Giant Australian birds which lived alongside emus may have become extinct due to slow breeding patterns and belated sexual maturity

Giant birds once roamed all around the world - often taking top spot as an apex predator in their environment. Nowadays, the last of these large, flightless birds are a little more timid - like the third-largest living bird the emu and the largest, the ostrich.

Research published in the Anatomical Record journal digs deeper into the mystery of why some of these giant feathered friends became extinct. Vertebrate palaeontologists from Flinders University and South Africa's University of Cape Town looked at the microstructure of the huge birds' fossil bones dug up in the Flinders Ranges near Alice Springs.

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Among Australia's mihirung (the Aboriginal name for the giant dromornithids) were Dromornis stirtoni and Genyornis newtoni.

Dromornis, or "thunder bird", was the largest and oldest of the mihirung, living seven million years ago, standing up to three metres tall and weighing in at 600 kilograms. Dromornis stirtoni is arguably the largest ever bird to live on Earth.

By comparison, Genyornis newtoni was a "mere" 250 kilograms and stood two metres tall. The most recent of the mihirung, Genyornis would have lived alongside the earliest emus, dying out around 100,000 years ago.

Studying the microstructure of the fossil bones the palaeontologists found the large size of these birds and their breeding cycle couldn't keep pace with environmental changes.

"Sadly for these amazing animals, which already faced rising challenges of climate change as the interior of Australia became hotter and dryer, their breeding biology and size couldn't match the more rapid breeding cycle of modern day emus which were able to keep pace with the more demanding environmental conditions," says Professor Anusuya Chinsamy-Turan, from the University of Cape Town.

"Questions, such as how long these gigantic birds took to reach adult size and sexual maturity, are key to understanding their evolutionary success and their ultimate failure to survive alongside humans," Chinsamy-Turan adds.

"We studied thin sections of the fossilised bones of these thunder birds under the microscope so we could identify the biological signals recorded within. The microscopic structure of their bones gives us information about how long they took to reach adult size, when they reached sexual maturity, and we can even tell when the females were ovulating."

The study indicates that Dromornis stirtoni took a long time to become fully grown and reach sexual maturity - up to 15 years.

Emerging later in the Pleistocene, when Australia's climate had grown drier and more seasonal, Genyornis newtoni became sexually mature after only a couple of years. However, being six times as big as modern-day emus, they still took several more years to reach full adult body size.

Co-author Trevor Worthy, associate professor at Flinders University, adds that mihirung lived alongside emus.

"In fact, they persisted together through several major environmental and climatic perturbations," he says. "However, while Genyornis was better adapted than its ancestors, and survived through two million years of the Pleistocene when arid and drought conditions were the norm, it was still a slow-growing and slow-breeding bird compared to the emu."

Despite the fossil record showing that late Pleistocene dromornithids' reproductive biology had responded to climate pressures by breeding earlier in their lives, they just could not keep up with the changing environment or the reproductive efficiency of large birds which survived. Emus, for example, grow to full size and breed within one to two years. This allows the population to more effectively rebound after droughts and food scarcity.

"The differing breeding strategies displayed by emus and dromornithids gave the emu a key advantage when the paths of these birds crossed with humans about 50 thousand years ago, with the last of the dromornithids goings extinct about 40 thousand years ago," Worthy adds.

"In the end, the mihirungs lost the evolutionary race, and an entire order of birds was lost from Australia, and the world."