple younglings
  • Do only produce offspring once or twice in life
• Social:
  • Live in small herds
  • Multiple males and females
  • Pairings don't stay together
  • Whole group/herd cares for younglings
• Character:
  • Do not easily scare
  • Protective of their group and younglings
  • Docile
  • Quite ferocious when incited
• Food:
  • Herbivores, feed on grass, mosses, bushes, etc.
  • Multiple stomachs, ruminate food
• Produce:
  • Coat/Wool: similar to bisons/goats
  • Meat: comparatively small yield when butchered, mostly stringy
  • Dairy: weird after-taste, very rich in nutrients

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Question: Does my thinking add up? Are there any big issues/contradictions in how this species looks, lives exists?

The question does not ask for the plausibility of a six-legged mammal in general.
Questions about the taxonomical branch and other similar creatures in the ecosystem will come later.
Questions on how it ends up in this specific niche of nature will come later but can be addressed in answers here as well.

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Addendum:
- Cannot swim, do not float: The idea behind this is related to big earth-animals such as e.g. Hippos. The BOBs are too heavy/dense in build as well as featuring a coat of fur/hair; thus they have difficulties in swimming, as in being in waters that are deeper as they can stand. They would/will still ford rivers and move into waters less deep than they can stand.
- Definition of swim according to the merriam-webster (emphasis mine):

a : to float on a liquid: not sink
b : to surmount difficulties: not go under

Answer 1

It doesn't matter that the milk tastes a bit funny, you're still going to drink it. The same with being a bit short on meat, if that's what there is, more so as a multiple birth animal, then that's what you'll eat, especially in an arctic environment. However most big beasts of burden have a lot of meat, it might be quite tough, but there should be a lot of it.

Reproduction rate is too low, once or twice in a lifetime isn't enough for what's fundamentally a prey animal, that'll need to be every couple of years at least. Unless your world has no predators and this species has an unbelievable survival rate for newborns.

Speaking of predators: How do they deal with them, do they have horns? Hooves? Both would come in useful.

Why do they live so long when they can only breed twice? Very few creatures live on past the end of their breeding cycle.

You should possibly also consider how females choose mates. Normally with herd animals it's one (or a small number of) dominant male(s) and the rest don't get much of a look in. The other part of this is of course, how do males impress females, do they fight, do they display, are they particularly charming in a bar, do they dance. Pair breeding is much more common in birds than mammals.

Swimming: Pretty much everything can swim, often badly but they can. Unless you have a particular plot reason for them not to, let them swim.

(Your world though, with a global belt ocean, the tides and storms are going to be epic!)

Answer 2

Honestly the existence of creature per-se is not problematic; In particular being located in "not-earth" so to speak.

I do not expect BOB to be a mutated mammal with extra limbs; those have problems walking, have a hard time getting food, and tend to die young, before having offspring. Edit: the condition is Polymelia, it is a defect in the development of the fetus and not a mutation, and thus and it is not hereditary.

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As, for why would this feature be selected, the advantage of the six legs is evidently on locomotion. In particular walking and climbing can be done with three contact points all the time, regardless of terrain. This is why six limbs make more sense on small creatures that climb trees.

The BOB could have a similar approach; if their original habitat was mountain or rocky desert (you say valleys, a rocky desert is a valley? Ok, good). At least that was the case during the initial stages of evolution, so the six limbs could have some advantage at that time. In our world, they would compete with goats, so I agree on the choice of fur. Note: being a desert doesn't mean it is hot, being far from the equator would make it colder.

I'll - as you may expect - handwave mammal glands as convergent evolution.

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Ok, so the BOB ancestors came from that terrain. But the modern BOB has been domesticated.

