A few years ago various corners of the Internet loved to ask whether someone would rather fight a horse-sized duck or a hundred duck-sized horses. Perhaps those asking knew Australia was in fact once home to a duck relative that weighed as much as a large horse. Anyone considering the merits of fighting Dromornis stirtoni (our suggestion, don't) might be curious to know of their potential opponent's intellectual capacities. Thankfully, answers are at hand.
The eight known species of the mihirung family – an Aboriginal term for "giant bird" – left no fossilized brains for us to study, but we have found a sampling of skulls. Dr Warren Handley of Australia's Flinders University has examined these and found them unusually revealing as to the shape of the brain inside. Since the location of brain regions devoted to different roles is fairly well conserved across animal species, there is much to learn from knowing where the big bits of mihirungs' brains were. Combining this with observations about other parts of the “demon duck of doom”, such as the size and location of its eye sockets, has proven revealing.
“Together with their large, forward-facing eyes and very large bills, the shape of their brains and nerves suggested these birds likely had well-developed stereoscopic vision, or depth perception, and fed on a diet of soft leaves and fruit,” Handley said in a statement. “The shape of their brains and nerves have told us a lot about their sensory capabilities, and something about their possible lifestyle which enabled these remarkable birds to live in the forests around river channels and lakes across Australia for an extremely long time.”
Gastroliths – stones many birds use to break down food – found with some fossils confirm the nature of their diet. The locations at which their bones have been found show the later mihirungs at least never ventured far from waterways.
The mihirungs survived from around 24 million years ago until around 50,000 years ago. The largest, and last surviving, D. stirtoni, grew to 3 meters (10 feet) tall with males weighing 600 kilograms (1,300 pounds). Although some estimates make Madagascar's elephant birds larger still, Handley told IFLScience that using consistent techniques to estimate weight, these were the heaviest birds ever to shake the Earth.
Once thought to have been ratites, a group that includes ostriches and cassowaries, mihirungs are now considered part of the chicken and duck family, a finding Handley's study confirms. As with the other great beasts of the Pleistocene, the cause of the mihirungs' demise is hotly debated, but Handley told IFLScience human arrival in Australia definitely contributed
For most of their time, adult mihirungs had no predators. They didn't get so big to avoid being eaten.
Instead, Handley told IFLScience they were a product of the phenomenon where some herbivores respond to a poor diet by becoming massive. Only through size could they get enough of the low nutrition food available to them to survive, and being so tall at least let them browse on fresh leaves. “Sexual selection would also have contributed,” Handley added, with the largest males winning fights to mate.