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Dogs (and raccoons) have the most neurons

Cat lovers are going to hate this: new research shows that cat brains have only about half as many neurons in the cerebral cortex as dogs. Because this is the part of the brain that allows for complex and flexible cognition, the expectation is that dogs have better cognitive capabilities than cats. The new findings come from Suzana Herculano-Houzel’s group, formerly at the Federal University of Rio de Janeiro, Brazil, now at Vanderbilt University, TN, USA.

While animal trainers and owners often agree that dogs do seem to have a richer behavioral repertoire and to be better than cats at problem solving, evidence of that is far from systematic, much less definitive. The reason is that counting neurons in brains (by turning them into soup, a method developed by Herculano-Houzel) has proved far simpler than putting a number on behavior and cognition so that they can be compared across species.

Still, numbers of neurons in the cerebral cortex have been telling a coherent story. Humans have by far the most of those, even though their cerebral cortex is nowhere near the largest: the African elephant cortex, for instance, has only one third as many neurons (5.6 billion) as the twice smaller human cerebral cortex (which has on average 16 billion neurons). Second place currently goes to gorillas and orangutans, tied at 8-9 billion neurons; chimpanzees, at 6-7 billion neurons, occupy third place in the ranking of Most Neurons in Cerebral Cortex.

The discrepancy between Most Neurons in Cortex and Largest Cortex is due to an evolutionary divergence of primates (ourselves included) from all other non-primate mammals examined so far. Whereas early mammals seem to have added neurons to their cortices that also became progressively larger, primates appeared on the planet with a number of distinctions that included flat fingernails (not claws), binocular vision, and neurons that no longer got larger as they became more numerous. As a result, over evolutionary time, non-primate brains increase in size much faster than primate brains as their cortices gain neurons.

The latest work from Herculano-Houzel’s group, just published in Frontiers in Neuroanatomy, added eight carnivoran species to the still-growing list of animal species whose brains the group has been turning into soup to count their neurons. The simplest expectation was that this new set of species – ferret, banded mongoose, cat, raccoon, dog, hyena, lion and brown bear – should have brains that scaled like any other non-primate brain. On the other hand, large carnivorans such as lions rely on hunting large game, which might impose a heftier cognitive requirement to outsmart prey whose longer legs can outrun them. In turn, others, like cats and dogs, have been domesticated for thousands of years, which is often presumed to alter the relationship between brain and body size in the direction of smaller brains for a given body size (or larger bodies for a given brain size). Would their numbers of neurons also have been affected?

The work, led by Ms. Débora Jardim-Messeder as part of her Master’s degree, shows that most of the examined carnivorans, including domesticated cats and dogs, match other non-primate species in the numbers of brain neurons they have for a given brain structure mass. Cats, with brains that are always smaller than those of dogs, have, as expected, fewer neurons, too: in the cerebral cortex, the authors found around 250 million neurons in the cat, a meager number compared to 430 million neurons in a small dog and an even larger 620 million neurons in a golden retriever. Because both small and large dogs still fit the relationship between cortical volume and number of neurons that apply to all other non-primate mammals, it seems that even though domestication and artificial selection led to dogs of a wide variety of sizes, their brains still scale like any other non-primate species.

That is not at all the case for the raccoon, one of two marked exceptions uncovered by the study. Despite having cat-sized brains, the two wild-caught raccoons had dog-like numbers of neurons in their cerebral cortex, around 400 million. So many neurons in such a small cortex puts raccoons on par with primates. Shrewd and cunning that these masked animals are, they were indeed once classified as monkeys before being acknowledged as carnivorans. Behavioral neuroscientist Prof. Kelly Lambert, from the University of Richmond, VA, who provided the raccoons for the study, has long been amazed by the behavior of raccoons and was pleased to see that they have appropriately large numbers of cortical neurons. Lambert studies rats in her own lab, though; when asked why she doesn’t study raccoons, since they are so clever (whatever that means or however that’s measured), her answer is “because the lab would have to be a maximal-security facility to keep those guys indoors!”.

