Bulk/robustness--how to define it?

[Infinity Blade]

A few times I've heard bulk being thought of as total body mass relative to total body length.

Is this really a good way to define bulk/robustness? I mean, would say, a lion, suddenly become more robust if you took away its tail? Shouldn't we be focusing solely on the torsos when comparing robustness? After all, that's more or less what people are thinking of when saying animal A is bulkier/more robust than animal B, right?

[theropod]

Well that really entirely depends on the specific comparison.
Often people compare two animals of comparable total length, although this is more rarely the case with extant mammals such as your example (because A: reliable body mass figures are pretty common, not really more difficult to find than length measurements and B: their tails are so puny).

Obviously it is important for the simple matter of size if one of them has a longer tail.
E.g. Brachiosaurus and Diplodocus. I think if you were to put them at torso length parity, the difference in bulk wouldn’t be so noticeable, but since Brachiosaurus has a proportionately shorter tail and longer torso, it’s a much larger, i.e. bulkier animal at their actual sizes.

Whereas when we are comparing two animals of comparable body mass, it really doesn’t matter whether one has, say a longer tail, at least as long as that tail has negligible mass (like most mammalian tails, or the whiplike tail-tip of a flagellicaudatan). The animals’ torsos are still similar in size and, all else being equal, robusticity too.

One aspect that I think is often overlooked is robusticity of the limbs. An animal may have a compact torso and short tail, but it’s not unusual for such animals to have slender, cursorial limbs. Another animal may be less robust in the torso, but have more robust limbs (Note that that is not to say cursorial limbs aren’t strong, they absolutely are, but in a specialized kind of way that primarily comes at the cost of flexibility and versatility). For example, a giraffe doesn’t strike me as an animal I would describe as robust, but its torso alone certainly is.

So I think sometimes it’s advisable to differentiate between bulk and robusticity. The former is directly linked to body size (i.e. mass), the other is linked to durability and, to an extant, strength. But since that’s bound to lead to misunderstandings too, maybe we should simply try to explain our point in more detail whenever we are talking about this subject.

[Infinity Blade]

Umm...okay, after reading your comment, let me see if I have the right takeaway and everything I'm about to say holds true.

"Bulk", as you've defined it, does with body mass. If two animals are at similar sizes, it doesn't mean anything in regards to the fight. "Robusticity", however, will, as it's the one that's actually linked to durability and strength (at least to an extent for the latter). But even thinking of the two terms this way may not always work.

It just became morning where I am, so maybe I'm not thinking straight right now. Correct me wherever I'm wrong.

[theropod]

Yeah, that was pretty much the idea. That doesn’t mean anyone has to roll with it though, and I might be getting it all wrong in terms of the meaning I’m ascribing to those words.

What’s more important imo is my last line, I think we should be a bit more elaborate in talking about this subject and stop just writing "Taxon A is bulkier/more robust than taxon B" (which is often followed by a "hence it wins this fight!!!!"). That in itself constitutes an overgeneralization.

E.g. Tyrannosaurids are more robust in the skull and axial collumn than Allosauroids, and at the same total length they are certainly bulkier in the sense I tend to use the word in (i.e. more voluminous), but if you compare their limbs the robusticity-situation is reversed, and Allosauroids usually have similar or larger body mass than Tyrannosauroids, so "bulkier" may not accurately describe the latter as compared to the former.

[dinosauria101]

More mass in the same area, I believe. Example: Diplodocus vs Apatosaurus



At length parity, Apatosaurus would weigh much more