Does size matter? Comparing big and small brains.

My husband has this really big head, and inside, probably a really big brain. But does that mean he is smarter than me - as he claims?

One of the longest-standing debates in neuroscience is whether larger brains equate to higher intelligence - as early theories assumed that absolute brain size or neuron count determined cognitive ability.

The blog post is about seriously big brains.

Small and big brain studies
Historically, neuroscientists have studied brains in genetically accessible organisms such as fruit flies, zebrafish, and mice.

They have pretty small brains compared to humans, but these small-brained creatures offer rapid breeding cycles and large offspring numbers, making them ideal for genetic studies.

The ability to precisely manipulate their nervous systems has led to groundbreaking discoveries and has revealed remarkable similarities in brain function between insects and mammals, particularly in navigation.

However, using very small brains as models comes at a cost, leaving large-brained species, such as elephants and whales, understudied. That raises questions about how much we can truly generalize findings from small to large brains that have fundamentally different brain structures and developmental patterns.

Big brains: Empty space or super neurons?
A major difference between small and large brains is neuronal density. Tiny brains, such as those of insects and small mammals, are densely packed with neurons, whereas large brains, like those of elephants and whales, have far fewer neurons per cubic millimeter compared to smaller brains.

This has sparked two competing theories:

The "empty-brain hypothesis" suggests that large brains are mostly space, with their lower neuron density limiting cognitive power. This could explain why humans outperform elephants despite the latter having larger brains than us.

The opposing "super-neuron hypothesis" argues that neurons in large brains are more complex and capable, compensating for their lower numbers by performing more advanced processing.

The role of glia cells and myelin in large brains
Brain cells - the neurons - are not the only important cells in the brain. Glial cells are the support cells and protectors for the neurons. The ratio in human brains is around 1:1 - or one support cell per brain cell.

More recent studies challenge this idea, showing variations in glia-to-neuron ratios across brain regions in humans - and there is a completely different ratio in other species. In elephants and probably in whales also, glia cells far exceed neuron numbers like 10-100:1, and the neurons themselves are unusually large. In Dolphins, the ratio is around 7:1.

Another crucial feature in large brains is myelin, the insulating fat sheath around neurons. In some large-brained animals, specific myelin patterns align with body structures, suggesting a role beyond simple insulation.

A striking example is the discovery of specific myelin patterns in the elephant brainstem that correspond to the folds of the elephant’s trunk (yes, the belly). This suggests that myelination is not just a passive feature of neural conduction but may also be actively involved in structuring the nervous system in a way that mirrors the complexity of an animal’s body and sensory needs.

This level of neural mapping is similar to the somatosensory homunculus in primates like us humans, where different body parts are represented in specific brain areas.

Does size matter?
Despite clear structural differences between small and large brains, their link to cognitive performance remains unclear. Earlier theories assumed that bigger brains meant more computing power, but modern studies challenge this assumption.

Larger-brained animals like apes and elephants display distinct intelligence and sophisticated social behaviors that are difficult to quantify in laboratory settings, and large-brained animals adapt through learning and cultural transmission rather than genetic changes as seen in small animals.

However, smaller creatures such as mice and rats have demonstrated surprising cognitive abilities, too. Some rodents even pass mirror self-recognition tests, which some primates fail.

Mirror self-recognition tests assess an animal’s ability to recognize itself in a mirror, often by placing a mark on its body and observing if it investigates the mark. Species like great apes, dolphins, and elephants pass the mirror test, while most monkeys and small mammals fail.

These findings suggest that brain size alone does not dictate intelligence. It may be shaped by other stuff like the complexity of the neural networks, the efficiency of synaptic connections between the brain cells, and even the ability to process and transmit cultural knowledge.

And I guess that's an answer for my big-headed husbands, too.

About the scientific paper:

First author:  Michael Brecht, Germany
Published: Current Opinion in Neurobiology, February 2025
Link to paper: https://www.sciencedirect.com/science/article/pii/S0959438825000121