Large bones of the extinct “thunderbird” or dromornithid, unearthed in the northern foothills of the Flinders Ranges and near Alice Springs, have provided new insights into their slow breeding patterns.
Studies of the microstructure of these giant Australian fossil bones by the University of Cape Town (UCT) and Flinders University vertebrate paleontologists indicate that their size and breeding cycle gradually changed over millennia, but ultimately failed to keep up with the changes. in the environment around them.
“Unfortunately for these amazing animals, already facing increasing challenges from climate change as Australia’s outback became hotter and drier, their breeding biology and size could not match the faster breeding cycle of modern (smaller) emus to keep up with these more demanding environmental conditions,” said Professor Anusuya Chinsamy-Turan, from UCT, South Africa.
“Questions, such as how long it took these giant 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 the microscope so that we could identify the biological signals contained within them. The microscopic structure of their bones gives us information about how long it took to reach adult size, when they reached sexual maturity, and we can even see when the females were ovulating.”
The research, published in The anatomical recordcompares the bones of the oldest and largest mihirung (the Aboriginal name for the bird), Dromornis stirtoni, who lived 7 million years ago, could grow up to 3 meters in height and weigh up to 600 kg – to the smallest of the ratites, Genyornis newtoni – the last species of mihirung – which lived alongside early emus, now the world’s third largest bird.
The study indicates that Dromornis stirtoni — arguably the largest bird to ever live on Earth — took a long time to grow to full body size and reach sexual maturity, possibly up to 15 years.
By the late Pleistocene of Genyornis newtoni, the climate was much drier with more seasonal variation and unpredictable droughts. These birds grew six times larger than emus with a body weight of about 240 kg, but grew to adult size more quickly than the first mihirung, probably within 1-2 years and started breeding soon after.
However, it took them several more years to reach adult body size and therefore their growth strategy was still quite slow compared to almost all modern birds that reach adult size in one year and are able to breed in the second year of their lives.
Flinders University Associate Professor Trevor Worthy of Flinders Paleontology adds that dromornithids were contemporaries of emus for a very long time before the last mihirung became extinct.
“In fact, they stayed together through several major environmental and climatic disturbances,” he says. “However, although Genyornis was better adapted than its ancestors and survived two million years of the Pleistocene when dryness and drought were the norm, it was still a slow-growing and slow-breeding bird compared to the emu.
“The different breeding strategies of emus and dromornithids gave the emu a significant advantage when these birds crossed paths with humans about 50 thousand years ago, while the last of the dromornithids became extinct about 40 thousand years ago.
“Eventually the mihirungs lost the evolutionary race and a whole order of birds was lost from Australia and the world.”
Although the bones of the late Pleistocene dromornithides show that their reproductive biology had responded to ever-changing climate pressures, breeding earlier than their ancestors, the strategy did not approach the reproductive efficiency shown today by large ratites.
For example, emu grows to full adult size and breeds within 1-2 years. This kind of breeding strategy allows their populations to recover when favorable conditions return after periods of drought or food shortages that can cause population decline.
Giant fossil flightless bird had a massive body but was still ‘bird brain’
Anusuya Chinsamy et al, Osteohistology of Dromornis stirtoni (Aves: Dromornithidae) and the biological implications of the bone histology of the Australian mihirung birds, The anatomical record (2022). DOI: 10.1002/ar.25047
Provided by Flinders University
Quote: Breeding biology of giant Australian mihirung birds headed for extinction (2022, Aug 23) retrieved Aug 23, 2022 from https://phys.org/news/2022-08-biology-giant-australian-mihirung-birds.html
This document is copyrighted. Other than fair dealing for personal study or research, nothing may be reproduced without written permission. The content is provided for informational purposes only.