If you are out in the woods these days, tread softly -- not only are there bears hibernating, but at this time of year, there are baby bears sleeping...
Bear cubs are born in mid-winter, and they are tiny. Truly tiny. Less than a pound tiny.
Why are they born so small? and why in winter?
The black bear schedule for mating and birthing seems to be arranged to maximize opportunities to eat when the eating is good. They mate in late spring and early summer before berries ripen. This is especially important where fall foods are lacking, as here in northeastern Minnesota. If females can’t find enough berries and hazelnuts (a summer nut), they can’t maintain their pregnancies.
To avoid having cubs when they would interfere with feeding, they have delayed implantation. The fertilized egg develops only to the blastocyst stage and does not implant in the uterine wall and begin developing into an embryo until November or early December.
By that time, the mothers are in dens and have reduced metabolic rates, which means less oxygen consumption. It also means there is no food intake to help nourish embryos in the uterus. Mothers are living off their fat and nourishing their embryos with glycogen from their muscles. Fatty acid molecules are too big to diffuse through the uterus to provide energy for the embryos. There is a limit how much glycogen they can burn up, so they give birth when the cubs weigh only ¾ pound and are smaller, relative to the mother’s size, than any other placental mammal.
Once born, the cubs breath on their own, and the mother can make milk for the cubs from fatty acids, conserving her glycogen. This all works out fine and doesn’t prevent the mother from traveling and eating because she is immobile in a den anyway, during the period of scarce food. It isn’t the cold weather that bears are escaping by denning, it is to conserve energy during the winter period when easily digestible foods are scarce to nil
There is an upside -- the momma bear gets to sleep right through the terrible twos, so to speak -- by the time Spring is here and the cubs are ready to come out of the den with her, they are already back-talking pre-teens...