Bird Brains Have as Many Neurons as Some Primates

If you own a parrot, you may owe it an apology. “Bird-brained” isn’t an insult: although a bird’s brain may seem diminutive, its small size only makes its mental capacities more impressive.

Scientists have known for some time that birds are capable of complex cognition—as tool-making crows and discriminating pigeons reveal. The question was how nature had gotten all that neural hardwiring into such a small package. Research published this month in the Proceedings of the National Academy of Sciences offers a new explanation: bird brains pack neurons more densely than those of other animals.

For a long time, the prevailing idea was that brain size most directly determined an animal’s intelligence and that neuron density didn’t vary much across species. In the last ten to twenty years that perception has changed, especially in mammalian brains where differences in neuronal density have been observed. Bird brains are so small that they clearly require some structural difference to allow them to carry out complex cognition in a compact space, but until now it wasn’t clear precisely how they differed.

The new study comes out of an international collaboration between universities in Prague, Vienna, Rio de Janeiro and Sao Paulo, and is the first to quantify the neurons in the avian brain. To do so, the researchers took samples from 28 different species and dissected them into anatomical sections. They then made homogeneous suspensions from each section, stained the nuclei, and divided each into tiny individual samples of suspended cells. These smaller samples each contain approximately the same number of cells, making it relatively easy to quickly and accurately count neurons. Using this method, an entire avian brain can be processed in one day in contrast to the old fashioned approach, which was much more labor- and time-intensive. (Historically, neuroanatomists had to cut very thin sections of the brain, staining them to show individual cells and counting every cell by hand. Across an entire brain, this method required a huge amount of time and had a high risk for human error.)

Although many in the field expected the bird brain could be densely packed, the extent came as a surprise to the study authors. “My expectation was simply that bird brains should be different from mammals in size and number of neurons,” says neuroscientist Suzana Herculano-Houzel, now at Vanderbilt University, one of the senior authors on the paper. “But we didn’t have any idea that the difference would be so extreme that in a parrot brain you would have as many neurons as in a mid-size primate.”

Previous research on bird intelligence in songbirds and parrots—birds with impressive vocal capabilities—has shown that there are particular pathways in bird brains that enable this complex cognition. But these pathways are made up of millions of neurons, which seemed impossible for such small brains to accomodate. “[It seemed like] there must be something that’s lost to make space for this pathway,” says neurobiologist Erich Jarvis at the Howard Hughes Medical Institute, who studies these neural pathways and did not participate in the new research. “This paper gives me an explanation: these birds didn’t lose neurons for another trait, they just packed things into a tighter space.”

For the Birds?

The findings by Herculano-Houzel and her colleagues are just part of a larger effort to better understand bird brain anatomy and intelligence. Until the mid-twentieth century many scientists held the bias that birds were incapable of advanced cognition.

In fact, even the names given to bird neuroanatomy signal their perceived simplicity. A century ago, neurobiologist Ludwig Edinger compared the neuroanatomies of various animals so that he could assemble a more cohesive view of how their brains evolved. But Edinger’s understanding of evolution wasn’t accurate; he believed that evolution happened linearly in time and space. He thought that primitive areas of the brain must be at one edge of the brain and over time the more complex parts developed in layers on top of each other.

Bird brains have a very different structure from most other intelligent animals, which led Edinger to think that they must not be as smart as mammals. So when he went about naming bird brain structures, he gave them names like “paleostriatum” meaning “old striatum,” referring to a fairly primitive area involved in instinctual behaviors and basic motor coordination. He assumed that the brain had evolved from the striatum. But in fact, we now know it evolved from the pallium, an area that carries out reasoning and planning.

In 2005 an international group of experts argued in Nature Reviews Neuroscience that the derogatory nomenclature was hindering scientific advances because it inaccurately reflected the brain’s sophistication. Their plea ultimately led to new terminology for use in the research community. This change reflects a modern, nuanced view of bird evolution to which Herculano-Houzel hopes her work contributes. She and her colleagues chose to study a variety of species so that they could “get a glimpse into evolution,” she explains. “The idea is that if you understand what changes and what doesn’t change between modern species, then you could do the detective work to figure out what the first bird brains were made of.”  

There’s still work to be done to prove that these densely packed neurons are directly impacting intelligence, says Jarvis. Nonetheless, he finds the new work impressive: “I hesitate to use the word breakthrough, but I think it’s a conceptual breakthrough.”

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