A new study led by Vanderbilt University scientist Dr. Suzana Herculano-Houzel has found that birds have significantly more neurons packed into their brains than are stuffed into mammalian and even primate brains of the same mass.
Birds are remarkably intelligent. Some parrots and corvids achieve primate-like levels of cognition. But how do they achieve impressive cognitive prowess with walnut-sized brains? This image shows a male Cape parrot (Poicephalus robustus). Image credit: Cyril Laubscher.
The study, published online this week in the Proceedings of the National Academy of Sciences, provides a straightforward answer to a puzzle that researchers have been wrestling with for years: how can birds with their tiny brains perform complicated cognitive behaviors?
“For a long time having a ‘bird brain’ was considered to be a bad thing. Now it turns out that it should be a compliment,” Dr. Herculano-Houzel said.
Dr. Herculano-Houzel and co-authors systematically measured the number of neurons in the brains of 28 avian species ranging in size from the tiny zebra finch to the emu.
“We found that birds, especially songbirds and parrots, have surprisingly large numbers of neurons in their pallium: the part of the brain that corresponds to the cerebral cortex, which supports higher cognition functions such as planning for the future or finding patterns,” Dr. Herculano-Houzel said.
“That explains why they exhibit levels of cognition at least as complex as primates.”
That is possible because the neurons in avian brains are much smaller and more densely packed than those in mammalian brains.
Parrot and songbird brains, for example, contain about twice as many neurons as primate brains of the same mass and 2-4 times as many neurons as equivalent rodent brains.
Not only are neurons packed into the brains of parrots and crows at a much higher density than in primate brains, but the proportion of neurons in the forebrain is also significantly higher.
The team argues that avian brains with the same or greater forebrain neuron counts than primates with much larger brains can potentially provide the birds with much higher ‘cognitive power’ per pound than mammals.
“One of the important implications of the study is that it demonstrates that there is more than one way to build larger brains,” Dr. Herculano-Houzel said.
“Previously, scientists thought that as brains grew larger neurons had to grow bigger as well because they had to connect over longer distances.”
“But 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. This keeps the average size of the neurons down.”
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Seweryn Olkowicz et al. Birds have primate-like numbers of neurons in the forebrain. PNAS, published online June 13, 2016; doi: 10.1073/pnas.1517131113
