If you could go back in time to near the end of the Miocene epoch, around 8 million years ago, and pop along to the north of Australia, you might see some peculiar, fast and – above all – massive birds sprinting across the horizon. These beasts were known as dromornithids, and their largest member, Dromornis stirtoni, was 3 meters (9.8 feet) tall and weighed around 500 kilograms (1,102 pounds) – 10 times that of an emu.
Having gone extinct around 20,000 years ago, along with plenty of other megafauna, much about these flightless birds is a mystery. With this in mind, a new study, published in the Journal of Vertebrate Paleontology, has perhaps solved one of the greatest enigmas of all: What on Earth was its sex life like?
As it turns out, it was something like that of a contemporary goose’s; they were likely monogamous, having picked out a mate for life. Both parents shared parental care and both aggressively defended their nests. The international team of palaeontologists, led by researchers at Flinders University (FU), managed to work all this out from a series of bone fragments.
“With their formidable height and huge size, especially of males that may have exceeded 600 kilograms [1,323 pounds], they were capable of protecting the growing chicks from predators, such as the dog-sized ancestral thylacines and marsupial lions that were around at the time,” Warren Handley, the lead author of the study and a palaeobiologist at FU, said in a statement.
Australia’s Northern Territory during the time of these giant birds was a warm, wet place with coastal forests. Piotr Krzeslak/Shutterstock
The highly fractured and fragmented bones of multiple D. stirtoni – also known as mihirungs or thunder birds – were found by the team in a shrinking watering hole in Alcoota within Australia’s Northern Territory. This remarkable site contained a treasure trove of fossils. Alongside the giant birds, the well-preserved remains of several long-gone marsupials, cow-sized herbivores and lion-like beasts could also be seen within the cornucopia of worn-down corpses.
Separating the D. stirtoni bones from the rest, the team initially thought they had discovered two new species of flightless bird. On closer examination, they realized that one set belonged to a male and the other to a female. Specifically, specialized tissue was found solely within the hollow, smaller bones of the females.
This tissue doesn’t just identify females from males – its existence within the medullary cavity is indicative of a female about to lay eggs. The medullary bone tissue, as it is known, serves as a calcium reservoir for building the hard parts of an eggshell.
At the very least, this showed that these demon ducks were sexually dimorphic, in that the males were noticeably larger than the females. The massive size of these birds also suggests that the less robust females primarily guarded the eggs; the presence of multiple males at the site would imply that they stuck around for much of their life, helping to guard any eggs or younglings. If this was the case, then it’s likely that these birds mated for life.
An artist’s impression of an adult guarding its young. James Robins/DeAgostini/Getty Images
Based on the behavior of modern equivalents, including the goose, the researchers assume that the males often fought using ritualized combat in order to assert dominance, and that the most dominant males “defended family groups and priority territories with a vengeance,” backed up by their physically strong female mates.
Although much of this is speculation based on a rather limited data set, palaentologists regularly compare extinct species to their modern counterparts in order to ascertain their behaviors. In any case, if what they claim is true, you wouldn’t have wanted to come face-to-face with one of these agile and aggressive dromornithids. They aren’t sometimes called “demon ducks of doom” for nothing.