Why did T-Rex have such teeny arms?

[Palo Verde]

Why did T-Rex have such teeny arms? They couldn't reach it's mouth to help it eat. They couldn't help it get up if it fell over. It just seems so impractical!

[TriPolar]

That is a question often debated. Some claim they are useful, that the arms were used to stand up or balance while eating at ground level or when knocked over. Another theory is that the loss of weight was necessary to maintain T form bipedal balance. Since T Rex was a successful dinosaur, the lack of longer arms wasn't a hindrance to it's existence.

[garygnu]

Very likely to be vestigial, or nearly so.
Just because an organ is not fully necessary for an animal doesn't mean it disappears completely immediately. These things take time.

[Colibri]

Originally Posted by garygnu

Very likely to be vestigial, or nearly so.
Just because an organ is not fully necessary for an animal doesn't mean it disappears completely immediately. These things take time.

Well, they weren't vestigial. The problem is, they were actually well-muscled and functional as appendages, although small ones. If an organ is vestigial and on the way to disappearing - the pelvis of whales, eyes of blind fish - you don't expect it to be functional at all. If T. rex's arms were vestigial, you would expect the musculature to be minimal and the joints to be non-functional.

T. rex arms

[Chronos]

One hypothesis I've heard is that they used them to hold onto each other for mating.

[Polycarp]

Those arms are not that small -- they're about the size of a full-grown man's, and muscled to match. The thing is, they're on a body built to a much larger scale.

One very common hypothesis is that they were used primarily as an anchor. When the T. rex rose from a resting position, the mighty hind limbs would move forward to lift the tail and butt of the enormous biped. The strong little forearms would grip the ground tightly to keep the animal from sliding forward while it got its hindlegs under its hips, and could then raise its chest and head by counterbalancing with the tail. Visualize your own posture as you attempt to get up after lying on your belly -- don't roll over or "sit out" but bring your kegs up under your hips like a sprinter. This is what T. rex was faced with, and he couldn't bang a gong for help.

[TriPolar]

Originally Posted by Polycarp

This is what T. rex was faced with, and he couldn't bang a gong for help.

A counter-position to Chronos' assertion that they used the short arms to get it on.

[Nunzio Tavulari]

So they could reach around for things that fell behind the washing machine.

[DrFidelius]

Toothpicks. Handy toothpicks.

[heathen earthling]

Dodging the functional question, T. rex had small arms because it was descended from previous tyrannosaurids that had small arms. The same trait was inherited by other closely related theropods such as Albertosaurus, Tarbosaurus and Daspletosaurus.

[pseudotriton ruber ruber]

They didn't need to box with God, that's why.

[CalMeacham]

Originally Posted by garygnu

Very likely to be vestigial, or nearly so.
Just because an organ is not fully necessary for an animal doesn't mean it disappears completely immediately. These things take time.

This isn't a useful way to look at things. "vestigial" organs aren't organs that are in the process of disappearing, and we just caught them at the wrong snapshot in time to see it. Such organs are still serving a purpose, and creatures which have these organs are presumably more successful than mutations lacking them.

In the case of what we think of as whale transitional forms, it's been speculated that the "vestigial" legs were, in fact, used for mating, to help the male hold on, as Chronos says some think the T. rex used its arms.

I haven't heard or read of that suggestion for T.rex arms before, but one that I have heard is that they might actually have helped the T. rex stand up. The huge legs can push the T. rex body forwards, but to prevent it from merely pushing the body along the ground, it has been suggested that the arms dug in and kept the body in place while it startede to rise. I don't know if that's really reasonable, but I do know that it has been, in al seriousness, suggested.


FWIW, in the original 1933 King Kong, shortly after the T. rex enters the scene, and just before we cut away to Kong haranguing Bruce Cabot in his cubbyhole under the cliff where the tree used to be, the T. rex pausees to scratch itself with one tiny arm as Fay Wray screams in terror. If nothing else, you'd think the arms would be good for scratching.

[Colibri]

Originally Posted by DrFidelius

Toothpicks. Handy toothpicks.

I know this is a joke, but T. rex couldn't even reach its mouth with its forelimbs.

