How do birds keep their balance?

Could it be with their butts?

We balance using special organs deep within our inner ears called semicircular canals (AKA semicircular ducts). Semicircular canals are three C-shaped tubes lined with tiny hairs and filled with a special fluid. When we move, that fluid sloshes around, moving the hairs and giving our brain information about how we’re oriented in 3-D space.

We balance using semicircular canals.
*Source: Wikimedia*

Birds also have semicircular canals — but they’re really, really good at balancing. So good, in fact, that some have suggested another region of their body is also helping them keep their balance — their butts!

Birds’ rear ends house a special organ called a lumbosacral organ (LSO), which is a modification of the spinal cord. Specifically, this organ is found in a region of the backbone called the synsacrum. The LSO is made up a few different pieces, but two notable ones are a general swelling of the spinal cord and some special nubbins that stick out from it called accessory lobes. But how is a blobby butt brain supposed to help with balance?

Birds balance using semicircular canals like us (A), but do they also use their butts to balance (C — LSO)?
*Source: Stanchak et al., 2020*

(Okay, fine, it’s not an actual brain. Though people claimed the dinosaur Stegosaurus had a butt brain for centuries. Turns out that was probably just a LSO, too.)

Quacking the case

In a new article out in Integrative Organismal Biology, K.E. Stanchak and colleagues from the University of Washington explore the hypothesis that the LSO plays a role in balance. The researchers started by taking micro-computed tomography (uCT) scans of synsacra (lower vertebral columns) from a bunch of different species of birds. The LSO occupies a cavity inside the synsacrum — so, Stanchak and colleagues created a 3-D endocast, or model of the internal shape, of each synsacrum to see what the shape of the LSO would be like in life.

An X-ray slice of a uCT scan, and the resulting synsacral endocast representing LSO shape, for a Humboldt penguin.
*Source: Stanchak et al., 2020*

Once they had all of their 3-D endocasts ready to go, the researchers measured a lot of different metrics to describe the shape of each species’ LSO. They found that all the species they study had an LSO, but that the shape of the LSO varied substantially among species. Ultimately, they found that species that do a lot of perching, like resplendent quetzals and tawny frogmouths, had really prominent transverse processes, indicating big accessory lobes (nubbins). They also found that flamingos, notorious for their ability to balance on one leg, had a really big LSO overall.

Perching birds have prominent nubbins sticking out of their LSOs (high LSTC Prominence).
*Source: Stanchak et al., 2020*

Because Stanchak and colleagues found that birds that perch do seem to have a more exaggerated LSO, they concluded that there is support for the hypothesis that the LSO plays a role in balance in birds.

Moving forward

In their paper, the authors raise several questions about how exactly the LSO helps birds balance. How do the different pieces of the LSO help birds sense both rotational and translational acceleration? Does the LSO contain specialized mechanosensory cells like the ones inside the ear’s semicircular canals? If yes, where are they located? How is information from the LSO combined with information from the semicircular canals when it reaches a bird’s brain? How does relative importance of the LSO compare with that of other balance boosters, like the passive mechanisms built into some species’ limb anatomy?

As these additional aspects of LSO function are unraveled, we can get closer to understanding when the LSO evolved and how it has contributed to the evolution of bipedal (two-legged) walking in birds — and in turn, to their current ecological diversity. Until then, make sure to keep checking out iobopen.wordpress.com for the latest in cool, organism-centered biology!

By Armita R. Manafzadeh

Armita R. Manafzadeh is a PhD candidate studying the dynamic arthrology of the archosaur hindlimb at Brown University. Her interests include functional morphology, vertebrate paleontology, and biomechanics.