Damn, He's Hot! Look At Those Tiny Arms! Or, Size Matters

This is really a question for PZ Meyers. Why did Tyrranosaurs have such dinky little front legs, so short they couldn't even use them to lift food to their mouths? It seems so dumb. I was considering it as I lay in bed last night and then I thought, aha, sexual selection! Their means of locomotion (rear legs) is such that they don't absolutely need front legs to get around. So maybe it was originally a signal, like, I'm so extrordinarily bad-ass that I can thrive even with these relatively wimpy arms! And then some kind of sexual selection feedback loop set in, and they were on a path to developing little nubbins and finally no arms at all, but they all got blown up by a meteor. The End. But then I thought, well, it can't really be the case that every time we see some apparent design flaw in a creature we're allowed to say about it, this is so stupid that only a really healthy, robust animal of this sort could survive, etc. You can't just claim sexual selection for everying, because chance ensures that lots of animals are just messed up from a teleological perspective. There's no guarantee that creatures will be optimally constructed; they just have to be good enough to stay alive and are saddled with all sorts of inherited templates which it is too late to change. So what do you guys think? Tell me what real scientists think so I can replace my imaginary version with a truer one.

Comments

[Cala]

Not a scientist, but my boyfriend and I have a theory, based on observation at the Royal Tyrell Museum in Canada:

When God was designing dinosaurs, he did so like a twelve year old spending skill points in D&D.

Thus, after giving the T. Rex huge teeth and making it REALLY big, God didn't have any points left to spend on arms.

This works for other dinosaurs, too. Ankylosaurus! Big club on the tail. Shit, we're out of points for brains. Oh well. Give it more armor.

Natural theology concludes that God is twelve.

[Matt]

It may depend on what 'arms' were before they were arms... Presumably some anatomical or developmental something-or-other got hijacked into 'armness' that turned out to be useful for mammals but not as useful for thunder-lizards, birds, snakes, etc.

[Matt Weiner]

Dunno, Matt, AFAIK the arms and legs come from those Homeobox genes that pretty much every organism incl. insects has. So the arms probably didn't develop from some other nubbin. I'm not a scientist though so I may be getting this totally wrong.

[Matt Weiner]

"Pretty much every organism" = really stupid thing to say. But, you know, lots and lots.

[belle waring]

I favor Cala's theory. so say we all.

[NeilS]

How did T Rexes do it? Perhaps the tiny arms were perfectly aligned for her G-spot?

Is there any modern analogue to tiny arms? I can't think of one.

[LowLife]

Soccer. T-Rex played lots and lots of soccer. There wasn't any ice so there wasn't any need for stick control it would have used in hockey.

[ettinauer]

I think it was Eddie Izzard who suggested they were for playing the piano.

[gordsellar]

Uh, later beings will ask, "Why didn't humans have wings? Those would have been sooooo useful. Evolution can't be true, or humans would have had enormous gigantic wings and slimmer figures (so the wings would work) instead of useless little shoulder blades buried under their flesh."

We haven't needed wings badly enough to develop them naturally. And T. Rex didn't friggin' need arms.

The answer is: SHARP POINTY TEETH. :)

But I like the AD&D suggestion, too.

[Rob G]

Um, cuz it didn't need them?

[Brad DeLong]

Why don't humans have prehensile tails?

[Adam Kotsko]

I think that we can easily synthesize the "skill points" scenario with the Homeobox genes scenario -- the latter being the places in which said skill points are accumulated.

[W. Kiernan]

Brad De Long ...say BDL, anybody ever tell you you have got a boss porno star name? asks: Why don't humans have prehensile tails?

Because after Original Sin &tm;, as Adam & Eve were guiltily fleeing the Garden of Eden, Jehovah attempted to stamp on them like skittering cockroaches, but he was soooo pissed off that he missed his shot and only managed to step on their tails. They kept running, of course, but their tails got left behind. Dang, that had to hurt. For technical details on how that trait was inherited by their various offspring, see here, or just ask that PZ Myers guy.

