In April, if the weather permits and you're in the right place in Yosemite backcountry, you might see the first bears of the season emerging from their winter dens. They move slowly at first, stiff-legged, their dark bodies silhouetted against patches of remaining snow. A bear that emerges in mid-April has just spent between five and seven months in a state that is almost, but not quite, hibernation. Scientists call it torpor. The distinction matters, because the bear's body has not simply been dormant. It has been radically reprogrammed.
That reprogramming, and the desperate metabolic consequences of it, explains why spring in Yosemite is the season when bears are most dangerous to humans and most vulnerable to starvation.
A bear enters its den in November or December, at the end of the fall feeding season when mast crops (acorns, pine seeds) have been picked clean and stored in the bear's body as fat. The den is usually in a hollow tree or a rock crevice, something protected and small enough that the bear can warm it with its own body heat. The bear eats nothing. Drinks nothing. Its metabolism drops.
An active bear's heart rate might be 70 to 100 beats per minute. In torpor, a bear's heart rate drops to 8 to 10 beats per minute. Its body temperature, normally around 38 degrees Celsius, can drop into the low 30s. Its kidneys essentially shut down. It recycles its own urea. It produces virtually no urine. It does not defecate. For five months, the bear's body operates at roughly 25 percent of its normal metabolic rate.
This is not sleep. The bear can wake if disturbed. But under normal circumstances, it remains in a state of profound torpor, its body a closed system burning stored fat at minimal rate.
Then, in spring, something triggers the emergence. Not hunger, exactly. The bear has stored enough fat that starvation is not an immediate concern when it wakes. What changes is that the days lengthen and the temperature outside the den begins to rise. The bear's circadian rhythm, responding to these light and temperature signals, begins to reset. Hormone levels change. The body's metabolism begins to increase. The torpor is interrupted. The bear wakes.
And it wakes desperately hungry.
The physiology of torpor emergence creates an immediate problem. The bear has been burning fat for five months. It has lost somewhere between 20 and 40 percent of its body weight. A healthy female black bear might weigh 150 pounds in fall and 100 pounds in spring. A large male might start at 300 pounds and finish at 180. That's not just weight loss. That's significant lean muscle loss as well as fat loss.
More immediately, the bear's digestive system has been dormant. The intestinal tract, the stomach, the liver: all have been operating at minimal capacity. When the bear emerges and begins to eat, these organs have to restart from an idle state. A fresh bear (one emerging from torpor) cannot immediately process large meals. Its digestion is sluggish. Its metabolism is still adjusting from the torpid state.
And yet the bear's body is now urgently demanding calories. The heart rate increases as the bear exits torpor. Metabolic rate climbs. The bear is no longer in energy conservation mode. It is in active mode, moving, exploring, searching for food. This shift from conservation to activity happens relatively quickly as the days lengthen and the bear's body temperature climbs.
The result is a period of acute metabolic demand with limited food availability. The first plants to emerge in spring at Yosemite's elevations are ephemeral herbs and shoots. Sedges come up along meadow margins. Grasses begin to green. A bear can eat these, and does, but the caloric density is low. A bear might need to consume hundreds of pounds of green vegetation to get the calories it got from pounds of acorn or pine seed.
The 1933 Nature Notes entry "Our Yosemite Bears" by C. A. Harwell specifically documents this spring transition, noting that bears in April and May are "extremely hungry" and "actively foraging from dawn," with frequent observations of bears in meadows, stripping vegetation. The bears were not desperate in an absolute sense. They were desperate in a relative sense: their body was demanding high-calorie food, and the landscape was offering low-calorie food.
When a bear emerges in spring, its behavior changes measurably. A torpid bear emerging from a den is not aggressive or particularly active. It's sluggish, moving slowly, often resting in protected locations near the den. But over days or weeks, as the metabolic transition completes and the body's temperature fully normalizes, the bear becomes increasingly active and increasingly focused on food.
