Getting fat is the most important preparation for birth.  After mating in late May or June, pregnant females focus on eating for the rest of the summer.  Females that don't become fat enough by fall cannot maintain their pregnancies. 

Around Ely, females that weigh 176 or more by fall maintain pregnancies, give birth in dens in mid to late January, and provide enough milk for their cubs to survive.  Females that weigh less than 148 pounds in fall do not give birth.  Females of intermediate weights may or may not give birth, and cubs from those females are less likely to survive. 

Newborn cubs are smaller, relative to their mother's size, than the young of any other placental mammal.  They are totally dependent on their mothers.  Newborn cubs have little fur, weigh less than a pound, and can barely crawl.  Three cubs is the most common litter size around Ely. 

Around Ely, temperatures at the time of birth in January can be 60 degrees (F) below zero.  Most dens are no warmer than the outside air because most den entrances are open.  Some "dens" are simply nests on the ground surface. 

By the end of February, cubs are 4 to 6 weeks old and usually weigh 2-3 pounds.  They have dense fur almost an inch long.  They can thermo-regulate to some extent, but they still cry for help staying warm and fed.  They can crawl, but they can't walk.  Their eyes are opening or fully open.  

On warmer days, with temperatures in the 20's, the cubs often sleep partially exposed on their mother's back. 

(see video below) 

Water in dens is a problem, especially for mothers with cubs.  Newborn cubs that are too young to walk in January, February, or early March can be killed from exposure or drowning if there is an early melt or hard rain. 

By mid-March, cubs have enough fur and mobility to deal with wetness with help from their mothers.  Some cubs take refuge on their mothers' backs.  Sometimes, the whole family can find a drier spot in the den.  Melt water that drips on cubs is licked off by the mothers.

(see video below)