How can birds - those animals with such tiny brains - perform complicated cognitive behaviors comparable to primates? A new study led by Dr. Suzana Herculano-Houzel from Vanderbilt University finally found an answer on that question: birds have significantly more neurons packed into their brains than are stuffed into mammalian and even primate brains of the same mass.
The scientists 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."
The neurons in avian brains are much smaller and more densely packed than those in mammalian brains. According to the study, parrot and songbird brains contain about twice as many neurons as primate brains of the same mass and 2 to 4 times as many neurons as equivalent rodent brains.
Further, the proportion of neurons in the forebrain of birds is also significantly higher.
"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."