Ounce For Ounce, Birds Have More Neurons In Their Brains Than Mammals

A few of the brains analyzed in this study. Brain mass (red), total number of neurons in millions (yellow), and the number of neurons in their forebrains (blue). Suzana Herculano-Houzel/Vanderbilt University
Janet Fang 14 Jun 2016, 14:00

Birds like crows and parrots have demonstrated their cognitive prowess time and time again. But despite being remarkably intelligent, birds have small brains. A macaw, for example, has a brain the size of a walnut. Now, researchers studying brains from dozens of species reveal that bird brains contain very large numbers of small, tightly-packed neurons. Ounce for ounce, birds have as many neurons in their brains as primates. The findings are published in Proceedings of the National Academy of Sciences this week.

Among their many cognitive feats, parrots and corvids (crows and ravens) have been known to make and use tools, problem solve, recognize themselves in mirrors, and plan for their future needs. Additionally, songbirds are capable of vocal learning – something only a few species (including humans) have a capacity for, and parrots not only learn human words, they can use them to communicate with us. Yet the brain of a common raven is 15.4 grams on average, and that of the hyacinth macaw is 24.7 grams. Small brains are expected to have a lower information-processing capacity.

A team led by Pavel Nemec of Charles University in Prague wanted to compare neuron numbers across birds and mammals of similar body sizes. They measured the number of neurons in the brains of 28 bird species: 11 parrot species, 13 vocal learning songbird species (including six corvids), and four other birds (an emu, barn owl, red junglefowl, and a pigeon). A few of these brains are pictured above.

"In designing brains, nature has two parameters it can play with: the size and number of neurons and the distribution of neurons across different brain centers,” study co-author Suzana Herculano-Houzel of Vanderbilt University said in a statement. “And in birds we find that nature has used both of them." The team found that bird brains have more neurons than mammal brains – and even primate brains – of similar mass, and they have higher neuronal densities too. The brains of parrot and songbird species contain about twice as many neurons as primate brains of the same mass and two to four times as many as that of rodent brains. Furthermore, compared to mammals, a greater proportion of bird neurons are located in the forebrain, the area associated with intelligent behavior.

While the relationship between intelligence and number of neurons isn’t firmly established, these findings demonstrate that there’s more than one way to design a brain. “Bird brains show that there are other ways to add neurons: keep most neurons small and locally connected and only allow a small percentage to grow large enough to make the longer connections,” Herculano-Houzel explained. “This keeps the average size of the neurons down.”

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