While adult bears are some of the largest land carnivores, their newborns are surprisingly tiny relative to their mothers when compared to almost any other mammal. This miniature size is a shared trait across all bear species, with birth occurring in the protective seclusion of a winter den. The specific weight, however, varies depending on the species, reflecting differences in adult size and habitat.
The Specific Weights of Newborn Cubs by Species
Newborn bear cubs are remarkably small, typically weighing between one three-hundredth and one five-hundredth of the mother’s body weight. An American Black Bear cub, for example, is born weighing approximately 280 to 450 grams (10 to 16 ounces), which is less than a pound. Given that a female black bear can weigh over 150 pounds, the size difference is dramatic.
Brown Bear cubs, which include the Grizzly Bear subspecies, are larger at birth, often weighing less than 500 grams (about one pound). Even for a massive coastal brown bear sow that may weigh hundreds of pounds, the newborn is extremely small.
Polar Bear cubs are the largest of the three North American species, with an average birth weight ranging from 454 to 680 grams (16 to 24 ounces). These cubs are born with only fine, woolly hair and closed eyes, completely dependent on their mother for warmth and nourishment inside the den. This birth weight scale is one of the smallest among all placental mammals.
Why Bear Cubs Are Born So Small
The minuscule size of bear cubs at birth results from a reproductive strategy adapted to the harsh winter environment. This process begins with delayed implantation, where the fertilized egg does not immediately implant in the uterine wall. Mating occurs in the summer, but the embryo remains free-floating, only implanting in the late fall, around November.
This delay ensures that fetal development, which lasts only about 6 to 8 weeks, occurs while the mother is already in the den. During this period, the mother is in a state of torpor, surviving solely on stored fat reserves and neither eating nor drinking.
Fetal growth requires a steady supply of protein and carbohydrates, which the mother cannot easily supply while fasting. By keeping the fetus small, the bear minimizes the metabolic cost of pregnancy and conserves limited protein stores for maintaining muscle mass. The birth of small young is a survival mechanism that allows the mother to fast throughout the winter.
Rapid Growth and Early Development Milestones
Once born, the small cubs enter a period of explosive growth fueled by their mother’s rich milk. Polar bear milk, for instance, can contain up to 33% fat, which is significantly higher than that of most land mammals. This high-calorie diet is essential for the cubs’ rapid development in the den’s insulated environment.
Black Bear cubs open their eyes between four and six weeks of age, and they begin to walk within the den around five weeks. By the time the family is ready to emerge in the spring (late March or April), the cubs have gained significant weight.
A Black Bear cub will weigh between 3 and 9 pounds when leaving the den, while a Polar Bear cub will be much heavier, reaching 22 to 33 pounds. This dramatic weight gain, which multiplies their birth weight by 10 to 20 times in just a few months, is necessary for survival outside the den. Brown Bear cubs will weigh up to 15 pounds by their emergence in late spring, ready to follow their mother to learn foraging skills.