Cat people and dog people long have sparred over which species is smartest.
Team Cat points to the feline's self-reliance and ability to hunt, which isn't great for wildlife but showcases the cunning predator lurking within lap kitties. Cats also clean themselves, relieve themselves in tidy litter boxes - or even toilets - and are generally better at food portion control than their canine housemates.
Team Dog cites the canine's ability to learn complex tasks, especially those that benefit humans. Dogs guide the blind, herd livestock, sniff out explosives and find survivors buried beneath earthquake rubble. They also have strong memories and an impressive capacity to understand human language.
But all this boasting may be unnecessary. The best way to measure cognitive ability is to tally each animal's neurons, says a study published in Frontiers in Neuroanatomy.
Those cells communicate via electrical charge and populate the brain and central nervous system. Neurons process information. While measuring intelligence is incredibly difficult, the researchers believe their method to quantify neurons in an animal's brain, especially in the cerebral cortex, is the most accurate tool to judge capacity for complex thought, said Suzana Herculano-Houzel, a Vanderbilt University neuroscientist.
So which animal comes out ahead?
"Dogs have about twice as many neurons as cats," said Herculano-Houzel, who wrote a book about brains called "The Human Advantage."
But an average dog is larger than the typical cat. Isn't it a given that dogs would have larger brains and more neurons?
The overall mass of one's gray matter isn't what's important, the study found.
The team also examined brains from a domestic ferret, a banded mongoose, a raccoon, a striped hyena, an African lion and a brown bear. While the brown bear's brain was triple the size of the dog's, the dog's had more neurons. The brown bear's neuron count was similar to that of the cat, whose brain is about 10 times smaller.
A cat has 250 million neurons in the cerebral cortex, compared with a dog's 530 million.
Both are dwarfed by the average human being, who clocks in at 16 billion cortical neurons.
But one of the more surprising research results has nothing to do with cats, dogs or people. It's about raccoons, those "trash pandas" long dismissed as vermin or vectors for the rabies virus.
Within the raccoon's cat-sized brain lurks a dog-like number of neurons. So many that if you looked only at neuron count and brain size, you might mistake the raccoon for a small primate.
"And that is saying a lot," Herculano-Houzel said. "Because something that we found previously is that there's a huge difference between how many neurons you find in a primate or in a non-primate brain of the same size."
Research method unproven
The link between neuron numbers and intelligence, however, isn't proven, noted Jessica Perry Hekman, a veterinary geneticist at the Massachusetts Institute of Technology and Harvard's Broad Institute.
"Which isn't to say it's wrong," Hekman said, "but that it's definitely something that they're just starting out with and collecting information on."
The study also had a small sample size. Except for dogs, which had two brains in the study, each other species was represented by only one brain. (Good brains are hard to come by, Hekman acknowledged.)
The study's comparison of domesticated, wild and zoo animals also could influence the results, Hekman said.
Experience affects brain development, especially in early life. Rats raised in pens with lots of enrichment, such as toys or complicated territory to explore, develop more synapses, or connections between neurons, than rats raised in barren pens.
So the life history of the analyzed hyena or mongoose might have played a role in its brain anatomy.
Hekman said an animal's number of synapses, rather than neurons, might be a more accurate measure of its intelligence.
But the biggest problem is that intelligence is a tough nut to crack. It might even be several nuts, as each species has distinct skills and challenges.
"I'm not even really sure we should call intelligence one trait," Hekman said. "It's a lot of different things."
Beast brains still puzzling
Even if this paper isn't the definitive guide to animal intelligence, it reveals some interesting data.
For instance, why did larger carnivores such as the lion and bear have fewer neurons than we'd expect for animals of their size?
Herculano-Houzel said she and her team guessed predators would have significantly more neurons than their prey, because hunting is a challenging way of life. Most lions' hunts fail, and every day is a battle to consume enough calories to make it to the next kill.
A wildebeest, by contrast, can fill up on plants at its leisure and form large herds that minimize its chances of turning into lion lunch.
Yet the team found that lions and hyenas had similar ranges of neurons as prey animals of relative size, such as the blesbok and kudu.
The science of comparing animals is evolving, and Herculano-Houzel said it would be great to consistently incorporate neuronal information with studies of behavior.
It also would be valuable to count the neurons of many brains from the same species to get a better range, as Hekman suggested.
The dog neuron counts came from only two animals, a mixed breed and a golden retriever. Who knows what sorts of differences one might find between Chihuahuas, mastiffs and corgis?
For the moment, it seems the jury is still out on whether dogs or cats are smarter - not that a few million neurons would change a pet owner's mind anyway.