If more nerve cells mean more smarts, then dogs beat cats, paws down, a new study finds. That harsh reality may shock some friends of felines. However, scientists say the real surprises came from the brains of less popular carnivores. Raccoon brains are packed with nerve cells, for instance. But brown bear brains are sorely lacking.
The researchers tallied the numbers of nerve cells, or neurons, in eight species. The ferret, banded mongoose, raccoon, cat, dog, hyena, lion and brown bear are all carnivores. Comparing how many neurons each hosted in their brains gave the scientists a better understanding of how different-sized brains are built. This neural tally appears in a December 12 Frontiers in Neuroanatomy paper. Ultimately, such data may also help reveal how brain features relate to intelligence.
For now, the multispecies nerve-cell count raises more questions than it answers, says Sarah Benson-Amram. She’s a zoologist at the University of Wyoming in Laramie. “It shows us that there’s a lot more out there that we need to study to really be able to understand the evolution of brain size and how it relates to cognition,” she says.
Suzana Herculano-Houzel is a neuroscientist at Vanderbilt University in Nashville, Tenn. She and her colleagues gathered brains from the different species. For each animal, they then whipped up batches of “brain soup.” This was brain tissue dissolved in a detergent. To this they added a molecule in this slurry that attaches only to neurons. This let the researchers count the neurons in each bit of brainy real estate.
The cerebral (Seh-REE-brul) cortex is the wrinkly outer layer of the brain that’s involved in thinking, learning and remembering. Based on work with other mammals’ brains, the team knew that there was a fairly predictable link between the size of a carnivore’s cerebral cortex and how many neurons were in it. (Primates are the exception. This group includes monkeys, lemurs, chimps, gorillas and humans. Primate brains tend to pack in lots of neurons without growing bigger.)
In the new study, the expected relationship between brain size and neuron number generally held up. But for some larger carnivores — with correspondingly larger cortices — the neuron count seemed really low. For instance, lions and bears both had fewer neurons than a golden retriever. This canine had 623 million neurons packed into its doggy cortex. (For scale, the human cortex holds roughly 16.3 billion neurons.)
The brown bear’s was surprisingly low. Its cortex is about 10 times bigger than a cat’s. Yet the bear’s cortex contained roughly the same number of neurons as a cat, some 250 million. “It’s just flat out missing 80 percent of the neurons that you would expect,” Herculano-Houzel notes. She suspects that there’s a limit to how much food a big predator like a bear can catch and eat, especially one that hibernates. That caloric limit might also cap the number of its neurons. After all, those cells burn a lot of energy.
Another surprise — but in the opposite direction — was the raccoon. Its cat-sized brain hosts a doglike number of neurons. This fits with the nocturnal mammal’s reputation as a clever problem-solver. Benson-Amram cautions, however, that it’s not clear how brain-neuron numbers relate to potential intelligence. Raccoons are very dexterous, she says. It’s possible that a beefed-up brain region that handles touch, which is part of the cortex, could account for the coon’s high neuron count.
Herculano-Houzel expected large predators such as lions to have lots of neurons. “We went into this study with the expectation that being a predator would require smarts,” she says. But in many cases, a predator didn’t seem to have more neurons than its prey. A lion, for instance, has about 545 million neurons in its cerebral cortex. But the blesbok antelope, one of its prey, has about 571 million, the researchers previously found. And that antelope’s high neuron count was packed into a cortex that was somewhat smaller than the lion’s.
It would be nice to know how neuron count relates to smarts. By counting neurons, “we’ve figured out one side of that equation,” Herculano-Houzel says. Those counts now need to be linked to animals’ thinking abilities.
