Which animal has the largest brain relative to its body size?

With a weight of 8 kg, the amber whale (Physeter macrocephalus) has the largest brain on average, but the total body weight of this animal is 45 tons and the ratio of brain to body weight is 1:5100. But which animal has the largest brain relative to its body size? A study published in 2009 in the journal Brain, Behavior and Evolution showed that a very small genus of ants has the largest brain relative to its body size.

Ants of the genus Brachymyrmex have an average body weight of 0.049 mg and an average brain weight of 0.006 mg. Therefore, the brain of this ant is approximately 12% of its body weight, and thus, the ratio of brain to body weight is about 1:8.

Why do animals evolve bigger brains?

In absolute terms, the size of an animal’s brain usually increases with the animal’s body size. Larger brain size is typically associated with three factors. Sophie Scott“Maternal investment, behavioral complexity and body size,” said Professor of Cognitive Neuroscience at University College London. Scott to Live Science “Having a bigger body means you need more control over it,” he said. Predators at the top of the food pyramid are usually large and benefit from larger brains due to the need for more complex behaviors such as the ability to lure prey.”

But brain size is not a perfect predictor of animal intelligence. According to a study published in Frontiers in Neuroanatomy in 2014, the African elephant brain (Loxodonta africana) has an average weight of 4.6 kg, which is three times larger than the human brain. According to Scott, their large brain size is partly due to their large cerebellum, which is used to coordinate muscle activity in their trunks and ears.

Ant / Brachymyrmex

The brain to body weight ratio of Brachymyrmex ants is 1:8.

Just as absolute brain size is not a good predictor of animal intelligence, brain-to-body ratio comparisons cannot be an indicator of intelligence either. According to a 2009 study published in the journal Frontiers in Human Neuroscience, humans and rodents have a roughly 1:40 brain-to-body weight ratio. However, the same study argued, if mice were human-sized, they wouldn’t be as smart because they have a smaller cortex (the outermost area of ​​the brain associated with the most complex mental functions) and fewer neurons than humans. Scott said:

If you look at the brains of a rabbit, a cat, and a small monkey, they are not that different in size, but their behavior will be very different because of the nature of the brain cells. If you examine a monkey, you see a primate brain with a larger frontal lobe and more curiosity-driven behaviors.

Scott explained that evolutionary adaptations change the structure of the brain, so that some areas increase in size and some neural connections are preferred. In humans, the size of the cerebral cortex and the density of cortical neurons relative to the size of the brain relative to the size of the body explains more about our intelligence. According to Scott: “Compared to other animals, we have a small body for the size of our brain.”

Cross section of sloth brain

A cross-section of a sloth’s brain on display in a museum

When comparing brains of different species, it is important to consider brain structure as well as brain size. Since the ratio of brain to body size does not take into account the evolutionary development of the cortex and the density of neural connections in that area of ​​the brain, scientists consider the brain quotient (EQ) as a more accurate measure of human intelligence. Brain coefficient is the relative size of the brain of a particular species compared to the expected brain size of other species with similar body size.

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One of the key influencing factors in EQ is the relative size of the cortex compared to the rest of the brain. According to an article published in the Encyclopedia of Behavioral Neuroscience, comparing animals based on their EQ provides a more accurate picture of their intelligence than brain-to-body ratio, although it is not as accurate as determining the absolute size and interaction of individual brain regions.

Then there’s a concept called Haller’s Law: the larger the animal, the smaller the brain-to-body ratio. Wolfila Grunenberg“As brain size is measured relative to body size, relatively speaking, the smallest animals have the largest brains,” said Professor of Neurobiology at the University of Arizona.

For example, ants have relatively small brains compared to anteaters, a group that includes bees, wasps (honeyless bees), bumblebees, and sawflies. Grunenberg said:

We think it’s because worker ants don’t fly. Flight requires a lot of visual processing, so many flying insects have large eyes, resulting in larger visual lobes. In some insects, such as millipedes, visual processing involves more than half of their entire brain.

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