The Secret Winter Lives of Lake Tahoe's
Black Bears
Keep Tahoe Bears Wild!
Spring is in the air at Lake Tahoe, and black bears are emerging from their winter dens. This includes mother bears with new cubs of the year that are only about 8 to 10 weeks old.
The reproduction cycle began back in the spring of last year, yet the mother bear has only been pregnant since around Thanksgiving. If the math doesn’t add up, here’s why: Black bears have a unique reproductive cycle that allows them to mate in the spring/summer but delay true pregnancy until the fall to increase the odds of success for a healthy pregnancy and fetus.
Black bears mate in the summer from June to August, and the adult females undergo a process called delayed implantation. The sperm from the male fertilizes the microscopic egg inside the female, which quickly develops into a small ball of cells called a blastocyst. At this point, the pregnancy process pauses with the blastocyst floating around in the uterus, suspending further development until the fall. This pause allows time for the female to gain the fat reserves she will need to sustain both herself and any cubs she may have during the upcoming winter hibernation, or torpor, as it’s more accurately called for bears.
Hibernation is tightly related to resource availability, and bears prepare for it in the fall by entering a period called "hyperphagia" (pronounced hi·per·fay·jee·uh). This period is marked by a substantial increase in feeding activity when bears are known to consume about 25,000 calories per day. All bears experience this feeding frenzy whether pregnant or not.
As natural food supplies dwindle and snow gradually blankets the Tahoe Basin, bears naturally go into hibernation to save precious energy and resources. This is because bears do not eat or drink during hibernation, nor do they urinate or defecate. Instead, they have developed the ability to live off acquired fat stores and recycle wastes back into usable proteins. During this period, their heart and metabolic rates drop significantly, and they can lose 25 to 40 percent of their body weight.
The stresses of hibernation are even more pronounced for females with cubs. To prepare for hibernation, the female’s body will carry out a self-evaluation in the late fall. If she has acquired the necessary fat reserves to sustain herself and her cubs throughout the entire hibernation period, the blastocyst will implant onto the uterine wall, and the fetus will begin to develop. In other words, true pregnancy begins. If the female has not accumulated enough body fat, the pregnancy ends and the blastocyst is reabsorbed by her body.
Typically before Feb. 1, a pregnant female will give birth in the den to a litter of one to four blind, naked cubs weighing less than 1 pound. The female hardly awakens from torpor during birth, becoming just alert enough to lick the cubs clean and move them into a position that keeps them warm and allows them to nurse. The cubs continue to nurse and grow, becoming more active through the remainder of hibernation. A female with new cubs of the year emerges from the den between March and May with cubs weighing around 5 to 7 pounds.
Throughout the cubs’ first year, they learn everything about how to be a bear from their mother, including where to find food, and what is dangerous and to be avoided. Cubs of the year are dependent on their mothers for several months and are taught how to forage on natural foods, including grass, berries and grubs. A female with cubs will be busy caring for and teaching her new cubs and will not mate again in the summer. Cubs of the year stay with their mother through the following hibernation and their first birthday, emerging from their mother’s den again the following spring as yearlings. Newly emerged yearlings are typically 50 to 150 pounds, and though they may be a bit awkward, they are well equipped and ready to go off on their own. Mother bears, once free of these “teenagers,” will once again find a mate and breed to continue the cycle of producing cubs every other year.
As omnivores, a bear’s diet is about 85 percent plant-based, with the remaining portion coming from insects, small mammals and carrion. Bears provide essential ecosystem functions, including helping to spread seeds through their scat, transporting pollen on their fur and tongues, cleaning up animals that died during the winter and aiding in nutrient cycling by digging for insects.
Though seeing a female bear with her new cubs can be a very exciting experience, bears play an important role in Lake Tahoe’s ecosystem and allowing them access to human food and garbage is detrimental to natural processes and their health. When mother bears teach their cubs to access human foods, they not only continue the cycle of human food-conditioning, but the cubs are much more prone to conflict, including being hit and killed by vehicles. Cubs taught to seek human food sources do not learn how to forage naturally. Instead, once they separate from their mother, they become dependent on human food sources and pass on the same unhealthy foraging habits to their own cubs.
You can support black bears during their reproductive process by giving females with cubs plenty of space and by securing attractants and human food sources that may lead new cubs down a path toward conflict. It is a shared responsibility to keep Tahoe’s bears healthy, safe, and wild!
For more information about black bear reproduction and cubs, visit January / February Is Birthday Time for Bears. For more information on coexisting with bears, visit BearWise.org or TahoeBears.org.
To report human-bear conflicts:
- In California, contact the CDFW at (916) 358-2917 or report online using the Wildlife Incident Reporting (WIR) system at apps.wildlife.ca.gov/wir.
- Non-emergency wildlife interactions in California State Parks can be reported to their public dispatch at (916) 358-1300.
- In Nevada, contact the NDOW at (775) 688-BEAR (2327).
- If the issue is an immediate threat, call the local sheriff’s department or 911.
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