Originally Posted by heathen earthling

Dodging the functional question, T. rex had small arms because it was descended from previous tyrannosaurids that had small arms. The same trait was inherited by other closely related theropods such as Albertosaurus, Tarbosaurus and Daspletosaurus.

While true, this doesn't offer any insight into why the trait developed in the first place.

Originally Posted by CalMeacham

This isn't a useful way to look at things. "vestigial" organs aren't organs that are in the process of disappearing, and we just caught them at the wrong snapshot in time to see it. Such organs are still serving a purpose, and creatures which have these organs are presumably more successful than mutations lacking them.

This isn't correct either. Some organs are vestigial, with no discernible function, and are in the process of disappearing. The energy cost in making them, however, has not yet produced enough selection against them to overcome the developmental program that makes them.

In the case of what we think of as whale transitional forms, it's been speculated that the "vestigial" legs were, in fact, used for mating, to help the male hold on, as Chronos says some think the T. rex used its arms.

This may have been true of some archaic whales, in which the vestigial legs were still external. It's not true of modern whales, in which the legs bones are internal and are truly vestigial, with no known function.

[Corcaigh]

P'raps he uses his arms to wave "hello" to his buddies?

[Scumpup]

They were used to wave threateningly.

[tdn]

They were used to play the ukulele.

[Exapno Mapcase]

Would you want to face a T. Rex? If the answer is no, then it doesn't really matter what its arms were like. They were just fine.

[WordMan]

It almost seems like they were a form of "land shark" - they had a powerful body/means to get around, a huge mouth and the various sensory tools to enable them to steer the mouth with their body to get food. Everything else is just window dressing

But, per Colibri, there is a difference between vestigial and "not a primary tool for this animal." The arms functioned and obviously did a few things sufficiently well to stick around evolutionarily.

Kinda like Humans' sense of smell??

[bup]

Originally Posted by Exapno Mapcase

Would you want to face a T. Rex? If the answer is no, then it doesn't really matter what its arms were like. They were just fine.

I disagree. It's still a good question. Useless appendages take energy, protein, and can get infected. Mother nature is too stringent to allow useless arms for very long. I think they were either useful or (although the fossil record doesn't support it) disappearing.

Maybe they were sexually appealing.

[panache45]

For giving two snaps and a hey.

[TriPolar]

Originally Posted by bup

Mother nature is too stringent to allow useless arms for very long.

Then how do you explain Johnny Damon?

[CalMeacham]

Originally Posted by Colibri

I know this is a joke, but T. rex couldn't even reach its mouth with its forelimbs.



While true, this doesn't offer any insight into why the trait developed in the first place.



This isn't correct either. Some organs are vestigial, with no discernible function, and are in the process of disappearing. The energy cost in making them, however, has not yet produced enough selection against them to overcome the developmental program that makes them.


This may have been true of some archaic whales, in which the vestigial legs were still external. It's not true of modern whales, in which the legs bones are internal and are truly vestigial, with no known function.

It's still the wrong way of looking at it. Nature doesn't have a mecghanism of "gradually disappearing over time" that requires some sort of half-life to disappear. In general, organs that are retained serve some purpose, which is why the aforementionedc "vestigial" external limbs have been explained in terms of reproductive usefulness.

[Colibri]

Originally Posted by CalMeacham

It's still the wrong way of looking at it. Nature doesn't have a mecghanism of "gradually disappearing over time" that requires some sort of half-life to disappear.

It's not a mechanistic process, but there are selective forces that will tend to cause organs that are not being used to disappear. Since the organs are not being used, mutations that interfere with their functionality will not be selected against. But it costs energy to make and maintain useless organs. Individuals that lose them will require less food and be at a competitive advantage against others that retain them. So it's really a balance between how strong this selective effect is versus the developmental programs that make the organs.

Admittedly this process can take a long time. The eyes of blind cave fish become non-functional long long before the eyes actually disappear. The developmental program produces a semblance of the eye structure, but the structures don't work. In this case, even if you lost the structure of the eye completely you would still need to fill the eye cavity with some other tissue, and the cost of making the eye structure may not be that much more than simply making other tissue.

Likewise with the hind limbs of whales. These have hung on for tens of millions of years, because the selective forces tending to reduce them are probably very slight.

