This small bird’s big egg sheds light on the extinct birds’ evolution.
Tucked away in a sand dune on King Island, off the southern coast of Australia, researchers have discovered the first almost-complete egg of the island’s extinct dwarf emu.
The small bird (Dromaius novaehollandiae minor) barely reached a metre in height and was less than half the size and weight of the mainland emu (D. novaehollandiae). Yet the King Island egg, reported in the journal Biology Letters, retained its large size.
This was likely because evolution favoured larger, more mature emu chicks who could survive on the island, the researchers suggest. Bigger chicks would be able to forage for limited food resources and maintain a warm body temperature against the cold.
“This scenario provides an interesting evolutionary response to island size, insular population and morphological plasticity in dwarf emus,” write Julian Hume, from the UK’s Natural History Museum, and King Island historian Christian Robertson.
They note a correlation between the degree of rapid evolutionary dwarfing and island size, comparing the King Island emu with those that split off on the larger Kangaroo Island (D. n. baudinianus) and Tasmanian (D. n. diemenensis) landmasses.
Knowledge of the birds’ life histories is rather checkered as they all faced rapid extinction at the hands of settlers. “All were victims of over-hunting by human colonists,” write Hume and Robertson.
They were first recorded by Europeans following a French expedition led by Nicolas Baudin, who transported two chicks to Paris. These two survived until 1822 while their remaining brethren had become extinct by 1810.
What is known was derived from Franꞔois Péron and his questioning of a sealer who lived on the island. The new egg discovery enabled Hume and Robertson to shed more light on the enigmatic birds.
Analysing it alongside 38 eggs from mainland emus, six from Tasmania and a unique one from Kangaroo Island, they were able to estimate body sizes. Results showed the King Island emus were 42% to 44% smaller than their mainland relatives, with the others in between.
The analysis also suggested that “dwarf emus had a comparable breeding strategy to mainland emu that includes a large clutch size, synchronised hatching of young to counter predator effects and thermos-regulation in hatchings to provide warmth”.
“It was only on the southern Australian islands that limited resources resulted in rapid dwarfing and retention of a large egg.”
They say more research is warranted, but further historical insights into the dwarf emus have been thwarted by destruction of the richest fossil remains by the development of a large golf course.
“[D]ue to their complete and rapid extinction, the true extent of these adaptations to a rapidly changing environment brought on by fluctuating sea levels is now impossible to determine.”