Mihirungs, giant flightless birds, were the largest birds to ever cross the boundaries of what is now Australia. They died out about 40,000 years ago. Now researchers can understand why.
Researchers claim that birds may have developed and reproduced too slowly to withstand the pressures imposed by humans' arrival on the continent on August 17.
Mihirungs are sometimes dubbed "demon ducks" due to their large size and close evolutionary connection with present-day waterfowl and game birds. The flightless, plant-eating birds lived for more than 20 million years.
Some species evolved into titans over a time. Take Stirton's thunderbird (Dromornis stirtoni), which lived about 7 million years ago, was 3 meters tall, and was capable of exceeding 500 kilograms in weight, making it the largest-known mihirung and a contender for the largest bird ever to live.
The majority of research on mihirungs has focused on their anatomy and evolutionary interactions with living birds. Little is known about the animals' biology, such as how long they took to develop and mature, according to Anusuya Chinsamy-Turan, a paleobiologist at the University of Cape Town in South Africa.
Chinsamy-Turan and colleagues at Flinders University in Adelaide, Australia, collected samples from 20 fossilized leg bones of D. stirtoni, from animals of different age groups.
"The microscopic structure of fossil bones generally remains intact, even after millions of years of fossilization," Chinsamy-Turan says, and it may be used to discover valuable clues about extinct animals' biology.
The team examined the thin bone slices under a microscope, revealing the existence or absence of growth marks. These marks reveal how rapidly the bone developed while the birds were alive.
According to the study, D. stirtoni took 15 years or more to mature into a mature form. It probably became sexually mature a few years before that, due to a time when it developed from a rapidly growing bone to a slower-growing form associated with reaching reproductive age.
These findings differ from the team's earlier investigation of the bones of another mihirung, Genyornis newtoni. That species — the last-known mihirung — was less than half the size of D. stirtoni. It lived as recently as about 40,000 years ago and was a contemporary of the continent's earliest human inhabitants.
Researchers claim that this speed at which mihirung species that had been separated by millions of years developed may have been a result of Australia developing a drier, more variable climate over the previous few million years. When resources are unpredictable, growing and reproducing rapidly can be advantageous.
Even so, the apparent jump in the developmental step of older mihirungs was still slower than that of the emus they inhabited. Emus develop rapidly, reaching adult size in less than a year and reproducing long after, laying large numbers of eggs.
G. newtoni became extinct shortly after hungry humans arrived in Australia, yet emus continue to thrive today, according to the research. Even though over millions of years, mihirungs as a group appear to have adapted to growing and reproducing faster than they used to, it was not enough to survive the arrival of humans, who likely ate the birds and their eggs.
According to Chinsamy-Turan, slow-growing animals face severe consequences due to their decreased capacity to recover from environmental dangers.
Other gigantic, extinct, flightless birds that were thought to have come to an end thanks to humans, such as the dodos of Mauritius (Raphus cucullatus) and the largest of Madagascar's elephant birds (Vorombe titan), have all grown fairly slowly (SN: 8/29/17).
“It is interesting to see this pattern repeating again and again with many large, flightless bird groups,” says Thomas Cullen, a paleoecologist at Carleton University in Ottawa who was not involved in the new study.
Modern ratite birds appear to be the exception in their ability to deal with the same pressures, according to the researcher. Other ratites that have survived till today, such as cassowaries and ostriches, also grow and reproduce rapidly (SN: 4/25/14).