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Chapter 1

The Puppy Brain

Puppy Brain Growth

If we want to understand how to predict who a puppy will grow up to be and influence the chances of them becoming a great dog, we have to first consider how their brains grow. When do puppy brains grow most rapidly and start to produce their individuality? When are their brains most plastic and influenced by experiences that affect their adult personalities? Should we start studying them when they are two or twenty weeks old?

Answering questions like these requires understanding how dogs mature compared to other animals. We know more about the brains of humans than any other species, and in many ways, puppy brains are similar to ours. Even though we tend to assume that our minds become more impressive as we get older, it is when we are young that our most extraordinary mental feats occur. In infancy, our minds are astonishingly plastic. We amass information at a rapid rate. In our first four years we learn thousands of words. The physical world begins to make sense—important concepts like gravity and the link between danger and pain. We develop morals, our emotions become more sophisticated, and we begin to think about the world and our relationship to others as we move about in it. Our psychology is shaped by who we interact with, where we spend time, and what we play with, whether it’s books, baseballs, or banjos.

No matter what we do with our lives as adults, it is this period of early cognitive development that should be considered one of our greatest accomplishments. Within months, we go from being helpless—unable to survive a few hours without shelter and care—to a walking, talking, culturally capable human.

There are many ways for animals to grow up. Baby snakes slither out of their eggs so perfectly formed that they do not need a moment of parental care. Axolotl salamanders never grow up, and their bodies remain in a juvenile aquatic state for the rest of their lives. Bluehead wrasse start life as female but can change into males. The list goes on.

For most mammals, the main difference in development is how ready they are for life just after birth. Many mammals are born to run. Antelope, for example, stand up within minutes, and in the span of a few hours, they can keep up with the herd; within months, they’ll be ready to strike out on their own—all useful skills when you are at the top of the predator menu. By contrast, other animals, like baby orangutans, cling to their mother twenty-four hours a day for years.

While most mammals fall somewhere between these extremes, both dogs and humans are toward the helpless end. Mammals who need more investment from their parents tend to grow larger than average brains. Larger brains usually belong to animals with more complex behavior and more flexible problem-solving abilities. Longer periods of parental help give the maturing brain time to grow in safety and allow experience to shape growth.

Since humans and puppies are born nearly helpless, our knowledge of how the human brain develops provides insight into when puppy brains grow the fastest and are the most plastic. The comparison reveals striking similarities and conspicuous differences. The similarities help explain why puppies can immediately become part of our families while the differences can inform our expectations of what a puppy might be capable of and when.

With around eighty-six billion neurons, the adult human brain is approximately three times the size of a chimpanzee brain. Like humans, dogs have a high number of neurons in their brains compared to other carnivore species. A large domestic dog, like Congo, has over twice as many neurons as a house cat. This is true of both the number of total brain neurons and the cortical neurons, which are engaged in complex problem-solving. Congo also has more neurons, and the associated cognitive computing power, than larger carnivores like African lions or brown bears. These size comparisons suggest dogs can potentially out-compute most other carnivores, but compared to humans, dogs are very limited in more sophisticated forms of cognition, such as reasoning that requires making an inference.

When compared to those of most mammals, dog brains, like human brains, are undeveloped at birth. The cortical layer of the mammalian brain consists of bumps and grooves known as “gyri” and “sulci.” Folding of the outer cortical layer allows brains to pack more neurons into smaller spaces. Puppies are born with smooth brains and relatively few neurons. Puppy brains only develop folds, cortical neurons, and the resulting cognitive abilities after birth and are entirely dependent on their mothers until this brain growth occurs. This leaves newborn puppies almost as helpless at birth as human babies, and just like our own babies, parental care is critical for puppy survival.

In some ways puppies are born even more helpless than humans. At least we are born with our senses functioning. We can see, and quickly develop a preference for, the face of our mother and people who look like her. We can recognize her scent and are less likely to cry when we smell her. We immediately recognize her voice and prefer it to any other. We can even tell the difference between her language and a foreign language. Our sense of touch is also present at birth, and every part of our body is sensitive to the physical world. In contrast, puppies are born with weak senses. Newborn puppies are blind, and their eyes do not open for two weeks. Although puppies can smell at birth, their olfactory cortex is not developed and their sense of smell is poor. Puppies are born with their ear canals shut; they open during the first two weeks of life. Puppies do not reliably start responding to sound until they are around twenty-five days old. Hearing is less developed than vision in newborn puppies—whereas in humans it is the opposite.

The one sense that newborn puppies can rely on is touch. Shortly after birth, puppies depend mainly on body heat to find their mother’s nipple. Puppies are also born with whiskers, which are specialized hairs with follicles full of nerves. They are located on a puppy’s muzzle, jaw, and above the eyes. The smallest particle sends tremors down a whisker, and the nerve endings send messages to the brain. Puppies immediately begin to use their whiskers to navigate in the dark, crawl through tiny spaces, and detect the location and speed of moving objects by the airflow.

However, the other senses quickly catch up. They mature at a pace that maps onto their rapid brain development. The occipital lobe, the visual center of the brain, is the fastest developing part of the puppy brain. By day twenty-five, puppies begin to see forms and start to orient toward visual stimuli like bright light. By their six-week mark, puppies can see, but it takes a few months for them to develop the full vision they will have as adults. Their olfactory bulb is more mature at two weeks and eventually grows into an incredibly complex structure—with olfactory neurons regenerating throughout adulthood.

Another advantage the youngest puppy has over a human newborn is their relatively well-developed motor cortex, which is involved in the control and execution of voluntary movement. While we flop around helplessly for months, unable to stand or even sit up, puppy muscle tone develops quickly. At only a few days old they can right themselves if they are on their side, and shuffle forward to find their mother’s nipple. By the third week they can sit up—then stand almost immediately after. By four weeks they can walk, and by six weeks they can right themselves when they are in danger of falling.

As puppies quickly strengthen their senses and motor abilities, the rest of their brain finishes what takes our brain years to accomplish. Gyrification, or the growth of cortical folds that allow for high neuron densities, is complete by six weeks of age. The full length of the corpus callosum, the part of the brain that connects the left and right hemispheres and allows them to communicate, reaches adult form at sixteen weeks. Likewise, the relative white to gray matter intensity and myelination of critical neuron networks reaches adult levels in dogs by sixteen weeks and is largely completed in the first year.

This overall pattern, in which puppies grow a more neuronally dense brain than many other mammals in the first few weeks after their birth, means that puppy brains have the potential to be heavily affected by experiences. It also means a puppy’s period of maximal brain plasticity takes place over a much narrower window of time than ours.

Like human brain development, puppy brain development is affected by experiences outside of the womb. Social experiences, in particular, heavily shape a dog’s brain development between weaning and the appearance of adultlike brain structures at around eighteen weeks. So the final period of rapid brain growth and myelination between eight and eighteen weeks—which is, in part, why it is believed to be the critical period of socialization.

The evidence for rapid brain growth during this ten-week period is also a reminder to be patient. Puppies are not working with a full deck of cards for months after we bring them home. Puppies do not pee inside on purpose or chew on furniture to make you mad. Their brains are just slowly catching up to our expectations.