Under The Baby Moon
By Dr. Jenn Ballard
As the last leaves fall from the trees and the air turns crisp, autumn is fading into winter and black bears are transitioning to a new season as well. Their autumn gluttony nearly complete, well-provisioned bruins now meander toward a season of rest; but for some, that rest will be interrupted by the improbable act of giving birth during the least hospitable time of year. The Osage (Wahzhazhe), who traditionally mark months according to social or natural features, call December the “Black Bears Give Birth Moon,” and rightly so. Bears stand out as the only group I’m aware of that gives birth during winter dormancy. In preparation for this, pregnant sows will be the first to start the winter denning cycle in late November or December, though that is not where their reproductive story begins.
Black bear breeding season occurs in early to mid-summer, usually June and July. While most wildlife time their reproduction to ensure young are born into optimal resources, bears follow a rhythm all their own. They seem to squeeze breeding into their busy schedules between den emergence and fall foraging, while their bodies put off actual pregnancy until much later. During the breeding season, most sows will experience multiple ovulation or “heat” cycles. Interestingly, these cycles seem to continue even if the female is already pregnant. This provides sows with ample opportunities to mate, and may afford them the advantage of breeding with multiple males. Giving birth to cubs with different sires in the same litter increases genetic diversity in the offspring, which can improve the odds of adding healthy new bears to the population. This polyestrous (multiple cycles) strategy would be problematic if it produced pregnancies at different stages, but bears accommodate with an adaptation known as delayed implantation.
Implantation is the process during which a developing offspring attaches to the wall of its mother’s womb; once there, a physical connection develops with the mother to exchange nutrients. Delayed implantation is exactly what it sounds like—a gap of time between when an egg is fertilized and when this connection develops. In the case of black bears, fertilized eggs develop into blastocytes, or a cluster of a few hundred cells, organized around a fluid-filled sac. Development then pauses completely. Potential offspring remain in this state of suspended animation for up to five months. Even pregnancies produced later in the summer have plenty of time to catch up to their peers before development begins again in earnest.
As late summer rolls into fall, black bears devote all of their time and energy to eating as much food as possible, a behavior known as hyperphagia. This helps them build up fat stores they will use during denning, when they will not eat or drink for months at a time. The body condition of sows during this time is closely linked to their reproductive output later in the year, but the exact mechanisms are difficult to study. It is believed that if a sow has insufficient resources to sustain herself and multiple cubs through the winter, her body may not allow all of the available blastocytes to complete their development. As a result, sows bred in the summer may produce anywhere from zero to five cubs depending on their age and nutritional condition, but for bears with reasonable food availability, the average litter size is two.
It's a common misconception that black bears give birth while sleeping, but they are actually not true hibernators and rouse periodically during denning, including to birth and care for cubs. While the total time from conception to birth can approach 210 days, the duration of true pregnancy following delayed implantation is around 60-70 days. That is similar to the gestation of a domestic dog, and, like a dog, bear cubs are born naked, blind, and only a fraction of the size of their mothers. They are not truly premature because they have completed the normal development for their species; instead, they are born immature (also called altricial) and need significant parental care. They will spend the remainder of the denning season nursing and sleeping in the relative security of their den and the warmth of their mother’s heavy winter coat.
When spring arrives, new mothers will be some of the last to emerge from their dens to ensure that their cubs are strong enough to climb trees and escape danger before venturing out. The cubs will remain with their mother for their entire first year, learning and benefiting from her protection. While caring for cubs, sows do not participate in the following year’s breeding season. Instead of producing a litter annually, female black bears den with their yearlings for a second time. It’s not until their second spring that mother bears force their yearlings to disperse and establish their own home range. Male offspring tend to be pushed farther away, while young females may be tolerated to remain in the area but not directly in their mother’s company. After their yearlings disperse, female bears will again participate in summer breeding and begin the process anew.
While early breeding has been documented in some cases, most sows do not participate in the reproductive cycle until their third summer, and they can continue to reproduce into their late teens and early twenties. Nonetheless, this process of late maturation, biennial breeding cycles, and small litter sizes results in a low reproductive potential for individual bears as well as slow population growth. High levels of maternal care are an adaptation intended to compensate by increasing cub survival. Wildlife species with low reproductive rates can be vulnerable to factors that reduce recruitment or increase adult mortality, and their populations may be slow to rebound if issues arise. Black bear managers in particular take this into consideration, looking at harvest rates, sex ratios, food availability, denning success, and many other factors to inform the goal of sustainable harvest.
Bridges AS, Vaughan MR, Fox JA. 2011. American black bear estrus and parturition in the Allegheny Mountains of Virginia. Ursus 22(1): 1-8.
Burns, LF. 2004. A History of the Osage People. The University of Alabama Press, Tuscaloosa, Alabama, pp. 210-211.
Himelright BM, Moore JM, Gonzales RL, Mendoza AV, Dye PS, Schuett RJ, Durrant BA, Read BA, Spady TJ. 2014. Sequential Ovulation and fertility of polyestrous in American black bears (Ursus americanus). Conservation Physiology 2. DOI: 10.1093/conphys/cou051.