This YouTube video of a three-month-old polar bear taking his first wobbly steps at the Toronto Zoo was viewed over 4.5 million times in the first four days of it being posted, and is sprouting up all over the Internet.
Something I noticed immediately is that the baby polar bear is mostly crawling backwards. Many (human) infants do the same – crawling backwards for a few weeks before they crawl forwards. Why is it common for polar bear cubs, human infants, and the young of many other four-limbed species to initially crawl backward?
Playing it backwards
A key to this puzzle is to understand that early physical development follows a cephalocaudal principle. In other words, muscle strength and ability to control movement progresses from head to foot. What this means is that early in life, the upper limbs are much stronger and more well-coordinated than the lower limbs.
This is clearly demonstrated in the polar bear cub. His front limbs are outstretched and already good at supporting his heavy head, while his back limbs are much more frail, often crumpling and collapsing beneath him.
As we see in the video, with stronger front legs pushing forward, and with much weaker hind legs, the cub’s movements shift his body backward. Notice especially the cub’s weak right hind leg, which remains almost anchored to the ground. This is the main reason the polar bear pivots to his left during the backward crawl.
Putting it all together
People are often surprised or puzzled to see infants moving in reverse. But this is a normal phase of learning to self-locomote.
For instance, although this baby coordinates her arms, each arm pushes up away from the body (which slides her back) rather than down toward the body. Although she’s moving fast, she’s still going backward.
To crawl forward, one must develop enough strength and control in the legs to propel the body forward, and arm movements must change from simply pushing away from the body to a more complex pattern that resembles reaching with alternating left and right arms.
The polar bear cub does the same thing. At first, he is mainly pushing outward with his front legs, which moves him backward. He then switches to bringing one front paw down in a swinging motion toward his belly, which could propel him forward. However, the momentum from the hind legs is still too weak, and he makes little forward progress.
The stronger upper body and weaker lower body trend of muscle development (cephalocaudal) does not apply throughout life. In humans, as babies develop weight-bearing abilities such as standing and taking first steps upright, the lower limbs rapidly become stronger and more controlled.
No two are exactly alike
As I often tell my Developmental Psychology students, if you look closely enough, every infant self-locomotes in a unique way. No two crawlers, rollers, or scooters are exactly alike. The placement of hands, angle of feet, order of movement of the four limbs, and a myriad of other factors, vary in subtle ways based on the baby’s own unique attributes and adaptation to its individual physical and social environments. Even when taking our very first steps in life, polar bear cubs and human babies show remarkable individual variation, ingenuity, and versatility.