Pandas, like all other mammals, give live birth. Pandas breed only once a year; mating season occurs from March to May, and females are in heat for 2 to 7 days. During this time, a female typically mates with several male panda bears. Once the female panda's egg is fertilized, it divides a few times, then floats freely in the uterus. Several weeks later, the embryo attaches to the uterine wall and develops. Baby pandas are typically born 8 weeks after the attachment of the embryo. A pregnant panda delivers one or two cubs in a hollow tree or rock cavity. This setting is a fundamental aspect of the panda birthing process to ensure the cubs are safe from predators. Baby panda bears weigh only 3 to 4 ounces at birth, which is approximately 1/900th of a fully-grown female's weight. If more than one panda is born, the mother typically abandons the weaker cub, leaving it to die shortly after birth. As a result, infant mortality is extremely high. Baby pandas are born blind and do not open their eyes for at least 45 days. Cubs are also born with white skin; within 2 weeks from birth, their skin turns gray and black hairs begin to develop. Young pandas become independent from their mother at roughly 1.5 years of age.