T. rex's Tiny Arms Might Have Helped Them Have Sex

The characteristic small arms of the T. rex might have been used to help them in the throes of passion.

Scientists in Argentina have discovered a new species of dinosaur predator that has similar little arms to a T. rex, which they think it may have used to have sex, leading them to suggest that T. rex evolved them for the same reasons.

According to a study, published in the journal Current Biology, the fossils of the newly discovered dinosaur species Meraxes gigas is the most complete of its kind found so far in the Southern Hemisphere. The huge dinosaur is thought to have been around 36 feet long, with a 4-foot skull, and lived around 100 million years ago. The fossil includes a nearly complete forelimb that appears very similar to those seen in T. rex fossils, measuring in at only 2 feet long: half as big as its skull.

t rex
Stock image of two T. rex. New evidence suggests that T. rex's tiny arms may have evolved for a specific purpose, and that purpose might be sex. iStock / Getty Images Plus

This led the scientists to believe that Meraxes used its skull and mouth as its main predatory weapons, and because its arms couldn't even reach its mouth, the little arms must have served some other purpose: such as sex.

"I'm convinced that those proportionally tiny arms had some sort of function. The skeleton shows large muscle insertions and fully developed pectoral girdles, so the arm had strong muscles," Juan Canale, lead author of the study, told the BBC.

"They may have used the arms for reproductive behavior such as holding the female during mating or support themselves to stand back up after a break or a fall."

T. rex arms were originally thought to be a useless trait. However, this discovery that Meraxes have the same kind of arms, despite not being closely related to the T. rex, has shed new light on the function of T. rex's little arms.

The researchers found via phylogenetic analysis that the little arms weren't an ancestral trait of a far-off common ancestor of Meraxes and T. rex, rather that they evolved in parallel in the two different lineages.

"This remarkable degree of convergence provides evidence that forelimb reduction was actively selected for in multiple lineages of large predatory theropods that independently evolved," said the authors in the paper.

Convergent evolution, which is when distantly related organisms independently evolve similar traits to adapt to a similar environment, is seen often in nature. For example, echolocation in bats and whales, long necks in sauropod dinosaurs like diplodocus and modern giraffes, and wings in birds and flying insects.

This implies that rather than T. rex's arms being a kind of evolutionary relic left over from previous generations, it evolved specifically for a purpose, that purpose possibly being the same as in Meraxes: sex.