BrainStuff Classics: Why Did the T. Rex Have Such Tiny Arms?

The Tyrannosaurus rex was a giant predator, but its arms weren't much bigger than a human being's. Learn a few hypotheses about T. rex's arms in this classic episode of BrainStuff.

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Episode Transcript

Speaker 1 (00:01):
Welcome to brain Stuff production of iHeart Radio. Hey brain Stuff,
Lorenvolk bum here with a classic episode from our former host,
Christian Sagar. This one is near and dear to me.
Our topic for the day is why the Tyrannosaurus Rex
have such tiny arms? A brain Stuff it's Christian Sagar.

(00:24):
With a name that literally means tyrant lizard king, you'd
assume that Tyrannosaurus rex would get a bit more respect,
but the giant predators disproportionately small arms have been the
subject of ridicule for decades. They are also a scientific puzzle.
More than one hundred years after discovery of this species,
experts still don't know why a huge animal, one that

(00:47):
could reach links of forty feet or twelve meters or more,
had four limbs that weren't much longer than an adult humans.
If the arms were limp muscle free pegs, it'd easy
to assume that they serve no purpose. However, the evidence
hints at a more complicated story. A few studies have
argued that, judging by the muscle scars left behind on

(01:10):
t rex limb bones, a full grown dinosaur could curl
more than two hundred and twenty pounds or one hundred
rams with each one of their biceps. Then again, this
isn't as impressive as it sounds. Thomas ore Holtz, a
vertebrate paleontologist at the University of Maryland, tells us that

(01:31):
some people like to overrate tyrannosaurus is upper body strength.
That figure from before only translates to about one point
to five per cent of the dinosaurs total body weight,
which may have been in the ballpark of around eight
tons or eight thousand kilograms. Holtz says that's like a
two hundred pound or nine man being proud of the

(01:55):
ability to curl two point five pounds or one kilogram.
On these grounds, A few experts have concluded that Tyrannosaurus's
arms were either functionless or seldom used, but not all
paleontologists by this idea. For his money, Kenneth Carpenter of
Utah State University thinks the little limbs made great hunting tools.

(02:19):
In two thousand eight, he and fellow paleontologist Christine Lipkin
compared the five Tyrannosaurus rex wishbones or ferculars that were
known to science at the time. Shaped like a giant boomerang,
the fercular sits between the shoulder blades. Three of the
five wishbones that Carpenter and Lipkin studied show telltale signs

(02:41):
of injury. Among these were stress fractures, which must have
re healed in life. So what does this mean? While
according to Carpenter, the four limbs were subjected to a
great deal of repetitive stress, which was not uniform or steady. Instead,
there were moments of x e straordinarily great force applied

(03:03):
to the arms. The most likely explanation is that t
Rex used its four limbs to grab hold of large
struggling prey. A plus sized thrashing victim could easily fracture
the carnivores wishbone or at least tear a few arm
muscles loose. Other ideas about the purpose of t rex

(03:24):
arms have nothing to do with subduing victims. One school
of thought involves naptime. Perhaps after a good night's sleep,
Torannosaurus used those arms to push itself up off the ground,
or maybe they had a sexier function. Henry Fairfield Osborne,
the paleontologists who named this species back in nineteen o five,

(03:45):
believed that males used their four limbs to grab hold
of their mates as Holts and others have admitted there
simply isn't enough evidence at this time to conclusively refute
or verify any of these notions. Much is often the
nature of paleontology. Today's episode was written by Mark Fancini

(04:08):
and produced by Dylan Fagan and Tyler Clang. For more
in this and lots of other topics, visit how stuff
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