A drought-induced shortage of food could make this a tough winter for young bears and reduce the number of cubs born to hungry hibernators.
As desperate bears try to fatten up for winter, they may have more conflicts with humans, but experts note that the starving bruins are in more danger from humans than vice versa.
Adult black bears are in little danger of starving during their long winter nap, according to Dave Garshelis of the Minnesota Department of Natural Resources and co-chair of the International Union for the Conservation of Nature's Bear Specialist Group (IUCN BSG). Bears are opportunistic survivors and can eke a living out of a parched landscape, but future generations of bears may be set back by a drought.
"The main thing affected during winter is reproduction," Garshelis said. "Bears mate in May-July, and blastocysts [an early stage in reproductive development] normally implant (in the uterus) in October-November, with births in January. However, if body condition is very poor, the blastocysts will not implant, or will later be naturally aborted, and reproduction will not occur."
Even if a cub is born, a hungry mother grizzly bear in Yellowstone National Park may end up raising a runt this year, due to a reduction in the quality and quantity of her milk.
"Lactation is energetically very demanding and is directly related to nutritional conditions that depend on the availability of foods," said Frank van Manen, team leader of the U.S. Geological Survey's Interagency Grizzly Bear Study Team. "Female grizzly bears in lower nutritional condition produce less milk and milk of lower quality. As a result, cubs of such females grow slower and have less body mass when they emerge from the den."
Mother bears and their new cubs are in greater danger of starvation in early spring, after the animals' metabolisms speed back up, but food sources are still scarce. Cubs over 18 months old that have been kicked out of the den will have to fend for themselves in the lean spring months. These youngsters are in the greatest danger of starvation.
"Yearlings are putting a lot of energy into growth as well as fat accumulation," said John Beecham, co-author of "A Shadow in the Forest -- Idaho's Black Bear" and chair of the Human-Bear Conflicts Expert Team of the IUCN BSG. "Although they may not starve during hibernation, in spring they could suffer because the first plants to sprout are grasses. Grass provides protein, but few carbohydrates. The yearlings may not get enough energy to survive until berries and other richer food sources are available."
The drought will probably not be much of a danger to adult bears, and experts agree that adult black bears are not likely to be a serious threat to humans. However, desperate bruins raiding trash cans and campsites may come into conflict with humans.
"Bears are very focused on food at this time of year," Beecham said. "The bears will be especially stressed this year. Humans need to help the bears avoid conflict by making efforts to make food difficult for bears to access. When bears become food conditioned, or accustomed to eating human food, it can be very difficult to break them of the habit. They then get bolder as they become more habituated to being in proximity to humans."
Grizzly bears can be a greater physical danger to humans, but the number of grizzlies captured in Yellowstone for being in conflict with humans has been within the normal range this year, according to van Manen. Hence, he doubts that campers are in any greater danger this year.
"Of course, campers and any other recreationists should always be very vigilant when in grizzly habitat and observe storage orders, carry bear spray, travel in larger groups and know how to respond in case of grizzly bear encounter," van Manen said.
Although humans are unlikely to end up in a bear's belly, starving and thirsty bears are more likely to become the prey of Earth's apex predator, Homo sapiens.
"When natural foods are poor, human hunters may be more successful for two reasons," Garchelis said. "One, bears are congregated in the few remaining places where there is food. Two, in several states, hunters can use bait and bears are more attracted to bait in food-poor years. Thus bear mortality may indeed be greater in poor-food years, because of a larger hunting harvest."
More than half of the United States is currently in a drought, but regional differences result in different conditions for bears.
"In Missouri, we are trapping bears as part of an ongoing research project and the bears we've captured are in good health," said Jeff Beringer of the Missouri Department of Conservation's and principal investigator for the Missouri Black Bear Project. "However, fall weight gain will be impacted by the production of acorns and other foods, which may be down as a result of the drought."
"In arid regions of the Southwest, my concern would be that water associated with human dwellings along with some succulent food resources could draw bears into more populated areas," said Jordan Schaul of the Alaska Wildlife Conservation Center and outreach coordinator for the IUCN BSG.
Bears in the western part of the U.S. may be in trouble if the drought continues into winter and snowfall is light, according to Beecham. In the west, bears depend on snow to seal and insulate their winter dens. In the spring, berries and other foods depend on snow run-off. Light snowfall can mean little food in the critical early spring period when bears most need calories.
The grizzlies of Yellowstone may be getting sufficient food despite the drought. Van Manen's research group observed the grizzlies feasting on a normal amount of army cutworms this year, and surveys suggest the production of nuts from whitebark pine trees will provide a normal amount of fall food.