Female bears nurse cubs for 2–3 months in the den without eating, drinking, urinating, or defecating. All milk energy comes from metabolizing stored body fat — a feat unmatched by any other large mammal.
Bear milk contains 20–25% fat — roughly 5× richer than cow milk. This paste-like, calorie-dense fluid allows cubs to grow rapidly from their tiny birth weight. It has almost no lactose, making it unique among mammalian milks.
Bears have three pairs of mammary glands: pectoral (chest), abdominal (mid-belly), and inguinal (groin). Typically only 2–3 cubs nurse at a time, so not all teats are actively used during a given lactation.
Grizzly cubs weigh just 300–500 grams at birth — about 1/300th of the mother's weight. This is one of the smallest offspring-to-mother ratios among placental mammals, necessitating intensive post-natal nursing.
With 20–25% fat, bear milk has the consistency of condensed cream or soft butter. It is so thick that it barely flows, delivering maximum calories in minimum volume to tiny cubs in the cold den.
Bear milk contains only ~0.5% lactose — among the lowest of any mammal. Energy comes almost entirely from fat, with substantial protein. This composition reflects the mother's reliance on body fat catabolism during hibernation.
During hibernation, bears metabolize stored adipose tissue to synthesize milk. A nursing mother may lose 40% of her body weight over the denning period, converting her fat reserves directly into nourishment for cubs.
Thanks to this calorie-dense milk, bear cubs grow from ~400 g at birth to 4–5 kg by den emergence — a 10× increase in just 2–3 months. This rapid growth rate is essential for surviving the first spring outside the den.