In general, organs that are retained serve some purpose, which is why the aforementionedc "vestigial" external limbs have been explained in terms of reproductive usefulness.

Not necessarily true. It's just that sometimes selection is so weak for eliminating them that they have persisted for a long time.

[Lukeinva]

Even though his arms (hands) couldn't reach its mouth they could still be used for eating as in holding prey. I would think.

[Son of a Rich]

Kind of a funny moment on King of Queens, when Doug wants access to a computer at the library, and drives away the girl using it by going on about T-rex's difficulty in masturbating with such small arms.

[Zebra]

They used them to flash gang signs. East Coast T-Rex in da HOUSE!




But really, they actually hunted with guns.

[Ludovic]

Originally Posted by TriPolar

A counter-position to Chronos' assertion that they used the short arms to get it on.

Originally Posted by Son of a Rich

Kind of a funny moment on King of Queens, when Doug wants access to a computer at the library, and drives away the girl using it by going on about T-rex's difficulty in masturbating with such small arms.

I see even other species have problems with all their young dudes.

[Really Not All That Bright]

If not useful for grabbing prey, they were probably handy for leaning on a sauropod carcass while munching on the top bits.

[madrugafenn]

Because that's where Jesus attached the saddle and the lasers (pew pew!)?

[cornflakes]

This is all a layman's guess (of course, I would like partial credit if it gets published. )

Looking at Colibri's pic (and the image in Wikipedia), the humerus attaches with a pretty sturdy ball and socket. There are large flanges on the scapula (clavicle?) which seem to imply that the arms could be swung forward with some force over a wide range of motion. That said, the curve of the claws seems to indicate that the arms were mainly used for ripping backwards and it seems that most artwork shows the latissimus as being well developed.

The arms may have been used for mating, but it seems like they are well positioned to protect the neck when fighting other tyrannosaurs. The neck seems to be the likely spot for a kill. The chest is protected by the ribs. The abdomen is relatively low to the ground, and it is protected by the legs and feet. By comparison, the neck is a large, soft, vital area that is out in the open.

Without the arms, there is no effective defense for the neck. A T-Rex would have to move their entire bodies to defend with their teeth, and using the feet to defend would likely result in their falling down in front of their opponent.

[Cheesesteak]

Balance, as shown in this highbrow documentary.





Ok, it's Caveman starring Ringo Starr, but the T-Rex vs Gravity scene is one of my all time favorites.

[Son of a Rich]

Originally Posted by cornflakes

This is all a layman's guess (of course, I would like partial credit if it gets published. )

Looking at Colibri's pic (and the image in Wikipedia), the humerus attaches with a pretty sturdy ball and socket. There are large flanges on the scapula (clavicle?) which seem to imply that the arms could be swung forward with some force over a wide range of motion. That said, the curve of the claws seems to indicate that the arms were mainly used for ripping backwards and it seems that most artwork shows the latissimus as being well developed.

The arms may have been used for mating, but it seems like they are well positioned to protect the neck when fighting other tyrannosaurs. The neck seems to be the likely spot for a kill. The chest is protected by the ribs. The abdomen is relatively low to the ground, and it is protected by the legs and feet. By comparison, the neck is a large, soft, vital area that is out in the open.

Without the arms, there is no effective defense for the neck. A T-Rex would have to move their entire bodies to defend with their teeth, and using the feet to defend would likely result in their falling down in front of their opponent.

How would those (relatively) small arms defend against gigantic T-rex jaws? Seems like there would end up being a lot of armless T-rexs*.

*what's the plural of T-rex?

[Chronos]

Quoth Colibri:

It's not a mechanistic process, but there are selective forces that will tend to cause organs that are not being used to disappear. Since the organs are not being used, mutations that interfere with their functionality will not be selected against. But it costs energy to make and maintain useless organs. Individuals that lose them will require less food and be at a competitive advantage against others that retain them. So it's really a balance between how strong this selective effect is versus the developmental programs that make the organs.

Y'know, folks talk a lot about the energy cost of useless limbs or organs, but I suspect that the entropy cost is a lot more significant. Organs become nonfunctional simply because it's a lot easier for one random mutation to break them than it is for another random mutation to fix them once broken. Even if, say, blind fish have no direct benefit over sighted fish, all else being equal, you'd still get fish losing their vision in lightless environments, just by the statistics.

