The ancient 'devil ducks' could have been doomed by their sluggish development.

• stirtoni leg bones
• flightless bird groups
• modest reproductive rates
• Dromornis stirtoni
• giant birds

Mihirungs, unable to fly, were the giant birds ever to roam what is now Australia. Around 40,000 years ago, the last of these heavy beasts became extinct. Now scientists may have a better sense of the reason.

Researchers explain in the August 17 issue of the Anatomical Record that the birds may not have been able to survive the continent's human population growth and expansion because of their modest reproductive rates.

Because of their enormous size and shared evolutionary ancestry with modern waterfowl and game birds, mihirungs have earned the nickname "devil ducks." Those bird-like creatures that fed only on vegetation lasted for about 20 million years.

In that period, several species achieved the status of titans. A good example is Stirton's thunderbird (Dromornis stirtoni). It was the biggest mihirung yet discovered, standing at over 3 meters in height and potentially weighing over 500 kilograms. It lived about 7 million years ago.

The anatomy of mihirungs and their ancestry have been the primary areas of study. Anusuya Chinsamy-Turan, a paleobiologist from the University of Cape Town in South Africa, notes that very little is known about the biology of these species.

Twenty fossilized D. stirtoni leg bones were collected by Chinsamy-Turan and his team from animals at different phases of life at Flinders University in Adelaide, Australia.

However, despite the apparent spring in their stride, the growth of more recent mihirungs remained slower than that of the emus with whom they shared their habitat. In less than a year, emus mature into enormous adults and begin breeding, depositing huge clutches of eggs.

The team suggests that this distinction is to blame for the extinction of G. newtoni and the survival of emus in Australia, respectively. Mihirungs seem to have evolved through millions of years to grow and reproduce more rapidly than in the past. However, this was not enough to ensure the species' survival until the advent of humans, who likely consumed the birds and their eggs.

Chinsamy-Turan warns that "slowly developing animals suffer catastrophic repercussions" due to their diminished capacity to recuperate from environmental stresses.

The scientists found that other big, extinct, flightless species assumed to have been wiped off by humans developed slowly (SN: 8/29/17). This includes the dodos of Mauritius (Raphus cucullatus) and the largest of Madagascar's elephant birds (Vorombe titan).

Thomas Cullen, a paleoecologist at Carleton University in Ottawa who was not involved in the current research, says, "It is extremely intriguing to see this trend recurring again and again with many huge, flightless bird groups."