A molera is a small opening in a puppy head, comparable to the fontanelle (soft spot) in a human infant skull. For most canines this opening closes up as they age, becoming solid. Some dogs, most notably chihuahuas, have moleras that never fully close. Although large moleras and moleras that remain unclosed after a puppy sixth month have a reputation for being a sign of hydrocephalus, they are often a regular occurrence in Chihuahuas, and not considered to be abnormal. Unclosed moleras are most common in the Chihuahua breed, but they have also been seen less commonly in other toy and brachycephalic breeds.

Dogs, like most mammals, start life with a soft spot in the middle of their skulls. This spot usually closes up within the first few weeks of life, but in some cases, it remains.

Dogs who retain unclosed moleras into adulthood rarely show any symptoms beyond the spot in the center of the skull where the head is soft to the touch. For chihuahuas, in particular, this is not considered to be a medical abnormality and has historically been accepted as a mark of purity in the breed.

• Skull does not fuse
• A soft spot remains in the center

Types

Chihuahuas are by far the breed most often affected by unclosed moleras, and the specific head shape of the chihuahua may alter the chances of the molera remaining open. Although breed standards for chihuahuas only list two varieties, long-haired and short-haired, many people split them into two groups, apple-headed chihuahuas and deer-headed chihuahuas. The apple-headed chihuahuas have a more rounded head with a 90-degree angle where the muzzle joins the forehead, whereas the deer-headed chihuahua head is longer and where the muzzle and forehead meet is at a more sloping 45-degree angle. Chihuahuas with the apple shaped head are more likely to have a molera that does not close than those with a deer shaped skull.

Like most mammals, puppies are born with the bones of their skull unfused in order to allow them passage through the birth canal. This leaves a hole open in the top of the skull that usually closes by the time the animal is four to six weeks old. In some dogs, this simply never closes. It is most commonly seen in Chihuahua and Chihuahua mix dogs, although it is occasionally seen in other small or brachycephalic breeds of dog.

In most cases, molera is easy to diagnose with a simple physical examination as it can easily be felt by palpation, however additional testing may be required under certain circumstances. In order to determine the size and positioning of the hole, x-ray technology may be employed to get a clearer picture of the bones of the skull. Although historical information states that dogs with unclosed bones in the skull are predisposed to hydrocephalus, however, studies have shown that there does not seem to be any connection between the presence and size of a molera and the development of hydrocephaly.

In some cases, the examining veterinarian may choose to do an ultrasound of the dog head as well. This form of imaging is frequently employed if there are any other physical signs pointing to hydrocephaly in order to better visualize the brain and its surrounding structures, particularly the ventricles.

Although this is not technically a medical abnormality, the lack of bone on the top of the head does leave these small animals open to trauma to the brain that other dogs may not be vulnerable to. It is essential to exercise extra caution when dealing with dogs with soft spots as it is somewhat easier to cause damage to the brain. This means that the dog should not be allowed to jump down off of furniture, out of your arms, or even off of high stairs.

It is also important to be selective about your pets playtime companions. It may be inadvisable to allow your pet to be handled by very young children as they could easily be too rough, and although large, exuberant dogs may sometimes safely play with small dogs, the opening in the skull makes affected animals particularly susceptible to bites and blows from other dogs, even those that occurred during play.

The prognosis for most dogs with molera is excellent. With just a few precautions these dogs can live very long and happy lives. Although other dog breeds may have a molera that never closes, the overwhelming number or animals with this condition are Chihuahua dogs. Not only is this not an abnormal condition for Chihuahua, but it was actually a required feature and listed as a breed standard in the 1923 breed standard guide for the American Kennel Club. The number of dogs with open moleras are declining due to changes in breeding practices. Moleras are more common in the smaller dogs, and the average size of Chihuahuas has been on the rise, and while soft spots are an acceptable feature for Chihuahuas, they are no longer required to meet breed standards.