Reproductive biology of Australian giant mihirung birds paved the way to extinction

Reproductive biology of Australian giant mihirung birds paved the way to extinction

Artist’s impression of Dromornis stirtoni, arguably the largest bird to ever live on Earth. Credit: Peter Trusler

Large bones of the extinct ‘thunderbird’ or dromornithid, discovered in the northern part of the Flinders Ranges and near Alice Springs, have yielded new insights into their slow reproductive patterns.

Studies of the microstructure of these giant Australian fossil bones by vertebrate paleontologists from the University of Cape Town (UCT) and Flinders University indicate that their size and reproductive cycle have gradually changed over millennia, but do not ultimately couldn’t keep pace with the environmental changes around them.

“Unfortunately for these amazing animals, who were already facing increasing challenges from climate change as Australia’s interior became hotter and drier, their biology and breeding size could not match the breeding cycle any longer. modern (smaller) emus to keep pace with these more demanding environmental conditions,” says Professor Anusuya Chinsamy-Turan, from UCT, South Africa.

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

“We studied thin sections of the fossilized bones of these thunderbirds under a microscope so that we could identify the biological signals recorded inside. The microscopic structure of their bones gives us information on how long they took to reach adult size, when they have reached sexual maturity, and we can even tell when the females have ovulated.”

The research, published in The anatomical filecompares the bones of the oldest and largest mihirung (the aboriginal name for the bird), Dromornis stirtoni, which lived 7 million years ago, was up to 3m tall and weighed up to 600kg , down to the smallest of the flightless birds, Genyornis newtoni – the last species of mihirung – which lived alongside the first emus, now the third largest bird in the world.

The study indicates that Dromornis stirtoni – arguably the largest bird to ever live on Earth – took a long time to reach full size and become sexually mature, perhaps up to 15 years.

During the late Pleistocene of Genyornis newtoni, the climate was much drier with more seasonal variations and unpredictable droughts. These birds grew six times larger than emus with a body mass of around 240 kg, but reached adult size faster than the first mihirung, probably within 1–2 years and began breeding soon after. .

However, it took them several more years to reach adult size and so their growth strategy was still quite slow compared to almost all modern birds which reach adult size in one year and can reproduce in the second year. of their life.

Co-author with Flinders University Associate Professor Trevor Worthy of Flinders Paleontology adds that dromornithids were coeval with emus for a very long time before the last mihirung disappeared.

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

“The differing reproductive strategies displayed by emus and dromornithids gave the emu a key advantage when the paths of these birds crossed with humans around 50,000 years ago, with the last of the dromornithids having died out there. is around 40,000 years old.

“Ultimately the mihirungs lost the evolutionary race, and an entire order of birds was lost from Australia and the world.”

Although the bones of Late Pleistocene dromornithids show that their reproductive biology had responded to ever-changing climatic pressures and that they reproduced earlier than their ancestors, the strategy did not approach the reproductive efficiency shown by large ratite birds today.

For example, emus reach their adult size and reproduce in 1 to 2 years. This type of reproductive strategy allows their populations to rebound when favorable conditions return after periods of drought or food scarcity that could lead to population declines.

Giant Flightless Fossil Bird Had Huge Body But Still Had ‘Bird’s Brain’

More information:
Anusuya Chinsamy et al, Osteohistology of Dromornis stirtoni (Aves: Dromornithidae) and the biological implications of the bone histology of Australian mihirung birds, The anatomical file (2022). DOI: 10.1002/ar.25047

Provided by Flinders University

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