On the evening of 13 August 1967, two women were attacked and killed by grizzly bears (Ursus arcto) in separate incidents within Glacier National Park (GNP). Following these incidents, there was speculation that due to odors associated with menstruation, women may be more prone to attack by bears than are men (Rogers et al. 1991).
The objective of this paper is to present the data available on this subject so that women can make an informed choice when deciding whether or not to hike and/or camp in bear country during their menstrual period.
In a study designed to test the hypothesis that bears are attracted to the odors of menstruation, Cushing (1983) reported that when presented with a series of different odors (including seal scents, other food scents, non-menstrual human blood, and used tampons), four captive polar bears (Ursus maritimus) elicited a strong behavioral response only to seal scents and menstrual odors (used tampons). Cushing (1983) also reported that free-ranging polar bears detected and consumed food scent samples and used tampons, but ignored non-menstrual human blood and unused tampons. This suggests that polar bears are attracted to odors associated with menstrual blood.
Herrero (1985) analyzed the circumstances of hundreds of grizzly bear attacks on humans, including the attacks on the two women in GNP, and concluded that there was no evidence linking menstruation to any of the attacks. The responses of grizzly bears to menstrual odors has not been studied experimentally.
Rogers et al. (1991) recorded the responses of 26 free-ranging black bears (Ursus americanus) to used tampons from 26 women and the responses of 20 free ranging black bears to four menstruating women at different days of their flow. Menstrual odors were essentially ignored by black bears of all sex and age classes. In an extensive review of black bear attacks across North America, no instances of black bears attacking or being attracted to menstruating women was found (Cramond 1981, Herrero 1985, Rogers et al. 1991).
Yellowstone National Park Bear-Inflicted Human Injury Statistics
Prior to 1979, most bear-inflicted human injuries in Yellowstone National Park (YNP) involved human food conditioned bears that were aggressively seeking human foods and injured people in the process. By 1979, human foods and garbage were no longer readily available to park bears and few human food conditioned bears remained in the population. During the 40 year period from 1979 through 2018, more than 118 million visits to YNP were recorded. These visitors spent over 26 million overnight stays camping in developed area roadside campgrounds, and over 1.6 million overnight stays camping in remote backcountry areas of the park. Although actual statistics are not available, thousands of menstruating women undoubtedly visited, hiked and/or camped within YNP over the last 40 years. From 1979 through 2018, 50 people were injured by bears (41 by grizzly bear, 6 by black bear, and 3 by bears where the species was not identified) within YNP, an average of only 1.3 bear-inflicted human injuries per year. Of these 50 injuries, 39 (78%) were men, and only 11 (22%) were women. Of the 10 incidents where women were injured, most (73%, n=8) involved surprise encounters with bears while the women were hiking, and were therefore not likely related to menstruation. One incident involved a female park ranger moving an injured bear that had been hit by a car, off of the roadway. In one incident a grizzly bear pulled a woman out of her tent at night and killed and partially consumed her. However, the woman was not menstruating at the time of the attack. In one incident a curious bear approached and bit a woman, but the woman was not menstruating at the time. There was no evidence linking menstruation to any of the 11 bear attacks on women. It is difficult to accurately compare the ratio of males to females that are injured by bears because the park does not keep records of visitor use in the park by gender. However, the bear-inflicted human injury data from YNP does not indicate any correlation between bear attacks and menstruation (Gunther and Hoekstra 1996).
Although there is no evidence that grizzly and black bears are overly attracted to menstrual odors more than any other odor, people who are still concerned can take additional precautions to further reduce the risks of attack.
The following precautions are recommended:
The question whether menstruating women attract bears has not been completely answered (Byrd 1988). However, there is no evidence that bears are overly attracted to menstrual odors more than any other odor and there is no statistical evidence that known bear attacks have been related to menstruation (Byrd 1988). For all park visitors combined, the chances of being injured by a bear are approximately 1 in 2.7 million visits. The probability of being injured by a bear is significantly lower for those visitors that don’t leave park developments, roadsides, or boardwalk trails (1 in 39.6 million visits), but higher for those camping overnight in roadside campgrounds (1 in 26.6 million overnight stays) or backcountry campsites (1 in 554,000 overnight stays). The greatest risk is incurred by people while hiking in the backcountry (1 in 232,000 travel days). Although the risks are low, visitors have been injured and killed by bears in Yellowstone National Park (Gunther and Hoekstra 1996). If you are uncomfortable hiking and camping in bear country for any reason, you should probably choose another area for your recreational activities. Your risk of bear attack is highest while hiking in the backcountry. You can reduce the risks by: 1) hiking in groups of 3 or more people, 2) staying vigilant for bears, 3) making noise in areas of poor visibility, 4) carrying bear spray and being proficient with its use, 5) staying on maintained, designated hiking trails, and, 6) not running during encounters with bears.
Byrd, C.P. 1988. Of bears and women: Investigating the hypothesis that menstruation attracts bears. M.S. Thesis, Univ. Montana, Missoula. 129pp.
Cramond, M. 1981. Killer bears. Outdoor Life Books. Charles Scribner's Sons, New York, N.Y. 301pp.
Cushing, B. 1983. Responses of polar bears to human menstrual odors. International Conference on Bear Research and Management 5:270-274.
Gunther, K.A., and H.L. Hoekstra. 1996. Bear-inflicted human injuries in Yellowstone, 1970-1994, a cautionary and instructive guide to who gets hurt and why. Yellowstone Science 4(1): 2-9.
_____. 2016. Yellowstone National Park bear-related injuries/fatalities. Bear Management Office Information Paper No. BMO-1. U.S. Department of Interior., National Park Service, Yellowstone National Park. 2pp.
Herrero, S.M. 1985. Bear attacks - their causes and avoidance. Winchester Press, New Century Publishers, Inc., Piscataway, New Jersey. 287pp.
Rogers, L.L., G.A. Wilker, and S.S. Scott. 1991. Reaction of black bears to human menstrual odors. Journal of Wildlife Management 55(4):632-634.
Last updated: September 10, 2019