In the opposite direction is the brain of the brown bear, with far fewer neurons than expected: although it was the largest carnivoran brain examined, at 315 grams (almost 10 times as large as a cat or raccoon brain), it only had about 250 million neurons in the cerebral cortex, like the cat, and almost 50% fewer cortical neurons than the raccoon. It seems unlikely that the particular bear brain examined was abnormal, for it showed no obvious signs of disease, and had exactly as many non-neuronal cells as expected for a mammalian brain of its size (and so did the raccoon brain, by the way).

The researchers suspect that having an enormous body to support is incompatible with sustaining a large number of neurons in the brown bear cerebral cortex. Because these animals are omnivores, they get fewer calories per gram of food ingested compared to other large but meat-eating carnivorans – and yet bears have even bigger bodies: the brown bear examined in the study weighed 350 kg, against the 180 kg of the female lion analyzed. In line with an insufficient diet to keep body and brain going, brown bears hibernate for about six months of the year, which decreases dramatically their energy cost during the winter. The authors speculate that the bear cortex might grow to its full size and then lose a very large number of its cortical neurons, as their energetic cost is not met.

The lion, the second largest carnivoran studied, shows a similar tendency. Although its body was 9 times as large as a golden retriever and its brain was twice the size of the dog’s, the female lion analyzed had only around 500 million neurons in the cerebral cortex, fewer than the golden retriever (and only slightly more than one of the raccoons). Prof. Herculano-Houzel, the senior author in the study, suspects that the meager number of neurons in the lion cerebral cortex, despite its respectable size, is explained by the same problem faced by other large meat-eating carnivorans: running is so energetically expensive that their brains only deem large prey as worthy of purchasing – but that also means that what they give chase to also runs very fast, on long legs, which makes hunting a costly and uncertain enterprise. “They are hailed as kings and queens of the jungle, but all that majesty seems to come at a steep energetic price. It must suck to be a large carnivoran”, says Herculano-Houzel.

The group is now working on a new approach to estimate how much energy the brains of different species cost – without having to bring live animals into the lab.

 

Jardim-Messeder et al. (2017) 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. Front Neuroanat, in press. 

Authors

Débora Jardim-Messeder1, Kelly Lambert2, Stephen Noctor3, Fernanda Marques Pestana1, Maria Eduarda de Castro Leal4, Mads F. Bertelsen5, Abdulaziz N. Alagaili6, Osama B. Mohammad6, Paul R. Manger7, Suzana Herculano-Houzel8,9,10

 

1Instituto de Ciências Biomédicas, Universidade Federal do Rio de Janeiro, Rio de Janeiro, RJ, Brazil, 2Department of Psychology, University of Richmond, VA, USA, 3Department of Psychiatry and Behavioral Science, University of California at Davis Schoold of Medicine, Davis, CA, USA, 4Biosystematics Section, Zoological Museum SNM, Copenhagen, Denmark, 5Centre for Zoo and Wild Animal Health, Copenhagen Zoo, Fredericksberg, Denmark, 6KSU Mammals Research Chair, Department of Zoology, College of Science, King Saud University, Riyadh, Saudi Arabia, 7School of Anatomical Sciences, Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, South Africa, 8Department of Psychology, 9Department of Biological Sciences, 10Vanderbilt Brain Institute, Vanderbilt University, Nashville, TN, USA.

 

 

Carnivoran brains in numbers

 

Species

Body mass

Brain mass

Number of neurons in the brain

Mass of cerebral cortex

Number of neurons in cerebral cortex

Ferret

800 g

5.4 g

404 million

3.1 g

39 million

Banded mongoose

1.5 kg

14.3 g

454 million

9.3 g

116 million

Raccoon

6.2 kg

34.2 g

2.1 billion

24.5 g

438 million

Cat

4.5 kg

34.8 g

1.2 billion

24.2 g

250 million

Dog, average

19.7 kg

86.5 g

2.2 billion

65.5 g

528 million

mutt

7.4 kg

58.4 g

1.8 billion

46.2 g

429 million

Golden retriever

32.0 kg

114.5 g

2.6 billion

84.8 g

627 million

Hyena

40 kg

99.2 g

3.9 billion

67.4 g

495 million

Lion

180 kg

199.0 g

4.7 billion

139.9 g

545 million

Brown bear

350 kg

315.0 g

9.6 billion

222.0 g

251 million