Blog 8: Evolutionary Tree

This is the modified evolutionary tree we created while studying the mammalian hibernation patterns of the Ursidae family (the bear family). Mammalian hibernation is defined as a type of hibernation that has a specialized, seasonal reduction in metabolism that is concurrent with scare food and cold weather. Therefore, bears that perform mammalian hibernation hibernate during the coldest periods of wintertime. If they did not, they would have a slim chance of surviving the brutal weather that winter brings. Our hypothesis was that bear hibernation is not ancestral. Therefore, it is not a trait found in the common ancestor.

Our data supports our hypothesis; bear mammalian hibernation is not ancestral. According to our data, only the Brown and American Black bear demonstrate mammalian hibernation; the Sun, Sloth, Andean bear, and Giant Panda do not mammalian hibernate to any extent; the Polar Bear and Asiatic Black Bear perform some characteristics of mammalian hibernation, but they do not fully mammalian hibernate. Clearly this behavior is not found in the common ancestor and therefore did not evolve over time. However, we discovered a reasonable answer as to why two bears in the Ursidae family fully mammalian hibernate, four do not at all, and two semi do. The bears that hibernate are bears that live in geographic regions where the winters are extremely cold. Thus, they hibernate for a few months in order to survive the winter months. They crawl into dens and remain in a tight ball until their hibernation period is over. The heart rate drops as well as the body temperature during this time. The bears that do not hibernate are bears that live in somewhat tropical regions where there is not a harsh winter. Therefore, there is no reason to hibernate if there is both food available and temperatures that don't reach below freezing all year round. For the two bears that semi-hibernate (the Polar and Asiatic black), it's slightly different. It's cold year round where Polar bears live, so the specie is already accustomed to surviving the cold temperatures. With that said, they do not perform mammalian hibernation like the Black and Brown bears do but rather remain in their dens for relatively long periods of time (several months). Asiatic black bears live in several different areas of southern Asia where there are two different climates with different temperatures. Therefore, only bears in the northern part of the region in their habitat mammalian hibernate whereas the bears in the southern part do not; some will sleep the whole winter, and some will sleep only during the harshest parts of it.  From our data, we concluded that bears use mammalian hibernation as a defense against the cold, harsh winter months. If bears live in areas where there aren't tough winters, then they don't hibernate. Therefore, the trait didn't start in the common ancestor (the Giant Panda according to our evolutionary tree) but emerged as the different bear species did.