Elephant Birds, The Biggest Avians to Walk the Earth, Were Practically Blind

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Illustration of giant nocturnal elephant birds foraging in the ancient forests of Madagascar at night. John Maisano for the University of Texas at Austin Jackson School of Geosciences.

The African island of Madagascar was, until relatively recently, home to the largest birds that ever lived.

Collectively known as "elephant birds", some species of these recently extinct, flightless avians, grew to heights of around 10 feet tall and could weigh up to 1,800 pounds.

The biology of these massive birds is mostly unknown, but scientists have long thought that they were primarily active in the daytime and had good eyesight like the vast majority of species in their bird group (ratites)—which includes ostriches, emus, rheas, cassowaries and the extinct moa, among others.

However, new research published in the journal Proceedings of the Royal Society B suggests that elephant birds may have been nocturnal as well as practically blind, much like their closest living relative, the kiwi—a chicken-sized, flightless bird found in New Zealand that is the only nocturnal ratite species.

For their study, a team led by scientists from The University of Texas at Austin digitally reconstructed the skulls of two species of elephant birds—which are unique to Madagascar—using scans of real fossils.

Because bird skulls wrap tightly around their brains, the researchers were able to infer the shape of the organ inside. Using their reconstructed skulls, they created digital "casts" of the elephant bird brains, while also doing the same for some close relatives, both living and extinct.

"One of the goals of our lab is to shed light on the group of birds that includes elephant birds," Christopher Torres, a researcher from UT Austin, who led the research, told Newsweek. "Elephant birds represent a really important gap in our knowledge about this really strange group.

"Brain reconstruction using high resolution digital data is a really useful way of gaining tremendous insight to the sensory world extinct birds occupied, and so was an ideal way to gain some insight to what the elephant bird lifestyle was like," he said.

The researchers found that the optic lobe—a region responsible for processing vision—in both elephant bird brain reconstructions was tiny, like the kiwi. In fact, the lobe was almost entirely absent in the larger of the two elephant bird species. This suggests their eyesight was poor, making it likely that they were nocturnal.

"No one has ever suspected that elephant birds were nocturnal," Torres said in a statment. "The few studies that speculated on what their behavior was like explicitly assumed they were active during the day."

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Illustration of giant nocturnal elephant birds foraging in the ancient forests of Madagascar at night. John Maisano for the University of Texas at Austin Jackson School of Geosciences.

The casts also indicated that the birds had large olfactory bulbs—regions where scent is processed—suggesting they had a heightened sense of smell to offset their poor eyesight.

In addition, the researchers noticed slight differences between the two elephant bird species studied, which helped to shed light on the different habitats that they both resided in. For example, the larger of the two had a bigger olfactory bulb—a trait which is associated with living in forests.

In contrast, the smaller species had a smaller olfactory bulb, possibly indicating that it lived in grasslands. While still tiny, its slightly larger optic lobe suggests it had somewhat keener vision, which means it may have been more active at dusk rather than the pitch black of night.

Andrew Iwaniuk, a professor at the University of Lethbridge and expert on bird brain evolution, who was not involved in the latest study, said the new findings were intriguing.

"I was surprised that the visual system is so small in a bird this big," he said in a statement. "For a bird this large to evolve a nocturnal lifestyle is truly bizarre and speaks to an ecology unlike that of their closest relatives or any other bird species that we know of."

The earliest known elephant bird remains are less than one million years old, but studies using ancient DNA recovered from specimens of the animal have suggested that their ancestors split away from their closest relative, the kiwi, more than 50 million years ago, according to Torres.

"We don't really know when or why they went extinct, although best guesses put the timing at less than one thousand years ago," Torres said. "The most commonly suggested causes for their extinction are human activity—for example, hunting and/or habitat destruction—and non-anthropogenic climate change.

"The latter of these is likely the most influential factor, though they are by no means mutually exclusive," he said. "Elephant bird extinction predates the point when humans were having a major impact on global climate. Madagascar's climate was still changing, however, which has been proposed to have led to restriction of the elephant bird's preferred habitat."

Some recent studies have suggested that elephant birds outlived initial contact with humans by many thousands of years, a vastly different story from the moa, which were apparently gone within a handful of centuries after humans first made it to New Zealand.

"Thus, if humans contributed directly to elephant bird extinction, it certainly wasn't catastrophic," Torres said. "Elephant bird habitat may have already been restricted to rather remote parts of Madagascar, possibly inhibiting human contact and minimizing the effects hunting may have played.

"The effects of human hunting were probably further reduced by the elephant birds' nocturnality: If humans were presumably hunting during the day, they were less likely to cross paths with birds that [were] mostly active at night!"

According to Torres, the latest research fundamentally changes the way we imagine elephant birds living their lives.

"Previously, in our mental pictures of the roles elephant birds played on Madagascar, the sun was shining," he said. "Now we know they were foraging in the moonlight, instead."

Furthermore, the findings also shine a light on how the birds became nocturnal in the first place. The study uncovered a previously unrecognized pattern where birds seem to pass through an initial phase of nocturnality characterized by increased sensitivity to light which enables sight in low-light conditions.

"Then, only after the loss of flight, do birds subsequently reduce the visual system and presumably rely instead on a variety of other senses," Torres said. "This last phase is the condition we infer for elephant birds and has been observed by others in kiwi.

"Our research also further emphasizes how important and variable the interplay between sensory systems has been during the evolution of elephant birds and their relatives," he said. "Even though so many of these big, flightless birds look really similar on the surface, they've had vastly different evolutionary stories, and elephant birds may have had the strangest and most unexpected one yet."

This article has been updated to include additional comments from Christopher Torres.