Giant flightless birds called mihirungs were the largest birds to ever cross what is now Australia. The animals, which weighed up to hundreds of kilograms, became extinct about 40,000 years ago. Now researchers might have a better idea why.
Birds may have grown and reproduced too slow to bear pressures of human arrival on the continent, researchers report August 17 in the Anatomical Record.
Mihirungs are sometimes called “demon ducks” because of their large size and close evolutionary relationship with modern-day waterfowl and game birds. Flightless herbivorous birds lived for more than 20 million years.
During that time, some species became Titans. Take Stirton’s Thunderbird (Dromornis Stirtoni). It lived about 7 million years ago, stood 3 meters tall and could exceed 500 kilograms in weight, making it the largest known mihirung and a contender for the largest bird that ever lived.
Most of the research on mihirungs has focused on their anatomy and evolutionary relationships with living birds. Little is known about the animals’ biology, such as how long it took them to grow and mature, says Anusuya Chinsamy-Turan, a paleobiologist at the University of Cape Town in South Africa.
So Chinsamy-Turan and colleagues at Flinders University in Adelaide, Australia, sampled 20 fossilized leg bones from D. Stirtoniof animals of different life stages.
“Even after millions of years of fossilization, the microscopic structure of fossil bones generally remains intact,” and can be used to decipher important clues about the biology of extinct animals, says Chinsamy-Turan.
The team examined the thin slices of bone under a microscope, detailing the presence or absence of growth marks. These markings provide information about how fast the bone grew while the birds were alive.
D. Stirtoni it took 15 years or more to reach full size, the team found. It probably reached sexual maturity a few years before that, based on the timing of a change from fast-growing bone to a slower-growing form thought to be associated with reaching reproductive age.
These results differ from those of the team previous analysis from the bones of another mihirung, genyornis newtoni. That species, the last known mihirung, was less than half the size of D. Stirtoni. He lived about 40,000 years ago and was a contemporary of the first human inhabitants of the continent. g newtoni it grew much faster than its giant relative, reaching adult size in a year or two and growing slightly larger in subsequent years and possibly reproducing then.
This difference in how quickly mihirung species developed that were separated by millions of years may have been an evolved response to Australia developing a drier and more variable climate in the last few million years, the researchers say. When resources are unpredictable, growing and reproducing quickly can be advantageous.
Still, that apparent vitality in the developmental pace of the later mihirungs was even slower than that of the emus they lived with. Emus grow rapidly, reaching adult size in less than a year and reproducing soon after, laying a large number of eggs.
This difference may explain why g newtoni it became extinct shortly after starving humans arrived in Australia, but emus continue to thrive today, the team says. Although for millions of years the mihirungs as a group seem to have adapted to grow and reproduce faster than before, it was not enough to survive the arrival of humans, who likely ate the birds and their eggs, the researchers conclude.
“Slow-growing animals face serious consequences in terms of their reduced ability to recover from threats in their environments,” says Chinsamy-Turan.
Scientists’ research on other giant, extinct, flightless birds thought to have been brought to an end by humans, such as the mauritian dodos (Raphus cucullatus) and the largest of the elephant birds of Madagascar (Titan Vorombe) — shows that they also grew relatively slowly (serial number: 8/29/17).
“It’s very interesting to see that this pattern repeats itself over and over again with many large groups of flightless birds,” says Thomas Cullen, a paleoecologist at Carleton University in Ottawa, who was not involved in the new study.
Modern ratite birds seem to be the exception in their ability to handle similar pressures, he says. In addition to emus, other ratites that have survived to this day, such as cassowaries and ostriches, also grow and reproduce rapidly (Serial number: 04/25/14).