By May, a spring bear is actively foraging for hours each day, moving extensively through meadows and along stream margins, investigating potential food sources. The bear is more likely to approach human settlements, to attempt entry into cabins or food storage areas, to persist in areas where human food or garbage is available. (Anyone who has lived in employee housing in the Valley has stories about spring bears at the tent cabins; this is the season when bear-box discipline matters most.) The motivation is not aggression. It's hunger.
The difference in bear behavior between a spring bear (just emerged, metabolically desperate) and a fall bear (recently fed, with adequate fat stores and no immediate metabolic pressure) is measurable. Fall bears are more predictable, more likely to avoid humans, more likely to be deterred by noise or barriers. Spring bears are less predictable, more likely to persist in attempting to access human food, more likely to escalate an encounter if they perceive a threat to a food source.
This is when bears are most likely to be killed by humans, not because they are aggressive, but because they are desperate, and desperation makes them bold.
If you see a bear in Yosemite in April or May, you are seeing a metabolically driven animal. Its body is in a state of transition and stress. It has not eaten a substantial meal in five months. Its digestive system is waking up. Its metabolism is accelerating. Its caloric demands are high. Its food options are limited.
A bear that is actively foraging, moving systematically through a meadow, turning over rocks and logs, is demonstrating this hunger. The intensity of the foraging behavior is a direct reflection of metabolic need. A spring bear forages more intently than a summer bear, because the spring bear has a more urgent need.
Additionally, a bear's behavior toward humans is different in spring. A spring bear that hears humans approaching is more likely to hold its ground, to continue foraging, or to move slowly away. A fall bear in the same situation is more likely to flee. This is not increased aggression. It's prioritization. The spring bear's food needs are greater. The bear is less likely to abandon a potential food source due to human presence.
The practical implication is that spring is the season to be most cautious around bears. Not because bears are more dangerous by nature, but because bears are more motivated, and more likely to take risks to obtain food.
C. A. Harwell's 1933 documentation in Nature Notes Vol. 10, No. 3 offers one of the most detailed accounts of Yosemite bear behavior from the park's early naturalist era. Harwell recorded specific observations of spring foraging, bear den locations, emergence timing, and interactions between bears and humans. The entries note the intense hunger behavior of spring bears, the shift in predictability and boldness as the season progressed, and the increased frequency of bear encounters in human-occupied areas.
The observations are not sensationalized. They're factual records of what naturalists observed: when bears emerged, what they ate, how they behaved. Modern behavioral studies of black bears in other Sierra Nevada locations have largely confirmed what Harwell documented: spring emergence brings a period of acute metabolic need, intense foraging behavior, and increased probability of human conflict.
The 1939 observations recorded in Nature Notes Vol. 18, No. 7 further document this pattern, noting specific instances of bears accessing stored food and human garbage, with particular intensity in May and early June.
Understanding the biology of spring emergence changes how you read a bear encounter. A bear is not stupid or reckless when it tries to access a campsite garbage can in May. It's responding to a biological imperative: metabolic demand created by torpor and emergence. The bear is not choosing to be bold. It's compelled by its own physiology.
This doesn't mean bears are not dangerous. Bears are large, strong, and potentially lethal predators. But the danger is not unpredictable. It flows directly from the conditions of the season: the bear's metabolic state, the availability of food, the bear's desperation. A bear in April is more dangerous than a bear in August, not because the April bear is inherently more aggressive, but because the April bear is hungrier and more motivated.
For humans in Yosemite, this means spring requires the most rigorous food storage practices. It means avoiding surprise encounters by making noise on the trail. May and June overlap exactly with peak season on the Mist Trail and the other Valley hikes, when bears are foraging the same lower meadows that visitors pass through. It means understanding that a bear's boldness in spring is an expression of metabolic urgency, not malice.
For the bears themselves, spring in Yosemite is often a period of high stress. If natural food is not readily available, bears may starve despite successfully emerging from torpor. If food is available (as it is in years with abundant mast crops the previous fall, or in locations where humans provide easy access to food), bears thrive. The harsh spring season has shaped bear behavior and ecology for centuries.