Some studies have found correlations between brain size, neuron number and problem-solving skills across various species. This includes one by Benson-Amram. But finding ways to measure intelligence across different species is challenging, she adds. “I find it to be a really fun puzzle. But it’s a big challenge to think, ‘Are we asking the right questions?’”
caloric An adjective that means having to do with calories. One calorie is amount of energy needed to raise the temperature of 1 gram of water by 1 degree Celsius. It is typically used as a measurement of the energy contained in some defined amount of food.
canine Members of the biological family of canids. These are carnivores and omnivores. The family includes dogs, wolves, foxes, jackals and coyotes.
carnivore An animal that either exclusively or primarily eats other animals.
cell The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.
cerebral cortex (pl. cerebral cortices) The outermost layer of neural tissue covering the front part of a vertebrate animal’s brain.
cognition The mental processes of thought, remembering, learning information and interpreting those data that the senses send to the brain.
colleague Someone who works with another; a co-worker or team member.
correlation A mutual relationship or connection between two variables. When there is a positive correlation, an increase in one variable is associated with an increase in the other. (For instance, scientists might correlate an increase in time spent watching TV with an increase in rates of obesity.) Where there is an inverse correlation, an increase in one value is associated with a drop in the other. (Scientists might correlate an increase in TV watching with a decrease in time spent exercising each week.) A correlation between two variables does not necessarily mean one is causing the other.
detergent A compound derived from petroleum products, often used for cleaning. Detergents work by breaking up and surrounding dirt particles or oily substances, so that they can be washed away with water.
evolution (v. to evolve) A process by which species undergo changes over time, usually through genetic variation and natural selection. These changes usually result in a new type of organism better suited for its environment than the earlier type. The newer type is not necessarily more “advanced,” just better adapted to the particular conditions in which it developed. Or the term can refer to changes that occur as some natural progression within the non-living world (such as computer chips evolving to smaller devices which operate at an ever faster speed).
feline Adjective for something having to do with cats (wild or domestic) or their behaviors.
ferret A mammal belonging to the family of animals that includes weasels, skunks, otters and badgers.
intelligence The ability to collect and apply knowledge and skills.
lemur Any of many primate species that tends to have a cat-shaped body and usually a long tail. They evolved in Africa long ago, then migrated to what is now Madagascar, before this island became separated from the east coast of Africa. Today, all wild lemurs (some 33 species of them) live only on the island of Madagascar.
mammal A warm-blooded animal distinguished by the possession of hair or fur, the secretion of milk by females for feeding their young, and (typically) the bearing of live young.
molecule An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).
nerve A long, delicate fiber that transmits signals across the body of an animal. An animal’s backbone contains many nerves, some of which control the movement of its legs or fins, and some of which convey sensations such as hot, cold or pain.
neuroanatomy The structure of the brain and nervous system. A person who studies this is a neuroanatomist.
neuron An impulse-conducting cell. Such cells are found in the brain, spinal column and nervous system.
neuroscientist Someone who studies the structure or function of the brain and other parts of the nervous system.
nocturnal An adjective for something that is done, occurring or active at night.
predator (adjective: predatory) A creature that preys on other animals for most or all of its food.
prey (n.) Animal species eaten by others. (v.) To attack and eat another species.
primate The order of mammals that includes humans, apes, monkeys and related animals (such as tarsiers, the Daubentonia and other lemurs).
species A group of similar organisms capable of producing offspring that can survive and reproduce.
tissue Made of cells, any of the distinct types of materials that make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues.
Journal: D. Jardim-Messeder et al. Dogs have the most neurons, though not the largest brain: Trade-off between body mass and number of neurons in the cerebral cortex of large carnivoran species. Frontiers in Neuroanatomy. December 12, 2017. doi: 10.3389/fnana.2017.00118.
Journal: S. Herculano-Houzel et al. Mammalian brains are made of these: A dataset of the numbers and densities of neuronal and nonneuronal cells in the brain of glires, primates, scandentia, eulipotyphlans, afrotherians and artiodactyls, and their relationship with body mass. Brain Behavior and Evolution. Vol. 86, December 2015. doi: 10.1159/000437413.