If terms like “bird brain” and “hare brain” are any indication, animal brains provide fascinating and diverse examples of one of the most complex organs known to science. Animal brains differ not only in overall size, but also in size in relation to the animal’s body mass.
At an average of 18 pounds (8 kilograms), the sperm whale (Physeter macrocephalus) has the largest brain, but has a total body mass of 45 tons (40 metric tons), giving it a brain-to-body mass ratio of 1:5,100. But which animal has the largest brain in relation to its body size?
A 2009 study in the journal Brain, behavior and evolution (opens in new tab) discovered that a particularly small genus of ants has the largest brain for its body size. brachymyrmex has an average body mass of up to 0.049 milligrams and an average brain mass of 0.006 milligrams. That means its brain makes up about 12% of its body mass, giving it a brain-to-body ratio of about 1:8.
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Why do animals develop bigger brains?
In absolute terms, the brain size of animals usually increases with the size of the animal. Larger brains are usually related to three factors: “maternal investment, complexity of behavior and the sheer size of the body,” said Sophie Scott, a professor of cognitive neuroscience at University College London.
“Having a bigger body means you have to control that more,” Scott told Live Science. “Apex predators are usually large. And because of a need for more complex behaviors, such as outsmarting your prey, they benefit from a larger brain.”
But brain size is not a perfect predictor of animal intelligence. The brain of an African elephant (Loxodonta africana) weighs an average of 10 pounds (4.6 kg), according to a 2014 study in the journal Boundaries in Neuroanatomy (opens in new tab), three times larger than the human brain. Their large brain size is due in part to their massive cerebellum, which is used to coordinate muscle activity in their trunks and ears, Scott said.
Just as absolute brain size is not a good predictor of animal intelligence, comparing brain-to-body mass ratios can also be a red herring. Humans and rodents have roughly a similar brain-to-body mass ratio (1:40), according to a 2009 study in the journal Frontiers in Human Neuroscience (opens in new tab). However, the same study claims that if a rat were the size of a human, it would not be as intelligent because it has a smaller cerebral cortex (the outermost part of the brain, which is associated with the most complex mental functions) and fewer neurons there. than people do.
“If you look at the brain of a rabbita cat and a little one monkeythey’re not that different in size, but their behavior will be very different because of the nature of the brain cells,” Scott said. “By the time you get to the monkey, you see a primate brain, with proportionally larger frontal lobe areas and more through curiosity-driven behavior.”
Scott explained that evolutionary adaptations change the structure of the brain to increase the size of certain areas and promote certain neural connections. In humans, the size of our cerebral cortex and the density of cortical neurons (the number of neurons present there) explain our intelligence more than the size of our brain relative to our body. Compared to other animals, “we have quite a small body for the size of our brain,” Scott said.
When comparing brains of different types, it is important to consider both brain architecture and brain size. Because the brain-body mass ratio doesn’t account for the evolutionary development of the cerebral cortex and the density of neural connections found there, scientists look to the encephalization quotient (EQ) as a more accurate measure of animal intelligence. The encephalization quotient is the relative brain size observed in a particular species, compared to the expected brain size of other species with a similar body size. An important factor in EQ is the relative size of the cortex compared to the rest of the brain. Comparing animals based on their EQ gives a more accurate picture of their intelligence than brain-to-body-mass ratio, according to the Encyclopedia of Behavioral Neuroscience (opens in new tab)although not as accurate as measuring the absolute size and interaction of individual brain regions.
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Then there’s a concept known as Haller’s rule: The bigger the animal, the smaller the brain-to-body ratio. “Because brain size scales relative to body size, the smallest animals have relatively the largest brains,” Wulfila Gronenberg, a professor of neurobiology at the University of Arizona, told Live Science.
For example, ants have relatively small brains compared to other hymenoptera, a class that includes bees, wasps, hornets and sawflies. “We think this is because … ant workers don’t fly,” Gronenberg said. Flying requires a lot of visual processing, so many flying insects usually have large eyes, leading to larger optic lobes. “In some insects, like a dragonfly, visual processing takes up more than half of their entire brain,” Gronenberg said.
Originally published on Live Science.