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Why do frogs have fused forearms?

Human forearms are made out of two bones – the ulna and the radius. These bones are separated in humans, giving us a range of movement that’s lacking in many other creatures. 

Goats, horses, and bats have semi-fused ulnae and radii, probably for running and flying, respectively. In frogs, these two forearm bones are completely fused, which is thought to be an adaptation for jumping. However, this explanation fails to consider the fullness of frog biodiversity. 

“Many people think frogs jump, and a lot of them do. But there are also frogs that live their entire lives underground; there are frogs that glide; there are arboreal frogs that mostly climb like monkeys; there are frogs who primarily live underwater; and then there are frogs that just mostly walk. Frogs have evolved to move in so many ways, but the radius and ulna bones are always fused together,” explained Dr. Rachel Keeffe with the Florida Museum of Natural History.

Dr. Keeffe wondered if there was a better explanation for the forelimbs of frogs. For instance, during mating, male frogs clasp females with their forelimbs and also fight other males for access to females. These activities could explain the beefier forelimbs.  

To test this idea, Dr. Keeffee used museum specimens to create forelimb models. She modeled the leg of a bullfrog, a frog known for its jumping prowess. Next, she developed other models with varying degrees of fusion between the ulnae and radii, including one model where the bones were separate, another where they were partially fused, and a third where there was a single bone. 

Dr. Keeffee ran software simulations on these different models to mimic the stress of jumping. Other simulations were used to mimic the stress of mating on a forelimb.  

Surprisingly, in both jumping and mating scenarios, less stress was put on the semi-fused model than on the fully fused forelimbs frogs possess.         

Rather than disproving her hypothesis, Dr. Keeffee interprets her results as a sort of evolutionary compromise. Although the bones have more stress when they’re fused, these bones are thicker and lighter, possibly for better jumping. 

As for mating, the stress is also more on the fused bones; however, with two unfused bones, the single tendon could be attached to only the ulna or radius, not both, making a weaker forearm than the fused bone. Dr. Keefe hopes to explore this idea with further model research. 

This study was published in the Journal of Anatomy.

By Erin Moody , Staff Writer

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