The Thunder Birds and the Bees

Every Spring in the nature preserve where I currently work, we are treated to the sight of a dozen or so tiny fluffy Canadian goslings. Everywhere they go, they are accompanied by both parents, who watch over them cautiously and confidently. Getting too close will earn you a display of head-bobbing and a stern hiss from mom or dad. The reproductive behavior of these birds is one of their most fascinating and endearing qualities.

Reproductive behavior is notoriously difficult to glean from fossils, which is a shame given how important sexual traits are in living creatures. How did ancient species select mates? Did they mate for life? Who watched the nest? All of these and more questions often go unanswered. But sometimes we get just the right data, as in the case of a new study on an ancient bird called a mihirung.

Skeleton of Dromornis stirtoni, one of the
largest birds to ever live. 10-feet tall, it
lived in Australia 8 million years ago.
Image from Wikipedia.

Mihirungs were enormous flightless birds that lived in Australia, and the biggest among them was 8 million year-old Dromornis stirtoni (“Stirton’s Thunder Bird”). Standing up to 10 feet tall and weighing more than half a ton, this was one of the largest birds of all time. Originally thought to be relatives of the ratites (ostriches, emus, cassowaries, etc.), recent research has revealed that they belong to the Anseriformes, the group of birds that includes ducks, swans, and those geese at the preserve.

In the new study, researchers analyzed fossils of this enormous species, taking detailed measurements of the shapes and dimensions of these birds’ enormous bones, and found something interesting: the morphology of the mihirung bones fell naturally into two groups, or morphotypes. The bones of one group were noticeably larger and beefier than the second group. Much of the time, this kind of difference might indicate that the fossils actually represent two distinct species, but the bones were otherwise so similar that researchers suspected another explanation.

Many animal species exhibit sexual dimorphism, major differences between sexes, such as the antlers of male deer or the larger breasts of female humans. In quite a number of species today, individuals of one sex are larger than the other, including many birds. Could this be the case with the giant mihirungs? The answer came from deep inside the fossils, in the form of medullary bone, a special type of bone that develops in birds only when they are about to lay eggs. As you can imagine, medullary bone only occurs in females, and in these fossils, it was only found in the smaller morphotype. This is a big clue that the smaller birds were the females, and that the Thunder Birds were sexually dimorphic!

This is a pretty cool discovery by itself, but it also opens the door to a lot of behavioral interpretation. Since sexual dimorphism is usually tied to sexual behaviors, the researchers compared the fossil data to modern birds. In many ratite species, such as emus and rheas, males are the smaller sex, and it is their job to care for nests of eggs. In species such as the musk duck, males are much larger than females, and compete for female attention, but have little or no role in parental care.

In the mihirungs, males are  only 15-20% larger than females, similar to most ducks and geese today. From this information, the researchers suggest the giant Thunder Birds had a similar mating system, including long-term coupling, shared parental duties, and aggressive defense of nests. Even the scientists of this study seem in awe of just how much you can infer (not for certain, of course, but with high likelihood!) using modern statistical techniques and data from living species.

And so my thoughts turn back to the geese at the preserve, and I wonder if a visitor to Australia in the springtime, eight million years ago, might witness a devoted couple of 10-foot tall “Thunder Birds” sticking close to their small (5 feet tall?) fluffy offspring, hissing and bobbing their heads at any creature that dared get too close. With parents like that, I doubt baby mihirungs would have much to worry about!

These are the geese I mention in the article, with mom and dad keeping an eye on
the younglings. Imagine this scene, but with adult birds weighing more than half a ton!

Video by me! 

References: 

Handley et al. 2016. Sexual dimorphism in the late Miocene mihirung Dromornis stirtoni (Aves: Dromornithidae) from the Alcoota Local Fauna of central Australia. Journal of Vertebrate Paleontology [Link - paywall]