The recently extinct Malagasy elephant birds included the largest birds ever discovered. Seven species are recognized across two genera, including the larger Aepyornis and the smaller, gracile Mullerornis. The neuroanatomy of these enigmatic birds is understudied but can shed light on their lifestyle. In a study published in the journal Proceedings of the Royal Society B, researchers from the University of Texas at Austin examined the neuroanatomy of two elephant bird species, Aepyornis maximus and A. hildebrandti, and found that the part of the elephant bird brain that processed vision was tiny, a trait that indicates they were nocturnal and possibly blind.
Elephant birds were large, flightless and lived in what is now Madagascar until a mixture of habitat loss and potential human meddling led to their demise between 500 and 1,000 years ago.
“Humans lived alongside, and even hunted, elephant birds for thousands of years,” said lead author Christopher Torres, a Ph.D. candidate at the University of Texas at Austin.
“But we still know practically nothing about their lives. We don’t even really know exactly when or why they went extinct.”
Ornithologists had previously assumed that elephant birds were similar to other big, flightless birds, like emus and ostriches — both of which are active during the day and have good eyesight.
But Torres and his colleague, University of Texas at Austin’s Professor Julia Clarke, revealed that elephant birds had distinctly different lifestyles through reconstructions of their brains.
By using CT-imaging data of the skulls, they were able to create digital brain reconstructions called endocasts.
In addition to the elephant bird skulls, they also created endocasts for close relatives of the elephant bird, both living and extinct.
In both elephant bird skulls, the optic lobe — a bundle of brain nerves that controls eyesight — was very small, with the structure almost absent in the larger species, Aepyornis maximus.
“The lobe had the most in common with that of the kiwi, a practically blind, chicken-size denizen of New Zealand, which came as a total shock because of the kiwi’s poor vision and nocturnal behavior,” Torres said.
“No one has ever suspected that elephant birds were nocturnal. The few studies that speculated on what their behavior was like explicitly assumed they were active during the day,” he said.
In addition to vision, the endocasts rendering of the olfactory bulb — the part of the brain that processes the sense of smell — helped shed light on the habitats where elephant birds lived.
The larger Aepyornis maximus had a large olfactory bulb, a trait associated with forest dwelling.
In contrast, the smaller Aepyornis hildebrandti had a smaller olfactory bulb, possibly indicating that it lived in grasslands.
The smaller species also appears to have somewhat keener vision, which means it may have been more active at dusk than during the pitch black of night.
“Details like these not only tell us about what the lives of elephant birds were like, but also what life in general was like on Madagascar in the distant past,” Professor Clarke said.
“As recently as 500 years ago, very nearly blind, giant flightless birds were crashing around the forests of Madagascar in the dark. No one ever expected that.”
Christopher R. Torres Julia A. Clarke. 2018. Nocturnal giants: evolution of the sensory ecology in elephant birds and other palaeognaths inferred from digital brain reconstructions. Proc. R. Soc. B 285 (1890); doi: 10.1098/rspb.2018.1540