The Brain Champions
Among all animals, the undisputed brain giants are sperm whales (Physeter macrocephalus). Their brains can weigh up to 9 kilograms (20 pounds).
Not only are their brains massive, but sperm whales also demonstrate impressive intelligence. A 2021 study found that after a few hunting encounters, the success rate of harpooners hunting them dropped by nearly 60%. This suggests that sperm whales may have learned to avoid hunters or could communicate this information to others, demonstrating higher levels of intelligence than previously thought.
Brain Composition and Proportions
In assessing brain size, we must also consider brain composition. The cerebellum is a brain structure responsible for movement, balance, coordination, and certain types of memory.
In sperm whales, the cerebellum comprises about 7% of their total brain mass, while in killer whales, it makes up twice that, at 14%. In humans, the cerebellum accounts for about 10% of our average brain mass, which weighs around 1.4 kilograms (3.1 pounds).
The overall body size of an animal also needs to be considered when evaluating the largest brains. By this measure, the winner is the ant Brachymyrmex patagonicus: its brain makes up a whopping 12% of its 0.049-milligram body mass, according to a 2009 study.
This aligns with Haller's rule, which states that the larger an animal's overall size, the smaller its brain-to-body ratio. "Because brain mass scales roughly with body mass, the smallest animals have the relatively largest brain-to-body mass ratios," Wulfila Gronenberg, professor of neuroscience at the University of Arizona, told Live Science.
Small Brains, High Intelligence
A dataset of 1,400 mammalian species (including some fossils) compared their brain and body sizes, revealing some surprising results. California sea lions, known for their impressive juggling abilities, have relatively small brains for their body mass.
In contrast, humans and dolphins have larger brains despite their smaller overall size. In chimpanzees and orangutans, despite being similar in size to humans, their brain sizes are only about one-third as large.
An interesting phenomenon is the domestication syndrome, where domesticated species have smaller brain sizes compared to their wild counterparts. For example, domesticated cattle have brains nearly the size of the now-extinct wild ancestor, the aurochs (Bos primigenius). Beef and dairy cattle, which have more frequent interactions with humans, have the smallest brain sizes.
Land Giants and Avian Intelligence
Among land animals, elephants have the largest brains and exhibit high levels of social intelligence, emotion, and memory.
Let's not forget about birds. Crows, in particular, are renowned for their intelligence. A study by researchers at Lund University found that crows are as smart as apes despite having much smaller brains.
"Brain size is not everything. We found that corvids, despite having much smaller brains, are as smart as great apes," said study author Can Kabadayi.
Summary
While brain size can be an indicator of intelligence, it is not the only factor. Brain composition, body size, and other factors must also be considered. Some animals, like sperm whales and crows, have impressive intelligence despite their brain size. On the other hand,domestication can lead to smaller brain sizes. Understanding the complexity of animal brains provides valuable insights into the evolution and diversity of intelligence in the animal kingdom.