A chihuahua’s brain is a small, walnut-sized organ that looks surprisingly similar to any other dog’s brain, just compressed into a rounder, more dome-shaped skull. It has the same folded surface, the same lobes, and the same basic architecture as a Labrador’s or a German Shepherd’s. What makes it visually distinctive is how the skull reshapes and repositions the brain tissue inside it, giving it a more compact, rounded profile than you’d see in larger breeds.
Size Relative to the Body
Dog brains across all breeds weigh between 50 and 130 grams, according to Harvard’s Canine Brains Project. A chihuahua’s brain falls near the lower end of that range, but relative to body size, it’s actually one of the largest brains in the dog world. This is a pattern across mammals: smaller species tend to have proportionally bigger brains compared to their body weight. So while a chihuahua’s brain is physically small enough to fit in your palm, it takes up a much larger percentage of the dog’s total mass than a Great Dane’s brain does.
Surface Folds and Structure
Like all dog brains, a chihuahua’s brain is gyrencephalic, meaning its surface is covered in ridges and grooves rather than being smooth. These folds, called gyri (the ridges) and sulci (the grooves), increase the total surface area of the outer brain layer. The canine folding pattern is actually more similar to the human brain than many people realize.
If you were looking at a chihuahua brain from above, you’d see a deep groove running down the center dividing the two hemispheres. On each side, you’d notice a prominent groove on the lateral surface where the frontal and temporal lobes overhang a deeper structure. Three arched ridges wrap around this groove, each separated by its own smaller groove, creating a layered, almost stacked appearance on the side of the brain. There’s also a distinctive curved groove near the top that bends toward the center with an S-shaped or inverted U appearance. These landmarks look the same in chihuahuas as in larger dogs, just scaled down.
How the “Apple Head” Skull Reshapes the Brain
The most visually striking thing about a chihuahua brain isn’t the brain itself. It’s what the skull does to it. Chihuahuas are bred for a brachycephalic, “apple-shaped” head: a skull that is short from front to back, wide from side to side, and tall with a domed forehead. This shape develops because the growth plates at the base of the skull close early, which limits how long the skull can grow. The cranium compensates by growing upward instead, producing that characteristic rounded dome.
This means the brain inside doesn’t sit in the same orientation as it would in a longer-skulled breed. In brachycephalic dogs, the brain is rotated along its side-to-side axis. Imagine tilting a brain slightly backward inside the skull. Researchers have noted that this rotation raises a genuine question about cause and effect: whether the skull shape forces the brain to rotate, or whether developmental changes in the brain influence the vault around it.
The short skull base also creates crowding. The front of the cranial cavity, including the thin bone plate that separates the nasal passages from the brain, is smaller than in longer-headed breeds. Brain tissue can press more tightly against these boundaries. In the back of the skull, the cerebellum (the structure responsible for coordination) can become indented or pushed into the opening where the skull meets the spine. The fluid-filled chambers inside the brain, called ventricles, also tend to be larger in dogs with rounder skulls.
The Molera: A Window in the Skull
Many chihuahuas are born with a molera, a soft spot on the top of the skull where the bone plates haven’t fully closed. It’s the same thing as the fontanelle on a human baby’s head. If you could look down at a chihuahua’s skull, you might see one or more small gaps in the bone, and through imaging, you’d see the brain’s membranes sitting just beneath the skin in those spots, without a hard layer of bone protecting them.
Chihuahuas with the most extremely round, domed skulls tend to have larger and more numerous soft spots. The Chihuahua Club of America has emphasized that a molera by itself is not a sign of disease. Neurologists at Cornell University and the University of Minnesota have confirmed that an open fontanelle does not automatically mean anything is wrong, and many clinically healthy toy breeds have them throughout life. About as many chihuahua puppies are born with a molera as without one.
What Hydrocephalus Looks Like on Imaging
Because of their skull shape, chihuahuas are one of the breeds most commonly associated with hydrocephalus, a condition where cerebrospinal fluid accumulates inside the brain’s ventricles and expands them. On an MRI or CT scan, a normal chihuahua brain already shows somewhat larger ventricles than a longer-skulled breed. But in hydrocephalus, the change is dramatic.
The fluid-filled chambers balloon outward, compressing the surrounding brain tissue into a thinner and thinner shell against the inside of the skull. In severe cases, the ventricles can extend forward into the olfactory structures at the very front of the brain, which doesn’t happen normally. The band of tissue connecting the two brain hemispheres gets stretched and pushed upward, while a structure called the fornix below it gets pushed downward. Researchers have also documented that in chihuahuas specifically, the fourth ventricle at the back of the brain can become massively enlarged, a sign that fluid flow is being obstructed as it tries to exit the brain.
Swelling around the ventricles, visible as a hazy border on imaging, is one of the most reliable signs that pressure inside the chambers is genuinely elevated rather than the ventricles simply being large for the breed. This distinction matters because many chihuahuas have ventricles that look big on a scan but function perfectly normally. Size alone doesn’t tell the whole story.
Blood Supply to the Brain
Five paired arteries feed the chihuahua’s brain, the same arrangement found in all domestic dogs. Three supply the main cerebral hemispheres (front, middle, and back), and two supply the cerebellum. All but the rear cerebellar arteries branch from a ring structure at the base of the brain called the Circle of Willis, which is formed by the internal carotid arteries meeting the basilar artery. This ring acts as a backup system: if one artery is compromised, blood can reroute through the circle to keep the brain supplied. In a chihuahua, this entire vascular network is miniaturized but structurally identical to what you’d find in a much larger dog.