Bear cubs are typically born during the winter months, usually between mid-January and early February, while the mother is deep within her den. This timing aligns with the mother’s denning period, providing a safe and secluded environment for the vulnerable newborns. Mating for bears generally occurs much earlier, during the summer months, often in May or June.
The precise timing of birth is regulated by a unique biological process called “delayed implantation.” After mating, the fertilized egg develops into a tiny cluster of cells, known as a blastocyst. Instead of immediately implanting in the uterine wall, this blastocyst remains dormant and unattached for several months. Implantation typically occurs in late November or December, but only if the female has accumulated sufficient fat reserves during the fall.
If the mother bear has not gained enough weight, the blastocyst may not implant, and the pregnancy will not proceed, which is a natural way of ensuring the mother’s survival and the cubs’ chances of thriving. Once implantation occurs, fetal development proceeds rapidly, with the cubs being born about two months later. During this period, the mother’s metabolism slows, a state sometimes referred to as torpor rather than true hibernation, allowing her to conserve energy while remaining responsive to her cubs.
Cubs at Birth
Newborn bear cubs are remarkably small and underdeveloped, especially when compared to the size of their powerful mothers. They are born blind, hairless, and largely helpless. These tiny cubs typically weigh less than a pound, often around half a pound, and measure approximately 9 inches in length, roughly the size of a stick of butter or a chipmunk.
Their extreme vulnerability necessitates intensive maternal care. They possess only a light covering of fur and can barely crawl.
Life Inside the Den
Once born, bear cubs enter a developmental phase within the den, where they remain with their mother until spring. During this time, the cubs grow rapidly, primarily by nursing frequently from their mother’s milk, which is rich in nutrients. The mother provides constant warmth, nourishment, and protection.
She keeps the cubs warm by hovering over them and using her body as insulation. The cubs do not hibernate; instead, they maintain a full metabolism to support their rapid growth. Milestones in their early development include opening their eyes, which occurs within a few weeks, and growing a thicker coat of fur. They also begin to gain mobility, transitioning from barely crawling to being able to toddle around the den. By the time spring arrives, typically in late March or early April, the cubs have grown considerably, often weighing between 4 to 6 pounds. At this point, they are ready to emerge from the den with their mother, venturing into the outside world for the first time.