[washerdreyer]

I am somewhat surprised this thread didn't just end after comment 1 gave the clearly correct answer.

[Kieran]

Why don't humans have prehensile tails?

They don't? Uh, never mind. Forget I mentioned it.

[Adam Kotsko]

Even if the explanation of Adam and Eve's lack of tails is incorrect, it remains the case that the story of the Fall is implicitly Lamarckian -- as punishment, the serpent loses its legs, a trait that its progeny inherits.

[Brad DeLong]

I mean, seriously, prehensile tails would be useful. Why, this morning, leaving the car with my laptop in my left hand, my car keys to lock the car in my right hand, the newspaper tucked between my chin and my chest, and the dog's leash... in my mouth... as I learned something new: this dog has peed on her leash, recently...

[M/tch M/lls]

Is there any modern analogue to tiny arms? I can't think of one.

Kangaroos?

[belle waring]

M/itc/h: kangaroos, wallabies etc. can reach their mouths with their hands, and nibble on stuff like they wuz giant squirrels.

[James Wimberley]

The bad-ass enormous fanged jaw must have been great for killing everything else but not so good for eating it tidily afterwards. You need a dainty hand to hold the serviette. Also to clean the bits of dead everything else stuck between the fangs, remove ticks, scratch under Lady T's ears. (Tortoises love this; someone else can try with crocodiles.) The arm doesn't consume much resources and is handy in a small way, so what's the payoff in eliminating it? Streamlining?

[Andromeda]

Conveniently, my in-laws have written a book on sexual selection.

[Patrick]

There doesn't need to be a payoff to eliminating the arms.

Mutations occur, and many of them screw up the creature that has them. Mostly these mutations go away after a generation, because if a creature has a messed up limb or organ and they actually need that limb or organ to function property to survive, they die and don't reproduce.

If the TRex wasn't using those arms for anything important, however, mutations which messed them up wouldn't affect its ability to pass on its genes. So those mutations would be expected to accumulate, and could very well result in wussy, useless little vestigial arms.

[Gary Farber]

"Why did Tyrranosaurs have such dinky little front legs, so short they couldn't even use them to lift food to their mouths?"

They used them for spelling.

[Daniel]

One of the dinosaur books I had when I was a kid said that the front legs were used to provide a little leverage, so it was easier for the T-rex to get up off the ground when it woke up. It didn't need them for anything else, since it has such awesome teeth and hind-legs, so the front legs are wussy-looking.

There were pictures to illustrate this, which would improve any theory.

[M/tch M/lls]

M/itc/h: kangaroos, wallabies etc. can reach their mouths with their hands, and nibble on stuff like they wuz giant squirrels.

Ah, but you're forgetting that acorns back in dinosaur times were much larger.

[saurabh]

I've never bought that "sexual selection for fitness disadvantage" theory. Males who have ridiculous features ARE at a fitness disadvantage, if traits are segregating independently. Horns so big I can't walk between trees doesn't mean I have higher fitness, it means I have lower fitness. If something else compensates for that, let's say greater neck-strength or general overall fitness, it's STILL going to mean i'm at a fitness disadvantage compared to some other moose with small horns who also happens to have greater neck-strength. So mate selection for ridiculously large horns shouldn't be advantageous for females. Unless there's some sort of pleiotropic effect - big horns means stronger bones, or something, because the same allele that produces one produces the other. In which case your sexual selection theory might hold water if small arms meant better balance, etc.

But I think the more sensible pattern for sexual selection would be runaway preference - when horns are little nubs, bigger horns means a fitness advantage. At this point, sexual selection makes sense. But once it's fixed, it runs out of control, since now bigger horns are preferred even if they're not advantageous. So, if at some point Tyrannosaur girls were selected to like smaller arms, because, say, it meant boys were more likely to be able to stand up to bite things from above, that might have resulted in runaway sexual selection to produce the tiny stubs that Tyrannosaurs ended up with. But there's no sexual dimorphism in T. reges, so I don't know that we should necessarily suspect sexual selection rather than plain-old natural selection for better balance.

