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American black bears (Ursus americanus)
can only be found on the North American
continent, ranging from as far south as Mexico
all the way to parts of Canada (e.g. Goodrich
and Berger, 1993). Within these areas, black
bears tend to migrate towards regions dense with
conifer and pine trees as well as ground
vegetation and a water supply (e.g. Sadeghpour
and Ginnett, 2011). In order to live in this
habitat bears have adapted in certain ways to
survive with regards to how they get food, why
they eat specific food, and where they migrate
in regards to food or predators.
A black bears diet is different with regards to seasons of the year, due to the fact that black bears hibernate in the colder months of the year (e.g. Goodrich and Berger, 1993). In the summer, black bears typically eat berries, grasses, roots tubers, insect nests, caribou, and moose (e.g. Bartareau et al., 2012). In order to obtain some of these foods, bears have adapted features which help them attain the nutrients more easily. For example, in order to get at the insect nests and roots, which are found under ground, bears have adapted sharp claws for digging (Bartareau et al., 2012). As the summer comes to a close, and the colder months start approaching bears add salmon to their diet along with tree bark, both of which are high in starch which will help them pack on the pounds in order to survive the winter months when they will not be able to feed, drink, or defecate (e.g. Iaizzo et al., 2012). In order to eat these things, the American black bear uses its sharp claws as well as its teeth, specifically its incisors, to strip the bark off of the tree, an adaptation that has helped it get the nutrients it needs for its dormant months (e.g. Schmidt and Gourley, 1992). Due to the fact that the incisors play such a significant role in feeding, it has been found that tooth size can change between organisms based on their diets (Miller et al., 2009). The variation in a black bears diet of both meat and vegetation allows it to have large teeth for eating the meat and vegetation (e.g. Evans et al., 2007). Lastly, bears have adapted certain advantages that help them obtain the slippery salmon that are a major part of their fall diet. Using their paws, black blears pin the salmon down and then pick up and carry the fish in their jaw to dry land where they proceed to eat it, a learned behavioral adaptation for energy consumption (Gende et al., 2001). So, why do black bears change their diet during specific times of the year? As stated earlier, bears hibernate which means they are dormant during the cold months. They do not leave their den and therefore, are not able to attain nutrients like food or water, they are not even able to defecate. So, how do they survive you ask? Well, American black bears have adapted to the changing climate and resource availability by increasing their intake of hard mast nutrients that are full of starch and protein that will keep them warm. They have adapted by evolving certain mechanisms, physiologically, that help them retain protein and water within their cells, helping them keep their lean body mass (e.g. Oftedal et al., 1992). If black bears had not learned to bulk up before dormancy, they would have died during the hibernating months due to lack of food resources outside of the den at that time, which is the whole reason why black bears hibernate at all, due to a lack of nutritional resources (Hayes and Pelton, 1994; Garrison et al., 2012). Another interesting adaptation that the American black bear has evolved in regards to hibernation is that they have special healing abilities which allow them to heal wounds while in hibernation even with slightly hypothermic body temperatures and a lack of nutrient intake (Iaizzo et al., 2012). This gives black bears a clear survival asset that will help them if they are wounded close to their time of hibernation. In several of the regions in which the American black bear lives, there are other predators too. A commonly occurring predator that lives alongside black bears is Ursus arctos (brown bear) (Garbeau et al., 2006). This has caused certain navigation adaptations to arise in the black bear species. Black bears are typically solitary creatures and, therefore, tend to avoid confrontation with other species. It has been found that black bears, females in particular, tend to navigate more carefully when coming out of hibernation, staying out of the way of their fellow brown bear territory (Young and Beecham, 1983; Garbeau et al., 2006). This is most likely due to the fact that the females are smaller than males in size and mass (Bartareau et al., 2012). This is an adaptation of males in terms of sexual selection because it has been found that males black bears that are larger more commonly mate with female black bears which is why there has been a positive correlation found between “fighting ability and mating access” (e.g. Kovach and Powell, 2003). In order to survive in their habitat, the American black bear has evolved several adaptations to aid them. They have claws and jaws which help them obtain the several different types of nutrients they need in order to hibernate. While in hibernation they lack nutritional resources which they have countered by evolving behaviors or knowledge of obtaining food with higher starch and protein concentrations to bulk up for dormancy. In the colder regions, black bears have adapted to the harsh cold climates by hibernating during low nutritional peaks, a survival skill necessary to prevent their species from going extinct. To most, these just seem like obvious things for bears to do, but like many things that organisms do, they are survival skills which their life depends on! |
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