[TriPolar]

Originally Posted by Cheesesteak

Balance, as shown in this highbrow documentary.

Ok, it's Caveman starring Ringo Starr, but the T-Rex vs Gravity scene is one of my all time favorites.

Actually I think balance is a pretty good reason for the size of the arms. It explains why the arms did not get any smaller or larger. If they helped to 'get it on', longer arms would not be a hindrance to that, nor would longer arms hinder eating, or rising from the ground. But I doubt the underlying sequence of events that eventually resulted in the short armed T Rex are all that simple. There's always the possibility that T Rex's long armed cousins were more fit, but were standing too close to an erupting volcano.

[Colibri]

Originally Posted by Chronos

Y'know, folks talk a lot about the energy cost of useless limbs or organs, but I suspect that the entropy cost is a lot more significant. Organs become nonfunctional simply because it's a lot easier for one random mutation to break them than it is for another random mutation to fix them once broken. Even if, say, blind fish have no direct benefit over sighted fish, all else being equal, you'd still get fish losing their vision in lightless environments, just by the statistics.

Yes, the entropy cost will cause them to become non-functional faster than the energy cost will cause them to disappear entirely, which is was part of my point about the eyes of blind cave fish.

[Scumpup]

One paleontologist that I know personally conjectured that they were used in caring for young. I don't know (does anybody?) if T-Rex cared for its young or not; but, if it did, then the relatively tiny arms seem more suited to the task than the jaws or legs.

[Really Not All That Bright]

Originally Posted by Scumpup

One paleontologist that I know personally conjectured that they were used in caring for young. I don't know (does anybody?) if T-Rex cared for its young or not; but, if it did, then the relatively tiny arms seem more suited to the task than the jaws or legs.

No tyrannosaur nest has ever been found, but lots of other theropods appear to have cared for their young, at least to the point of incubating the eggs.

[cornflakes]

Originally Posted by Son of a Rich

How would those (relatively) small arms defend against gigantic T-rex jaws? Seems like there would end up being a lot of armless T-rexs*.

*what's the plural of T-rex?

Maybe so, but they look like they could have torn into the sinuses or taken out the eyes of other T-Rex (singular is the same as plural, but you probably already googled it just like me.) Also, they probably would have been used as part of a combined defense. They may have been defense against smaller predators. In any case, it does look like they were well developed for slashing.

If their only purpose was sexual reproduction, then like other secondary sexual characteristics I would expect that we would see greater development in one gender (e.g., the male T-Rex.)

[Blue Blistering Barnacle]

Please set me straight if I am wrong about this. What I wonder about is why this happens over and over when these theropods get big. It's not just tyrannosaurids, but also the South American abelisaurids (such as Carnotaurus). They've all got weeny arms, in my opinion.

Or maybe T Rex is a code word for all the giant carnivores in this discussion. Allosaurus has pretty short arms, but not so extreme as T Rex. Spinosaurus seems to have decent arms- maybe he needed them to fish? Of course, the much smaller raptors have great arms (but they needed them to evolve wings with).

I once read a suggestion that the arms may have been proportionally longer and more important to juveniles. I have never heard that since and I don't recall any evidence being put forward. I really liked that idea, but I suppose juvenile skeletons must have been discovered which refuted the idea. (Also, Nanotyranosaurus also has teeny arms).

[Sr Siete]

He had arms because they were useful to grab onto things. They were small because he was a large bipedal creature, making the ability to keep their balance a useful survival trait.

[Blue Blistering Barnacle]

Originally Posted by Scumpup

One paleontologist that I know personally conjectured that they were used in caring for young. I don't know (does anybody?) if T-Rex cared for its young or not; but, if it did, then the relatively tiny arms seem more suited to the task than the jaws or legs.

I saw a documentary in which a mother T-Rex smuggled aboard a freighter to rescue its baby which had been kidnapped to LA. It wreaked all kind of havoc, but once re-united with its young, it waddled off peacefully into the credits.

[njtt]

Originally Posted by cornflakes

T-Rex (singular is the same as plural, but you probably already googled it just like me.)

When I Googled it, the plural was Reges.

