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The Mule Deer (Odocoileus hemionus) is a deer indigenous to western North America; it is named for its ears, which are large like those of the mule. Two subspecies of mule deer are grouped into the black-tailed deer. Unlike the related white-tailed deer (Odocoileus virginianus), which is found throughout most of North America east of the Rocky Mountains and in the valleys of the Rocky Mountains from Idaho and Wyoming northward, mule deer are found only on the western Great Plains, in the Rocky Mountains, in the southwest United States, and on the west coast of North America. Mule deer have also been introduced to Argentina and Kauai, Hawaii.
Mule deer migrate from low elevation winter ranges to high elevation summer ranges. Although not all individuals in populations migrate, some will travel long distances between summer and winter ranges. Researchers discovered the longest mule deer migration in Wyoming spanning 150 miles from winter to summer range Multiple US states track mule deer migrations. Mule deer migrate in fall to avoid harsh winter conditions like deep snow that covers up food resources, and in spring follow the emergence of new growth northwards. There is evidence to suggest that mule deer migrate based on cognitive memory, meaning they use the same path year after year even if the availability of resources has changed. This contradicts the idea that animals will go to the areas with the best available resources, which makes migratory paths crucial for survival.