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If you have noticed a hole in your Chihuahua's head, you are likely very concerned about it. Yet, there are good chances that hole is nothing major to be worried about. 

Turns out, that incomplete ossification of the bones in your Chihuahua's head is even mentioned in the breed standard for this breed. 

That hole in the head indeed, has even a name. It's called "molera" and it's a main characteristic of the Chihuahua dog breed. Not all Chihuahuas have it though.

Discover why Chihuahuas have a hole in the head, and most of all, what you need to be wary of. 

Please note: If your Chihuahua has a different "hole" in the head such as the result of an injury, please see the vet at your earliest convenience. 

What is A Chihuahua's Molera?

The molera is a characteristic of the Chihuahua dog breed. To be more exact, that 'hole," is the result of an incomplete ossification of the bones in the center of the Chihuahua's skull, right in between the frontal and parietal bones. 

Technically speaking, this soft spot is known as "open fontanelle," also known as "bregmatic fontanelle."

The American Kennel Club mentions that Chihuahuas must have a  well rounded "apple dome" skull, with or without molera. 

However, the United Kennel Club's  (UKC) standard for the breed clearly notes that  an open fontanelle, a hole or gap in the center of the skull is means for disqualification in the show ring. 

The Federation Cynologique Internationale (FCI) also considers Chihuahuas with an open fontanelle a disqualifying fault. 

Picture of an open fontanelle (Public Domain) Gray (1918) Anatomy of the Human Body

Picture of an open fontanelle (Public Domain) Gray (1918) Anatomy of the Human Body

The Purpose of a Chihuahua's Molera 

That "hole" though isn't there just for decoration! Turns out, it carries a very important function that aids in a Chihuahua puppy's survival.

Basically, that soft spot, allows the head to pass through the mother's birth canal more easily. This anatomical characteristic therefore makes it easier for a female Chihuahua to give birth.

In some cases, Chihuahua moms may need a C-section because of their conformation. This is especially true with apple-head chihuahuas compared to the dear-headed ones. 

On top of allowing an easier birth, the molera allows proper bone expansion after birth so to better accommodate the enlarging brain.

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When Does a Chihuahua's Molera Close?

 Once the puppy is born, the molera should gradually close on its own generally by the age of 9 to 12 weeks.

In certain breeds like the Chihuahua or miniature dachshund though, it may remain permanently open without any particular consequences, explains veterinarian William D. Fortney.

While moleras therefore usually close on their own, they can remain open permanently in some Chihuahuas. When the soft spot fails to close, it is referred to as a "persistent, bregmatic fontanelle" (BF).

While a persistent bregmatic fontanelle is supposedly not a major problem, it's best to avoid pushing on the area as there is no protection for the brain. 

If the open fontanelle is large, some owners have found it helpful to protect these vulnerable spots with helmets.

Relationship to Hydrocephalus 

In the past it was thought that a domed head and the presence of a soft spot was associated with a medical condition known as "hydrocephalus," where cerebrospinal fluid accumulates in the brain making it swell and triggering neurological signs. 

However, according to a study conducted by Greene and Braund in 1989:

 "Many clinically normal toy breeds and brachycephalic (short faced) breeds also may have open fontanelle without associated hydrocephalus. There did not appear to be any relationship between the presence or size of the fontanelle and the concomitant presence of hydrocephalus."

The Canine Inherited Disorders Database website seems to agree, according to the website, “an open fontanel is not diagnostic per se of hydrocephalus, as it may occur in a normal healthy dog. "

What Recent Studies Reveal 

While the presence of a molera  in a dog's head is not a sure sign of hydrocephalus, there is recent research that has found connections between persistent fontanelle and health issues. 

A recent study conducted in 2021  found some possible connections between  persistent fontanelle with other structural abnormalities. 

Furthermore,  an association was found between lower body weight in Chihuahuas and a higher number of and larger persistent fontanelles. 

Both these findings question the ethics of selective breeding of Chihuahuas that have very low body weights. More studies are needed on this.

References:

  • Kiviranta AM, Rusbridge C, Lappalainen AK, et al. Persistent fontanelles in Chihuahuas. Part I: Distribution and clinical significance. J Vet Intern Med. 2021
  • Kiviranta, A-M, Rusbridge, C, Lappalainen, AK, Junnila, JJT, Jokinen, TS. Persistent fontanelles in Chihuahuas. Part II: Association with craniocervical junction abnormalities, syringomyelia, and ventricular volume. J Vet Intern Med. 2021

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