If we are considering it to be a word fully adopted into English, rather than Latin, then surely it would be rexes.

[Colibri]

Originally Posted by Blue Blistering Barnacle

Please set me straight if I am wrong about this. What I wonder about is why this happens over and over when these theropods get big. It's not just tyrannosaurids, but also the South American abelisaurids (such as Carnotaurus). They've all got weeny arms, in my opinion.

Or maybe T Rex is a code word for all the giant carnivores in this discussion. Allosaurus has pretty short arms, but not so extreme as T Rex. Spinosaurus seems to have decent arms- maybe he needed them to fish? Of course, the much smaller raptors have great arms (but they needed them to evolve wings with).

There's no particular correlation between the size of the theropod and the degree of reduction of the arms. Carnotaurus of South America had arms that were even more reduced than Tyrannosaurus, (Carnotaurus on left) but it wasn't nearly as big. (In this case, the arm bones are much more reduced than in tyrannosaurs, and may have been non-functional. This is an instance in which the arms may in fact be vestigial.)

Spinosaurus on the other hand was possibly the largest theropod of all, but as you say had well developed arms.

Originally Posted by Sr Siete

He had arms because they were useful to grab onto things. They were small because he was a large bipedal creature, making the ability to keep their balance a useful survival trait.

But why were they so much smaller than the arms of most other theropods? That's the question.

[Malleus, Incus, Stapes!]

Originally Posted by Scumpup

One paleontologist that I know personally conjectured that they were used in caring for young. I don't know (does anybody?) if T-Rex cared for its young or not; but, if it did, then the relatively tiny arms seem more suited to the task than the jaws or legs.

Don't crocodiles carry their hatchlings in their jaws? Seems that lots of big sharp teeth don't necessarily get in the way of carrying babies.

[Earl Snake-Hips Tucker]

Originally Posted by njtt

. . .If we are considering it to be a word fully adopted into English, rather than Latin, then surely it would be rexes.

It made Merriam-Webster's main entries, so I'd say that qualifies.
ETA: One thing to consider about the tiny arms is that it is thought that the "hands" could not rotate downward, that they pretty much always faced each other. (IIRC)

[NoClueBoy]

From some other Doper, a long time ago in a similar thread:


To throw them up in the air,

And wave them like he just don't care!




Going green by not searching.

[Sr Siete]

Originally Posted by Colibri

There's no particular correlation between the size of the theropod and the degree of reduction of the arms. Carnotaurus of South America had arms that were even more reduced than Tyrannosaurus, (Carnotaurus on left) but it wasn't nearly as big. (In this case, the arm bones are much more reduced than in tyrannosaurs, and may have been non-functional. This is an instance in which the arms may in fact be vestigial.)

Spinosaurus on the other hand was possibly the largest theropod of all, but as you say had well developed arms.



But why were they so much smaller than the arms of most other theropods? That's the question.

"Much smaller" is a relative term. Theropods tend to have all small arms. Some had a straighter posture than others, meaning that a Spinosaurus probably used his arms to run four-legged, will T-Rex'es were pretty much just bipedal.

[Colibri]

Originally Posted by Sr Siete

"Much smaller" is a relative term. Theropods tend to have all small arms. Some had a straighter posture than others, meaning that a Spinosaurus probably used his arms to run four-legged, will T-Rex'es were pretty much just bipedal.

It would have been impossible for Spinosaurus to have "run four-legged"; the arms are much too small for that, and are not articulated in such a way to make quadrupedal locomotion feasible for any distance. T. rex was not just "pretty much" bipedal; it was obligately bipedal.

T. rex's arms are proportionally smaller than those of most other theropods, making the question of why they were like that of some interest. Just saying that all theropods had small arms doesn't contribute anything to the discussion.

[Fear Itself]

Originally Posted by TriPolar

Then how do you explain Johnny Damon?

Hah!

[electronbee]

Has anyone examined the tendon and muscle indentations left on the bones? I'm just wondering if the typical range and direction of movement had been determined.

Also, maybe there were flaps, like wings, attached to the abdomen and the lower part of the arms. Not to fly, but to spread open the flap in a display of mating courtship the way that some ground dwelling birds do with their wings. This is also why I ask about the examination of the bones as the possibility of a flap might be apparent.