What humans don’t know about the massive elephant bird could fill a book. And even what we know might need some serious revision.

New information published in the Proceedings of the Royal Society B takes what little we know and turns it on its head. The fossil record has already established that these extinct creatures were 10 feet tall and lived alongside humans. Now we know they were doing all that while practically blind and living in darkness.

According to a brain reconstruction study published Tuesday, it’s fair to say that the giant elephant bird wasn’t so different from the still-living, 18-inch-tall kiwi bird. The kiwi is the elephant bird’s closest living relative, and it too is flightless, nearly blind, and nocturnal. That is about where the resemblance ends — but just knowing the similarity fills in a crucial gap in knowledge.

“They are truly crazy creatures”

Chris Torres, a Ph.D. candidate at the University of Texas at Austin and lead author of this study, tells Inverse that while these birds only went extinct in the past millennium and coexisted with humans on Madagascar for over 9,000 years, their biology has been poorly understood.

“They are truly crazy creatures,” Torres says. “Studying elephant bird biology is crucial to helping us understand things like what life was like for gigantic birds that aren’t around anymore, what the ancient Malagasy ecosystem was like, and the evolution of the larger group that includes elephant birds as well as ostriches, kiwi, and relatives.”

Even within the family known as elephant birds, there was significant diversity: three genera of elephant bird, which comprise four different species of the giant avian. Through the use of CT-imaging data of elephant bird skulls, the scientists digitally reconstructed the brains of two of these species, Aepryonis maximus and Aepryonis hildebrandti. Then they compared these reconstructions, called endocasts, to other endocasts based off the skulls of close relatives to the elephant bird, like the kiwi bird.

Bird skulls are a bit different those of humans — their bony structure is wrapped tightly around the brain, and each curve and turn corresponds to a different brain structure. Because of this quirk of nature, Torres and his colleagues were able to determine that the elephant bird’s optic lobe was quite small.

This was astonishing to the researchers because the optic lobe is one of the most important regions of a bird’s brain for processing visual input. No one had ever suspected that elephant birds were nocturnal, yet here was a small optic lobe; an indicator that this 1,000-pound creature was something that ancient humans would have encountered in the darkness of primeval forests.

“We noticed that, among living birds, optic lobes were only tiny in species that are both flightless and nocturnal, like the kiwi and kakapo,” Torres says. “Thus, we hypothesize that elephant birds, which were very obviously flightless, were also nocturnal.”

The elephant bird was a big boy with a tiny optic lobe. Wikimedia Commons

But a small optic lobe doesn’t just suggest that these birds were nocturnal — it also indicates that they may have been blind. The optic lobe is an external feature of the brain that plays a major role in the tectofugal visual pathway, the dominant of the two major visual pathways in birds. As Torres noted, the extreme reduction of optic lobes is something that’s only been seen in nocturnal, flightless birds like the kiwi and kakapo. Nocturnal birds that fly, meanwhile, have a very sensitive visual system that allows them to navigate in low-light conditions.

Which makes sense — if you’re an owl flying through the forest, you need to be able to see. Flightless birds on islands, however, don’t necessarily need to see because environmental elements have driven an alternate evolution where other senses have been prioritized. In 2017, scientists found that some kiwi birds aren’t just partially blind, they are entirely blind, and they seem to be doing perfectly fine. They survive by using their senses of touch, smell, and hearing — something the elephant bird might have done as well.

The question that remains is this: What environmental elements drove the elephant bird to be blind and nocturnal? A nocturnal lifestyle is normally an evolutionary response that happens when it’s too dangerous to come out during the day or when what you want to eat is only coming out at night. But strangely enough, the elephant birds were herbivores with no natural predators. Torres says scientists haven’t pinpointed the answer yet, but he speculates that at least two factors could be at play.

“First, elephant birds probably inherited some degree of nocturnality from the ancestor they shared with kiwi,” he explains. “Second, competition between species may have caused some species to continue further down the nocturnal evolutionary pathway than others.”

Elephant birds might have also relied on their nocturnal lives to avoid the pesky issue of humans. Torres says while this explanation is less likely that the others, it’s still possible that being nocturnal at least helped them avoid the hunters for a life-saving spell.

Abstract:

The recently extinct Malagasy elephant birds (Palaeognathae, Aepyornithiformes) included the largest birds that ever lived. Elephant bird neuroanatomy is understudied but can shed light on the lifestyle of these enigmatic birds. Paleoneurological studies can provide clues to the ecologies and behaviours of extinct birds because avian brain shape is correlated with neurological function. We digitally reconstruct endocasts of two elephant bird species, Aepyornis maximus and *A. hildebrandti, and compare them with representatives of all major extant and recently extinct palaeognath lineages. Among palaeognaths, we find large olfactory bulbs in taxa generally occupying forested environments where visual cues used in foraging are likely to be limited. We detected variation in olfactory bulb size among elephant bird species, possibly indicating interspecific variation in habitat. Elephant birds exhibited extremely reduced optic lobes, a condition also observed in the nocturnal kiwi. Kiwi, the sister taxon of elephant birds, have effectively replaced their visual systems with hyperdeveloped olfactory, somatosensory and auditory systems useful for foraging. We interpret these results as evidence for nocturnality among elephant birds. Vision was likely deemphasized in the ancestor of elephant birds and kiwi. These results show a previously unreported trend towards decreased visual capacity apparently exclusive to flightless, nocturnal taxa endemic to predator-depauperate islands.