Mega beasts are intriguing and equally mysterious is their vanishing from the earth, making them interesting subjects of study. One recent research as per sciencenews.org report investigated mihirungs – giant flightless and plant-eating birds – that weighed hundreds of kilograms and roamed Australia more than 20 million years ago and became extinct 40,000 years ago.
Details of the study published in Anatomical Records suggest that slow growth and reproduction made these birds incapable of keeping pace with the pressure exerted by the arrival of humans on the continent causing their extinction.
These birds are called “demon ducks” too because of their huge size.
As they evolved, some varieties of this duck turned out to be really big. Standing 3 metres tall and weighing more than 500 kilograms, Stirton’s thunderbird or Dromornis stirtoni was found 7 million years ago – giving it the distinction of being the biggest-known mihirung and also in running for the largest bird ever to live.
Majority of the studies on mihirungs concentrate on their anatomy and how they evolved as compared to other birds while very little is known about their biology – how they grew and became mature. Hence, Anusuya Chinsamy-Turan from South Africa’s University of Cape Town along with others from Adelaide’s Flinders University looked at 20 leg bone remains of D. stirtoni belonging to different stages of life.
According to Chinsamy-Turan: “Even after millions of years of fossilization, the microscopic structure of fossil bones generally remains intact.” The thin bone slices reflect the growth marks which show how fast the birds grew when alive.
To reach full size, D. stirtoni took 15 years and more. It is thought that it achieve sexual maturity a few years before, going by the change in bone growth from rapid to slow, which points at attaining reproductive phase.
In contrast the team’s past study of another mihirung, Genyornis newtoni – half D. stirtoni’s size — showed that this species’ growth was quicker and it attained adulthood in one or two years and then started reproducing after growing a bit more. This species was found 40,000 years ago and it was contemporary of Australia’s early human beings.
This species’ fast growth was probably a response to the changing environment in Australia which became drier and variable. Thus, with uncertainty looming large, the creature took growth and reproduction as the way out.
In fact, as compared to emus, even G. newtoni were slower as the former grew even quicker attaining adulthood in less than a year and starting to reproduce not long after that by laying many eggs. This contributed to G. newtoni becoming extinct with the arrival of people while emus are still around.
Even though Mihirungs took to achieving faster growth and reproduction, they could not keep pace with humans who ate them as well as their eggs. Explaining this Chinsamy-Turan said: “Slowly growing animals face dire consequences in terms of their reduced ability to recover from threats in their environments.”
Summing up the finding of the research Thomas Cullen a palaeoecologist at Ottawa’s Carleton University who was not involved with the new study observed: “It is very interesting to see this pattern repeating again and again with many large, flightless bird groups.”
