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An illustration of dwarf emus and sea elephants at Sea Elephant Bay on King Island. This illustration was inspired by a woodcut print of the bay from the early 1800s.
The impressively large egg of a dwarf emu — a short and stocky bird that went extinct around 200 years ago — has been unearthed from a sand dune on an island between Australia and Tasmania, a new study finds.
The cracked and empty eggshell is missing a few pieces, but it's a "rare" and "unique" discovery, said study lead researcher Julian Hume, a paleontologist and research associate with the National History Museum, London. It's the only known nearly complete egg from King Island of Dromaius novaehollandiae minor, a dwarf emu that was roughly half the size of the Australian mainland emu (Dromaius novaehollandiae), the only surviving emu Down Under, he said.
The dwarf emu's egg is nearly the size of a regular emu's egg, perhaps because its chicks needed to be big enough to maintain body heat and strong enough to immediately forage for food after hatching, just like the kiwi does today, Hume said.
The islands off southern Australia used to be home to three emu subspecies: the smaller Tasmanian emu (D. n. diemenensis) and two dwarf emus, the King Island emu and the Kangaroo Island emu (D. n. baudinianus).
During the last ice age, when sea levels were lower, these islands were connected to the Australian mainland. Once the ice age's glaciers melted and sea levels rose around 11,500 years ago, the islands became separated, Hume said. When these emus became isolated on their respective islands, they rapidly shrunk in an evolutionary process known as insular dwarfism. The smaller the island, the smaller these emus got.
Image 1 of 2
The dwarf emu (bottom) was 44% smaller than the mainland emu (top), but their eggs were roughly the same size.(Image credit: Julian P. Hume)
A virtual image of the extinct dwarf emu next to today's living emu.(Image credit: Julian Hume & Christian Robertson, Biology Letters (2021))
Egg hunt
While doing fieldwork, Hume and his team met study co-author Christian Robertson, a natural historian on King Island who has a vast collection of emu remains. His collection's crown jewel was an emu egg from King Island.
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"He found all the broken pieces in one place, so he painstakingly glued them back together and had this beautiful, almost complete emu egg," Hume told Live Science. "The only one known in the world [from the King Island dwarf emu]." When Robertson invited Hume to study it with him, Hume said, "Yes please."
During the last ice age, these islands were connected with Australia's mainland. Once sea levels rose around 11,500 years ago, the emus on the different islands became isolated and evolved into new subspecies.
(Image credit: Julian Hume & Christian Robertson, Biology Letters (2021))
The team analyzed the egg's dimensions, as well as the measurements of 36 eggs from the mainland emu, six from Tasmania and one from Kangaroo Island, and femurs from each type of emu. Despite the adult emus' size differences, their eggs were remarkably similar: The mainland emu's egg weighed 1.3 lbs. (0.59 kilograms) and had a volume of about 0.14 gallons (539 milliliters), while the King Island dwarf emu's egg weighed 1.2 lbs. (0.54 kg) and had a volume of 0.12 gallons (465 mL), the team found.
From left to right: The egg of a mainland emu, Tasmanian emu, Kangaroo Island emu and King Island emu. All the emus, except the mainland bird, are now extinct. Scale bar, 10 mm.
(Image credit: Julian Hume & Christian Robertson, Biology Letters (2021))
So, as the dwarf emu shrank over time, retaining a large egg size must have been evolutionarily advantageous, Hume said. Similarly, the kiwi, a bird native to New Zealand, lays the largest egg in comparison with body size — one egg can take up to 25% of its mother's body. "That tactic is because the kiwi has to produce a chick that is ready to go," in terms of being able to feed itself and be big enough to maintain body heat, Hume said.
"That's exactly what the King Island emu was doing," Hume said. Large chicks may have also stood a better chance against predators, including the quoll, a carnivorous marsupial, he said.
The King Island dwarf emu went extinct within about five years of humans arriving there, Hume said. The last surviving King Island dwarf emus — a male and female taken to Paris — died in 1822.
The study was published online Wednesday (May 26) in the journal Biology Letters.
Laura is the managing editor at Live Science. She also runs the archaeology section and the Life's Little Mysteries series. Her work has appeared in The New York Times, Scholastic, Popular Science and Spectrum, a site on autism research. She has won multiple awards from the Society of Professional Journalists and the Washington Newspaper Publishers Association for her reporting at a weekly newspaper near Seattle. Laura holds a bachelor's degree in English literature and psychology from Washington University in St. Louis and a master's degree in science writing from NYU.
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