Correlation brain size and muscle volume

I have 3 dogs that weigh ≈ 5 kg, ≈ 20 kg and ≈ 45 kg. The smallest dogs whole head would fit in the mouth of the biggest dog. Although I haven't measured it exactly but by just looking at the dogs I would expect the weight of their brain to approximately correspond to their body weight, that is the 20 kg dog's brain is 4 times heavier than the smallest dog and the 45 kg dog's brain is 10 times heavier.

Meanwhile, all three dogs seem to have similar intelligence.

What is all the extra weight of the brain of the bigger dogs used for?

You can expand this question and note that an average mouse brain weigh 0,5 g while an elephant weighs around 5 kg. Elephants might be smarter than a mouse but not 10 000 smarter. Besides, the brain of a human is much lighter than an elephant and we obviously are much smarter.

Why do bigger animals need bigger brains than smaller animals to accomplish approximately the same intelligence? Is it something along the lines that they have more muscle fibres and every single musle fibre needs a few neurons to control it?

• No real expertise in neurobiology, but I know that there are factors to consider other than just mass. For example, the proportion of the brain that is cerebral cortex varies considerably. Also, the density of synapses (connections between neurons) is also not constant. You may find this article helpful: mosaicscience.com/extra/does-brain-size-matter Jul 12, 2019 at 18:03
• You may be interested in this question: biology.stackexchange.com/questions/61476 and the links therein. Jul 13, 2019 at 17:46
• Jul 15, 2019 at 11:08
• Sorry to get philosophical, but if we do not understand consciousness, then how can we define 'intelligence' and 'smartness' in objective, measurable ways? Furthermore, we can assume abstract ideas like intelligence are physically real, albeit this does not mean we are necessarily correct. Sep 28, 2023 at 10:03

I like the muscle_fiber/more_neuronal_mass theory. It makes a lot of sense having a better networked neural structure if you have more efferent processes to look at (in the form of more muscle fiber to actually move). After a quick search, it doesn't seem like this is a very thoroughly researched field. This review from 2014 states that:

• there seems to be a positive relationship between whole brain volume and white matter volume and muscle size
• grey matter (GM) volume had no direct relationship with muscle size, yet some areas of the GM were linked to muscle size (more specifically the right temporal pole and the bilateral ventromedial prefrontal cortex)
• no relationship exists associating grip strength and whole brain volume
• gait speed is positively linked to whole brain volume

The relationships between regional areas of the brain and their functions seem to be something dynamically non-linear enough to prove quite complex to classify properly.

The "extra weight", represented by a larger cerebral cortex, is where the neural integration happens and a study (and possibly others) confirms that cortical thickness is correlated with general intelligence in both hemispheres (samples = [children, adolescents, young_adults]).

References:

Kilgour AH, Todd OM, Starr JM. A systematic review of the evidence that brain structure is related to muscle structure and their relationship to brain and muscle function in humans over the lifecourse. BMC Geriatr. 2014 Jul 10;14:85. doi: 10.1186/1471-2318-14-85. PMID: 25011478; PMCID: PMC4105796.

Menary K, Collins PF, Porter JN, Muetzel R, Olson EA, Kumar V, Steinbach M, Lim KO, Luciana M. Associations between cortical thickness and general intelligence in children, adolescents and young adults. Intelligence. 2013 Sep;41(5):597-606. doi: 10.1016/j.intell.2013.07.010. PMID: 24744452; PMCID: PMC3985090.

A fast rummage through the web tells me (alot) that:

The most intelligent dog is a medium dog breed, the border collie.

Big dogs are generally more intelligent than small dogs, only 1/20 of the most intelligent dog breeds are small dogs.

bigger dogs have better short-term memory, able to focus better and have better self control, coordination end executive functioning.

They also have: More surface area with nerves and skin cells more muscle fibers.

Primate studies suggest that bigger brains have better executive functioning and not other types of intelligence.

• I don't think the causality goes in that direction. Working dogs are smarter than other dogs, especially working dogs that need to interact with humans. Medium size is a good size for working dogs that should be handled by humans, while their size still makes them efficient in their function.
– d-b
Sep 28, 2023 at 22:19
• poodles are rated in the top 20 most intelligent breeds, and bulldogs are towards the less intel, I don't know about wolves and foxes, that's tricky to test for. Oct 2, 2023 at 6:34