Bears fast completely for several months during winter denning, neither eating nor drinking. This requires specialized physiological management for both solid and liquid waste. The bear employs two distinct biological processes that allow it to remain confined and conserve energy without fouling its den or poisoning its body. These measures essentially pause the digestive and metabolic waste elimination systems.
The Unique State of Bear Torpor
Bears do not enter “true” hibernation like smaller mammals such as ground squirrels. True hibernators experience a profound drop in body temperature, often requiring energy-intensive awakenings to urinate and restore metabolic balance. Instead, bears enter a deep sleep known as torpor, or winter denning. Their body temperature only decreases slightly, usually remaining within 12°F of their normal 100°F to 101°F.
This minimal temperature reduction allows the bear to remain relatively alert and capable of a rapid, full awakening to defend itself. The bear’s heart rate drops significantly, slowing from 40–50 beats per minute to as few as 8–12 beats per minute. Respiration can also slow to one breath every 45 seconds. Since physiological systems are suppressed but not shut down, the bear must manage the small amount of waste still produced during fasting.
How Solid Waste is Managed (The Fecal Plug)
To prevent defecation during denning, bears form a physical barrier known as the “fecal plug” or “tappen” within their lower intestine. This dense, hardened mass accumulates over weeks and acts as a seal, preventing the passage of waste. The plug is not composed of compacted feces, but rather a mixture of residual material and intestinal secretions.
The composition includes hair and undigested plant fibers licked off the bear’s fur during grooming, sloughed-off intestinal cells, and sometimes bits of bark or bedding material. The intestinal walls absorb the fluids from this accumulating matter, causing it to harden into a compact mass. This mass can measure 7 to 15 inches in length and 1.5 to 2.5 inches in diameter. Upon emerging from the den in the spring, the bear expels this dry, fibrous tappen, signaling the resumption of normal digestive functions.
Internal Recycling of Metabolic Byproducts
The bear must also manage liquid metabolic waste, as it does not urinate or drink during denning. Failure to do so would lead to kidney failure and dehydration. The primary toxic waste product of protein metabolism is urea, which normally must be excreted. Bears possess a unique adaptation that allows them to recycle the nitrogen from this urea instead of eliminating it.
Urea is shunted back into the bloodstream, where it is broken down, likely with the assistance of gut microbes. The freed nitrogen is then utilized by the bear’s body to synthesize new non-essential amino acids. This recycling process prevents the toxic buildup of urea while simultaneously maintaining muscle mass, sustaining lean body tissue during fasting.