Giant kangaroos that once weighed up to 250kg in Australia may have been able to hop, new research shows. Modern kangaroos hop by using their strong achilles tendon, which stores energy. Dr Megan Jones from the University of Manchester led the study. She explained, “When [modern kangaroos] hop, the achilles tendon gets really, really pulled and stressed.” This helps them hop efficiently. But if kangaroos grow bigger without changes, the tendon could snap. Unlike earlier research that guessed based on today’s species, Jones and her team studied actual fossils. They examined giant kangaroo species called sthenurines and Protemnodon, with sizes up to 250kg, far bigger than today’s red kangaroos that max out at 90kg. The researchers measured the strength of the achilles tendon and the fourth metatarsal bone in the hind foot. Jones noted, “It’s no use if their tendon is fine, but their bones are going to start breaking [if they hop].” Their results showed bones and tendons were strong enough to support hopping. Some experts thought thick tendons might prevent hopping, but the study pointed out that other animals with thick tendons, like kangaroo rats, hop well. Jones said giant kangaroos may have hopped, but probably not over long distances or fast speeds. “It is entirely possible that, as well as using hopping more infrequently, or over shorter distances, the giant kangaroos may have reduced stresses by hopping more slowly,” the researchers added. Jones clarified the study only shows hopping was possible, not that it was the only way these kangaroos moved. Some sthenurine kangaroos might have walked on their tiptoes instead. Dr Gilbert Price from the University of Queensland, not part of the study, praised the fossil-based approach. “It shows that the giant kangaroos changed their proportions in ways that made hopping mechanically possible, even if it was less efficient than in modern species,” he said. He added the study didn’t claim they hopped like modern kangaroos, just that it was an option. Price also said understanding these kangaroos’ biology could explain why they went extinct. “Bringing the focus back to their biology and ecology isn’t just extra detail, but is central to understanding what happened.”