Bear Cubs in Winter and Spring

We have a few months before we are going to see any bear cubs wandering around New Jersey. But on another blog, I wrote about the February Full Moon as being the "Bear Cub Moon."

It may seem like some kind of magic to most people that a mother bear goes into a hole she dug in the ground in the fall and emerges in the spring with a couple of new, fuzzy, walking cubs. This magic trick is some very amazing biological engineering. 

The hidden-from-view birth of a bear cub is fascinating. Bears actually mate in the late spring or early summer, but the mother doesn’t technically become pregnant right away. Through a process called delayed implantation, the fertilized egg remains as a tiny ball of cells (a blastocyst) floating in her uterus for months. 

It only implants and begins to grow in the late fall, and only if the mother has gained enough weight. If she didn't eat enough berries or fish to survive the winter, her body would simply reabsorb the embryo, ensuring she doesn't try to raise cubs she can't support. 

Cubs are typically born in January or February, right in the heart of winter. While we call it hibernation, the mother isn't "dead to the world." She is in a state of light dormancy. She is alert enough to wake up, give birth, lick the cubs clean, and nudge them toward her belly to nurse. 

Because the cubs are so tiny, labor is relatively quick and much less physically taxing than it is for humans. 

Newborn cubs are incredibly primitive—often described as "fetal-like." They weigh less than a pound (about the size of a stick of butter or a teacup) and are born blind, nearly hairless, and totally helpless. 

newborn bear cub      nps.gov

Because they are so small, researchers often call the time in the den an "external pregnancy." Once the cub is born, it soon begins to nurse. A newborn cub’s physiology changes from one that couldn’t survive on fat in the womb to a system that can better metabolize fat. The mother provides all the nourishment that the cub needs, and the den offers warmth and protection. 

For the next several months, the den acts as a surrogate womb for the rapidly growing cub. Instead of growing inside the mother where they would drain her protein and mineral stores, they grow outside her body, fueled by her fat-rich milk.

Perhaps, the most amazing part is how they survive for months without a snack or a bathroom break: Mother bears produce milk that is incredibly high in fat (up to 30% fat), which allows the cubs to grow from less than 1 pound to about 5–10 pounds by spring. 

The mother bear does not eat, drink, urinate, or defecate for months. Her body is a closed loop; she recycles her own urea (waste) back into protein to maintain her muscle mass while she sleeps. 

The den acts as a giant incubator. The mother curls around the cubs, using her body heat to keep them at a steady temperature while the snow piles up outside.