What’s the Reason Behind the T. Rex’s Surprisingly Small Arms?

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"Whatever the arms are for, they certainly aren’t meant for passing potatoes," jokes paleontologist Kenneth Carpenter of Utah State University in an email. He’s referencing a humorous scene from Gary Larson’s iconic comic "The Far Side," where a family of Tyrannosaurus is depicted at dinner. Despite his efforts, the father dinosaur struggles to pass a bowl of potatoes to his wife, quipping, "Remember, my forearms are just as useless as yours."

Such jokes are common today. Despite its name, which translates to "tyrant lizard king," the Tyrannosaurus rex often faces mockery due to its disproportionately small arms. These limbs have baffled scientists for decades. Even over a century after its discovery, experts remain puzzled as to why such a massive creature — measuring up to 40 feet (12 meters) or more — had forelimbs barely longer than those of an adult human.

If the arms were weak and devoid of muscle, one might assume they were useless. However, evidence suggests a more nuanced reality. Some studies indicate that, based on muscle scars found on T. rex limb bones, a fully grown dinosaur could lift over 220 pounds (100 kilograms) with each bicep. While this seems impressive, it’s relatively modest in context.

Thomas R. Holtz, a vertebrate paleontologist at the University of Maryland, notes that some tend to exaggerate the Tyrannosaurus' upper body strength. "Sure, if a human could curl that much, they’d be considered superhuman," he explains via email. However, this strength only amounts to about 1.25 percent of the dinosaur's total body weight, which could have been around 8 tons (8,000 kilograms). "It’s like a 200-pound (90-kilogram) man boasting about curling 2.5 pounds (1 kilogram)," he adds. "So, while the arm was stronger than ours, it was relatively weak for the animal itself."

Based on this, some experts argue that the Tyrannosaurus' arms were either non-functional or rarely used. However, not all paleontologists agree. Carpenter, for instance, believes these small limbs were highly effective hunting tools.

In 2008, paleontologist Kenneth Carpenter and his colleague Christine Lipkin analyzed five Tyrannosaurus rex wishbones, also known as "furculars," which were documented at the time. Resembling a large boomerang, the furcular is positioned between the shoulder blades. While modern birds use this bone as a spring to aid in flight, the massive T. rex was grounded. Interestingly, three of the five wishbones examined by Carpenter and Lipkin displayed signs of injury, including stress fractures that had healed during the dinosaur’s lifetime. Additionally, one specimen had a humerus ("upper arm bone") with scars suggesting a torn muscle tendon.

Carpenter interprets these findings as evidence that the forelimbs "experienced significant repetitive stress, with occasional bursts of extreme force." He and Lipkin believe the most plausible explanation is that T. rex used its forelimbs to grasp large, struggling prey. A powerful, thrashing victim could have caused fractures in the wishbone or torn arm muscles.

Holtz notes that the prey-wrangling theory is complicated by the fact that juvenile Tyrannosaurus "appear to have had proportionally longer arms" compared to adults. It’s possible that younger dinosaurs relied on their forelimbs to subdue prey before outgrowing this behavior, as Holtz suggests.

Some theories about the purpose of T. rex arms don’t involve hunting. One idea relates to rest—perhaps Tyrannosaurus used its arms to lift itself off the ground after sleeping. Another theory suggests a reproductive role. Henry Fairfield Osborn, who named the species in 1905, proposed that males used their forelimbs to grip their mates during mating. For a massive, two-legged dinosaur, maintaining balance during such activities might have been challenging.

However, as Holtz and others have acknowledged, there isn’t enough evidence to definitively confirm or dismiss these ideas. This uncertainty is a common aspect of paleontological research.

One thing is clear: the small arms of Tyrannosaurus weren’t unique. Many large predatory dinosaurs had disproportionately tiny arms. Close relatives like Asia’s Tarbosaurus and North America’s Daspletosaurus shared this trait.

Interestingly, many small-armed predators weren’t closely related to T. rex. For instance, Carnotaurus, a horned predator from South America, had such underdeveloped arms that it likely couldn’t bend its elbows. Similarly, Gualicho, a recently discovered dinosaur from Argentina, also had notably small forelimbs, despite being distantly related to T. rex.

In summary, the exact purpose of Tyrannosaurus rex’s short arms may remain a mystery. However, this trait evolved independently in multiple carnivorous dinosaur lineages, suggesting it served some advantageous function—though what that was, we still don’t know.