Grizzly Bear - Slow Life
Fitness is maximized by allocating resources to survival or reproduction and the function over which allocation will be biased depends on levels of extrinsic mortality17. Under high extrinsic mortality, fitness can be optimized by higher investment in reproduction, leading to selection for faster life histories5,13, similar to what would be expected under natural selection. This acceleration in the pace of life has been documented in several harvested populations, which suggests that harvest should lead to r-selection2,13,16,18. Moreover, even in the absence of any harvest preferences, individuals can differ in vulnerability to harvest, depending on behavior, harvest methods, and regulations16,19,20,21. Indeed, in several sport hunting systems, the killing of females with dependent offspring is either illegal, discouraged, or avoided by hunters to protect the female segment of the population or because of the potentially lowered survival of orphaned offspring that can cause ethical, as well as demographic, issues22,23,24,25. In such systems, reproducing females are less vulnerable to hunting and thus should enjoy an artificial selective advantage that is accentuated with increasing hunting pressure. This type of harvest selectivity could promote longer periods of mother-offspring associations and slower life histories, with potential consequences for population dynamics. Although the potential selective and demographic effects of the protection of females based on reproductive status have already been acknowledged20,22,23,24,25,26,27, these effects have rarely been quantitatively assessed using empirical data16.
Grizzly Bear - Slow Life
Here, we test whether a hunting regulation that prohibits the killing of females with dependent offspring can induce selectivity on female reproductive tactics at the individual level and evaluate the effect of such selectivity on population processes. We use more than 20 years of exceptionally detailed individual-based data on survival and reproduction in a heavily hunted population of brown bears (Ursus arctos) in Sweden28, where two distinct maternal care tactics have been documented29. We start by documenting the temporal trend in the duration of maternal care and contrasting survival probabilities between females providing either short (1.5-year tactic) or long (2.5-year tactic) maternal care. Longer maternal care entails a loss of reproductive opportunities in species where breeding is not resumed until current offspring are weaned30. Therefore, we compare two demographically and evolutionary meaningful proxies of fitness31 that integrate information on survival and reproduction, i.e., asymptotic population growth rate (λ; the annual per capita rate of population increase32) and net reproductive rate, R0 (number of females an individual is expected to produce over its lifetime31,33), between the two maternal care tactics to quantify the difference in fitness between these tactics. Finally, we complement this analysis by evaluating the fitness pay-off of each maternal care tactic under various plausible scenarios of hunting pressure to determine if hunting can drive the relative occurrence of maternal care tactics in the population.
Humans as predators are a dominant agent of mortality in wildlife populations1 imposing a selective landscape that vary both in its strength and phenotypic targets14,15,18,20,35. Here, we show that a hunting regulation based on female reproductive status can improve the survival prospects of female brown bears that provide longer maternal care, thereby promoting slow life histories, with consequences for population processes.
Grizzlies also directly regulate prey populations and help prevent ungulate overgrazing. Studies show that the removal of wolves and grizzly bears in Grand Teton National Park caused populations of their herbivorous prey to increase, which decreased the density of plants in the area. The decrease of plant density led to a decrease in the population sizes of migratory birds. Grizzly bears, like all carnivores, are keystone predators, having a major influence on their ecosystems.
Grizzly bears are most often found on upper elevation slopes, in avalanche chutes, and in lower elevation wetlands. Female grizzly bears need about 50 to 300 square miles of habitat, and males require 200 to 500 square miles.Grizzly bears usually have overlapping ranges with several bears sharing an area.
Although sometimes portrayed in the media as voracious predators, grizzly bears are normally reclusive creatures. Grizzly bears are intelligent, curious, and have excellent memory, particularly regarding where food sources are located.
In the heat of the day, grizzly bears will rest in day beds in dense vegetation, including willows, alders, dense forest, and tall grass. Most grizzly bears spend their time alone except when breeding or raising cubs.
Cubs are born in the den in late January or early February and remain with the female for 2 to 3 years before the mother mates again and produces another litter. Grizzly bears have one of the slowest reproductive rates among terrestrial mammals, due to their late age of first reproduction, small average litter size, and the long interval between litters: it may take a single female 10 years to replace herself in a population.