I’ll go with CGPGrey’s domestication checklist (which is based on the book Guns, Germs, and Steel):

• Feedable: If the thing eats other animals, it is not good. You would have to hunt or domesticate those other animals to feed it. So you want Herbivores! (✓)
• Friendly: If it will kill you it is not good, it is going to run away it is not good. You said “Docile”, sounds good to me. (✓)
• Fecund: Animals with reproductive cycles too long or with too little offspring are not good. You say “Multiple younglings” I wonder how many is multiple because it only reproduces once or twice in 30-40 years ※. (✓)
• Family friendly: The creature stays in packs; they do not roam alone when they leave the protection of their parents. Instead they stay in family. They will accept the masters as if they were part of the family, leaders of the pack. This is in line with “Live in small herds“and “Protective of their group and younglings”. (✓)

※: The fact that their live cycle is larger than a generation of the masters is a bit problematic, because it means that the advance in domestication that a single person can archive is limited. Consider making them have more offspring or short their life cycle. This would imply that for domestication there must be people dedicated to do it, and thus there must be another benefit of having them around. There is the “they domesticated themselves” option: if they keep coming to the masters for food, because they can’t get enough is the wild (we said, desert, right?).

I have reason to believe that modern BOB was domesticated to be bigger and bulky (i.e. ancestor BOB is smaller and more agile). The reason is that bigger herbivores are harder to catch. Sure, they can be friendly, but you feed them and then they go away when they are no longer hungry… and trying to catch them will trigger their “Quite ferocious when incited”.

Oh - you say - but elephants, no elephants aren’t really domesticated. I mean, the influence of humans in their evolution is minimal, and they don’t depend on us. Instead we say that they are tamed, and it takes a lifetime, that’s why they are not common for farming. So make the BOB ancestors smaller. It should be noted that “Do not easily scare” is a feature of big herbivores that can fight back to the predators (which goes well with “Quite ferocious when incited”).

On that note, these BOB aren't exactly pray animals. They fight back.

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Pete is right about the meat, we have a big animal (taller than an oxen) and with lots of muscle (it is a carry beast, and has six limbs).

About the milk I have no problem with it not being tasty, in particular because I’m handwaving it as convergent evolution. Regardless people would find a recipe for it, mix it with some fruits. As Separatrix puts it “you're still going to drink it”.

Really, why did people domesticate this thing if they aren’t going to eat it? They could be good leaving them in the wild, and taming a few for work if they needed. No need to breed them, just leave it happen in nature.

About swimming, it is ok if they can’t. But I tell you, they will still cross some rivers, because they are big enough and stable enough.

I want to point on the multiple stomach subjects, that that is a misconception about ruminants, they have one single very specialized stomach with multiple stages. This adaptation is to break cellulose, by hosting microorganism capable of that task in their rumen.

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Addendum: On taxonomy. Unless the world is dominated by humans and these humans come from Earth, and they are convinced that the live comes from panspermia... the taxonomical classification used in this world is devoid of all the species on Earth. They are independent, and you should make up the classification names in some language of natives.

As per choice of language, the reason to use a language with no native speakers - being the case of Latin (some greek and some made up words based on people names) in our world - is that meaning of the world wills no longer changes by natural language evolution.

Answer 3

I'm not sure how you can expect a single answerer to address everything in what looks more like a collaborative question, but here goes with my initial thoughts.

Meat
You've stated that there's not much meat on these BOBs, but that doesn't really make sense to me. They're large enough to bear a burden, they have broad backs and six legs. Muscles are usually the main edible parts of a herbivorous animal, and this BOB should have plenty of them. You said there's little meat yield when butchered, so how does that work out? You might want to find another reason why they can't be a food source.

Apart from that, I can't see much to argue with.

Answer 4

This answer focuses on only part of your BOBs, namely, their evolutionary history. There have been suggestions that six-legged animals will have evolved from a remote ancestor that was a bottom dwelling marine organism. This creature can be labelled 'benthic' which is only fancy way of saying its bottom dwelling.

The majority of animal life on Earth, which includes humans, are tetrapods because our remote ancestor was a free swimming teleost fish. Its four fins eventually became four limbs. Its descendants colonized the land and that's the main reason animal life is four limbed.

Bottom dwellers can have six fins or similar structures and these could evolve into six limbed animal life. It's not unreasonable to expect animal life in your construct world to be all six limbed. Although it is possible there could have been two independent branches of animal life, so there could be both four-limbed and six-limbed animals sharing the world.

Hopefully this will add some versimilitude to their evolutionary backstory.