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.