The extinct dwarf emus that once roamed Kangaroo Island were bigger than their cousins on King Island but a bit smaller than extinct Tasmanian emus.
Emus that lived isolated on Australia’s offshore islands until the 19th century, including Kangaroo Island, King Island and Tasmania, were smaller versions of their larger mainland relatives – and their overall body size correlated to the size of the islands they inhabited.
Published on April 4 in the journal Biology Letters, this was the surprise finding of a University of Adelaide study that analysed the DNA and bone measurements of the now extinct small emus.
So why did emus on Kangaroo Island and the other islands go the way of the dodo?
Lead author Dr Vicki Thomson is an Australian Research Council Discovery Early Career Research Award Fellow at the University of Adelaide’s School of Biological Sciences.
Dr Thomson said it was most likely the very early settlers including sealers that hunted down the small emus to supplement food supplies.
“There is not much information known about how or when they went extinct,” she said.
“From some of my searching of the literature, it looks like some Tasmanian emus may have lasted until 1865 or so in the wild, but the Kangaroo Island emu might have gone extinct as early as 1827 and the King Island emu as early as 1805.
“On at least the smaller islands, Kangaroo and King Islands, we know they were hunted by the sealers for food, shot or killed by specially trained dogs.”
Australia’s iconic emu is the only living representative of its genus.
The study revealed that the small stature of the now extinct island emus evolved relatively quickly.
While the King Island emu had already been shown to be of the same species as the Australian mainland emu, little was known about the evolutionary relationships of the others.
“Our results have shown that all the island emus are genetically closely related to the much larger mainland emu,” Dr Thomson said.
“The leg bones, a measure of overall body size, show size differences from the smallest, found on King Island, to the largest, on the mainland, with Kangaroo Island and Tasmanian emus in between.
“The smallest, the King Island emus, are typically two-thirds of the size of our mainland emus, with others ranging upwards according to the size of their island.”
Co-author Dr Kieren Mitchell is a post-doctoral research associate at the University's Australian Centre for Ancient DNA.
“This suggests that island size, and presumably the associated reduced food resources available, may have been important in causing smaller body size in island emus,” Dr Mitchell said.
Dr Thomson said more work however needed to be done to confirm the role island size plays in the smaller stature of the extinct island emus.
“We do know that prior to European arrival, Kangaroo Island, King Island and Tasmania had these smaller bodied emus and they would have been isolated from the mainland after sea-levels rose around 10 to15,000 years ago.
“The change in body size thus appears to have happened quite quickly, and independently on each island.”
Researchers used both ancient and modern museum specimens for their research.
Dr Thomson said the Kangaroo Island bone remains were found in Kelly Hill caves, while on remains on King Island they were found in sand dunes.
“Most of the Tasmanian emu remains were from skins held by the British Museum of Natural History, from live animals transported back to Europe in the early days after their discovery,” she said
“However, we also have one set of definitely Tasmanian emu bones held by the Queen Victoria Museum and Art Gallery.
“When I say definitely Tasmanian emu, we know that because they were radiocarbon dated to 799 to 929 years ago.
“As we know some mainland emus were transported to Tasmania we needed to ensure they were definitely Tasmanian emus.”
Kangaroo Island amateur historian Steve Berzel has uncovered a fantastic image of emus by Charles Alexandre Lesueur; artist of the French voyage of Discovery 1800-1804.
The image appears in 'Atlas by Messrs. Lesueur and Petit', 1816. "New-Holland: Isle Decres, Cassowary".
Whether the emus depicted are actually from Kangaroo Island, which was called Isle Decres by the French, or King Island is up for debate.
“There is a hot topic currently circulating that the drawings of the female emu and chicks are from King Island, according to King Islanders that is, yet it has recently found its way onto the Wikipedia site. One contemporary scientist apparently backs this,” Mr Berzel said.
The Wikipedia entry for King Island emus has the following entry:
Péron's 1807, three-volume account of the expedition, Voyage de découverte aux terres Australes, contains an illustration (plate 36) of "casoars" by Charles-Alexandre Lesueur, who was the resident artist during Baudin's voyage.
The caption states the birds shown are from "Ile Decrès", the French name for Kangaroo Island, but there is confusion over what is actually depicted. The two adult birds are labelled as a male and female of the same species, surrounded by juveniles. The family-group shown is improbable, since breeding pairs of the mainland emu split up once the male begins incubating the eggs. Lesueur's preparatory sketches also indicate these may have been drawn after the captive birds in Jardin des Plantes, and not wild ones, which would have been harder to observe for extended periods.
The Australian museum curator, Stephanie Pfennigwerth, has instead proposed that the larger, light-ruffed "male" was actually drawn after a captive Kangaroo Island emu, that the smaller, dark "female" is a captive King Island emu, that the scenario is fictitious, and the sexes of the birds indeterminable.
They may instead only have been assumed to be male and female of the same species due to their difference in size. A crooked claw on the "male" has also been interpreted as evidence that it had lived in captivity, and it may also indicate that the depicted specimen is identical to the Kangaroo Island emu skeleton in Paris, which has a deformed toe.
The juvenile on the right may have been based on the Paris skin of an approximately five-month-old King Island emu specimen, which may in turn be the individual that died on board le Geographe during rough weather, and was presumably stuffed there by Lesueur himself. The chicks may instead simply have been based on those of mainland emus, as none are known to have been collected.