Female grizzly bear stop breeding in their mid-to late 20s. The typical female may give birth to a maximum of 10 cubs over her lifetime, half of which usually die within a year. Surviving cubs usually remain with the mother for 2 1/2 to 3 1/2 years, during which time the mother will not mate.
For 3 to 6 months during winter, grizzly bears enter dens and hibernate during periods of low food availability, deep snow, and low air temperature. Grizzly bears in the lower 48 states spend up to 4 to 6 months in dens beginning in October or November.
During hibernation, grizzly bears do not eat, drink, urinate, or defecate. Denning grizzly bears exhibit a marked decline in heart and respiration rate, but only a slight drop in body temperature. Due to their relatively constant body temperature in the den, denning grizzly bears can be easily aroused and have been known to exit dens when disturbed. Both males and females have a tendency to use the same general area year after year but the same exact den site is rarely used twice by an individual.
In preparation for hibernation denning, grizzly bears consume excess amounts of food and the bears may gain more than 3 pounds a day, deposited as fat. The excess fat also serves as energy to the bear upon emergence from the den, when food is still sparse.
Grizzly bears are opportunistic omnivores. In Washington and Idaho a typical grizzly bear diet is less than 10% fish or meat, and much of the meat is carrion from winter-killed deer and elk. In areas where animal matter is less available, grasses, roots, bulbs, tubers, and fungi are important parts of the grizzly diet.
During years when there are shortages of natural food sources, conflicts between humans and grizzly bears are more frequent, resulting in higher numbers of human- caused grizzly bear mortalities due to defense of life or property and management removals of nuisance bears.
In North America there are two subspecies of brown bear (Ursus arctos): the Kodiak bear, which occurs only on the islands of the Kodiak Archipelago, and the grizzly bear, which occurs everywhere else. Brown bears also occur in Russia, Europe, Scandinavia, and Asia.
Grizzly bears are omnivores. The most commonly eaten kinds of plants are fleshy roots, fruits, berries, grasses, and forbs. If grizzly bears are on the hunt, their prey can include fish (especially salmon), rodents like ground squirrels, carrion, and hoofed animals like moose, elk, caribou, and deer. They are especially good at catching the young of these hoofed species. Grizzly bears can also target domestic animals like cattle and sheep and cause economically important losses for some ranchers. The National Wildlife Federation has a program on National Forest lands surrounding Yellowstone Park to prevent attacks on domestic livestock by purchasing the grazing allotments from ranchers.
Winter can be very tough for many species of wildlife, because the season brings harsh weather and little food. Grizzly bears hibernate in warm dens during the winter to minimize energy expenditure at a time when natural foods are not available and to permit their tiny young to be born in a warm and secure environment. Throughout the summer and autumn, grizzly bears build up fat reserves by consuming as much food as they can find. In late fall or winter, the bears find a hillside and dig a hole to serve as their winter den. When inside the den, grizzly bears slow down their heart rate, reduce their temperature and metabolic activity, and live off stored fat reserves. Pregnant females give birth in the dens and nurse their cubs until they are large enough to venture outside in the spring as snow melts and new food become available.
The mother cares for her young for at least two more years, feeding and protecting them. When the cubs are two and a half years old, they typically separate from their mother. In areas with little food, the cubs may stay with their mother longer. Typically separation happens when the female enters breeding condition and attracts males, which can be a threat to the cubs. At around five years of age, grizzly bears reach sexual maturity.
Grizzly bears are federally listed as threatened. They were excessively overhunted by humans, and now there are less than 1,500 grizzlies left in the United States south of Canada; there are also about 31,000 in Alaska. The National Wildlife Federation is fighting for grizzly bears to make sure they have room to roam and can safely coexist with humans.
Slow to Grow. Grizzly bears are one of the slowest reproducing land mammals in North America. Females do not breed until 6-8 years old, and cubs will stay with mom until they are 2-3 years old. On average, litters are only 1-3 cubs, making every female bear extremely important to the survival of the species! 041b061a72