Bear Jam: Grizzly bear mother and cubs crossing a congested road in Grand Teton National Park (a)


Western grizzly bears (Ursus arctos) generally roam in solitude, avoiding humans and human-frequented areas [15]. Since the establishment of the US National Park system, however, the frequency of human-grizzly encounters has increased-- permanent campgrounds, in particular, are a bear's pantry [9]. Human-food-conditioned grizzlies use their keen sense of smell and scavenging memory to opportunistically source food [15], prying open car doors [9] to get at a morsel within or plucking it from an easily torn tent.

With the national and state park systems came a web of roads, routing heavy tourist traffic through grizzly territory.  Until the early 1970s, Yellowstone's roads offered the same attraction as campgrounds-- human food handouts and easily pillageable roadside trash cans made for a large number of roadside, human-food-conditioned bears. Since then, rangers have instituted bear-proof waste containers and outlawed the ecologically harmful and dangerous practice of bear-feeding.

Throughout the 1980s, rangers tried to deter grizzly activity within 400 m of paved roads, but this practice ultimately increased the frequency of human-bear confrontations, and reduced usable grizzly habitat by an estimated 130 sq miles--- thus, rangers re-strategized, turning to more dynamic methods such as bear-sitting (careful watch of roadside bears and, most importantly, management of human activity in their presence) and no-stopping zones [3].


The presence of human-food-seeking bears on roads has decreased with better public education and bear-proof disposal systems.  However, in the past thirty years, a strange behavior has arisen: non human-food-conditioned bears are often seen collecting natural forage by the roadside [3], despite the noise and threat of tourist traffic.

Map of the Greater Yellowstone Ecosystem (b)

Yellowstone rangers report that the majority bears spotted by the roadside are females with cubs [18], a perplexing observation. In 2013, the mother grizzly demographic comprised only ~80/740 individuals in the Greater Yellowstone Ecosystem (GYE) [4], a gargantuan area spanning over 28,000 square miles [10].  Since roads cover very little of the GYE's total area, it is significant that the relatively sparse population of mothers with cubs would choose to forage in these potentially dangerous areas of high human activity.

Why do Mothers and Cubs Frequent Roads?

Prevailing Theory:

The disproportionate presence of females and cubs in the sub-optimal road habitat might be explained as a counterstrategy to "sexually-selected infanticide."

  • SSI: Infanticide is most likely committed by roaming, non-parental or "immigrant" males. Immigrant males will kill cubs fathered by another bear in order to advance a female’s estrus and mate with her, thereby increasing their own lifetime reproductive fitness [8].
  • Though grizzlies are listed as threatened under the US Endangered Species Act [14], grizzlies are frequently and legally shot and killed by humans in self-defense in the GYE. A large proportion of grizzlies shot and killed in the GYE are adult males [5], and removal of adult males from a population increases the likelihood of NPI—one study conducted in 2000 showed that in the Kananaski Range (Alberta), legal hunting of adult male grizzlies was correlated with both a high proportion of younger adult males capable of NPI and an abnormally low average litter size (1.4 cubs/female) compared to the Selkirk Mountains of British Columbia (2.2 cubs/female) where trophy hunting is less common. Additionally, Kananaski females with cubs of the year were more likely to seek out sub-optimal habitats [16].
  • A recent GPS tagging study conducted in Alberta found that, during spring, sub-adult females and females with cubs were significantly more likely to cross and use roads than adult males [11]. Generally, this demographic of younger bears and females with offspring is known to choose more secure areas, devoid of lone adult males and females, over food-rich areas [1].

Alternate Theory:

Roads are essentially continuous clearings, which could allow for more convenient natural foraging.

Cubs in the breakdown lane (c)
  • In Summer and Fall, mother grizzlies with cubs of the year strongly prefer habitats on the edge of uncut forests [14], possibly because they are easier to travel.