The crouched stance developed to compensate for the growth of larger forelimbs that eventually made flight possible, according to new research that digitally “fleshed out” fossils to show physical changes over the eons as bipedal dinosaurs evolved into birds.
Birds and humans are the most common bipedal species in the modern world, but their legs are strikingly different. Humans are basically straight-legged, which allows their bones to support their resting body weight. But bird legs are bent into a zigzag, putting them in a crouched position that requires much more muscular effort to stand.
“It’s more efficient to bear weight passively, in a straight line down your long bones [like] a pillar,” said Vivian Allen of the Royal Veterinary College‘s Structure and Motion Lab at the University of London, and co-author of a study published this week in the journal Nature.
“In a crouch you have to use your muscles a lot more to resist gravity. Think about how much easier it is to hold something when you’re standing up straight than it is when you are crouched down. So if you were designing an animal, this seems slightly odd from the perspective of mechanical principles.”