[Bill Tozier]

People, please. If they had big, heavy arms, they'd tip over forwards. Look at those huge heads. They'd be scraping around with their tails in the air, catching nothing but a big mouthful of dirt.

[Anne]

My dad has a (non-comedy) theory about this actually. He thinks T Rex had a lifestyle like an bipedal alligator: wade into river when big herds of awkward slow herbivores are crossing, brace self on bottom of river with awesome legs, use giant jaws to grab and overpower prey, then dead prey floats and you can use little arms to manipulate it as you eat at leisure.

The main rationale for this is, they're so big that it would be disastrous if they fell while running. So, better to have a lifestyle where they don't need to run to catch prey. But if they're scavengers that only eat pre-killed things, there is no need for the giant jaws.

(Dad makes a better case for this than I do, and has done a lot of -- amateur -- research to see if it makes sense. I don't have any idea if any actual paleontologists have considered this theory though. If anyone knows, I'd be interested to hear!)

[Julian Elson]

Well, here's what I found in the D&D system reference documents on t-rexes:

TYRANNOSAURUS
Huge Animal
Hit Dice: 18d8+99 (180 hp)
Initiative: +1
Speed: 40 ft. (8 squares)
Armor Class: 14 (–2 size, +1 Dex, +5 natural) touch 9, flat-footed 13
Base Attack/Grapple: +13/+30
Attack: Bite +20 melee (3d6+13)
Full Attack: Bite +20 melee (3d6+13)
Space/Reach: 15 ft./10 ft.
Special Attacks: Improved grab, swallow whole
Special Qualities: Low-light vision, scent
Saves: Fort +16, Ref +12, Will +8
Abilities: Str 28, Dex 12, Con 21, Int 2, Wis 15, Cha 10
Skills: Hide –2, Listen +14, Spot +14
Feats: Alertness, Improved Natural Attack (bite), Run, Toughness (3), Track
Environment: Warm plains
Organization: Solitary or pair
Challenge Rating: 8
Treasure: None
Alignment: Always neutral
Advancement: 19–36 HD (Huge); 37–54 HD (Gargantuan)
Level Adjustment: —
Despite its enormous size and 6-ton weight, a tyrannosaurus is a swift runner. Its head is nearly 6 feet long, and its teeth are from 3 to 6 inches in length. It is slightly more than 30 feet long from nose to tail.

Combat
A tyrannosaurus pursues and eats just about anything it sees. Its tactics are simple—charge in and bite.

Improved Grab (Ex): To use this ability, a tyrannosaurus must hit an opponent of up to one size smaller with its bite attack. It can then attempt to start a grapple as a free action without provoking an attack of opportunity. If it wins the grapple check, it establishes a hold and can try to swallow the foe the following round.

Swallow Whole (Ex): A tyrannosaurus can try to swallow a grabbed opponent of up to two sizes smaller by making a successful grapple check. The swallowed creature takes 2d8+8 points of bludgeoning damage and 8 points of acid damage per round from the tyrannosaurus’s gizzard. A swallowed creature can cut its way out by using a light slashing or piercing weapon to deal 25 points of damage to the gizzard (AC 12). Once the creature exits, muscular action closes the hole; another swallowed opponent must cut its own way out.

A Huge tyrannosaurus’s gizzard can hold 2 Medium, 8 Small, 32 Tiny, or 128 Diminutive or smaller opponents.

Skills: A tyrannosaurus has a +2 racial bonus on Listen and Spot checks.

I'm wondering what Belle means by these dinosaurs being saddled with a bunch of inherited templates though. Are you talking about something like a celestial, phrenic, half-dragon tyrannosaurus? I fail to see how that's a weakness. Of course, the level adjustments would suck if the tyrannosaurus wanted to take class levels, with the huge ECL and all that, but from my understanding, it wouldn't unless it were Awakened by a druid or something.

[jb]

that is by far the nerdiest thing i've read this month. and i love it.

but i can't recall... does awakening also adjust ecl?