[10] The 10 valid subspecies, based on the third edition of Mammal Species of the World, are:[5] The most noticeable differences between white-tailed and mule deer are ear size, tail color, and antler configuration.

Mule deer antlers are bifurcated; they "fork" as they grow, rather than branching from a single main beam, as is the case with white-tails.

The mule deer is the larger of the three Odocoileus species on average, with a height of 80–106 cm (31–42 in) at the shoulders and a nose-to-tail length ranging from 1.2 to 2.1 m (3.9 to 6.9 ft).

[24][25][26][27][excessive citations] Unlike the white-tailed, the mule deer does not generally show marked size variation across its range, although environmental conditions can cause considerable weight fluctuations in any given population.

[28] In addition to movements related to available shelter and food, the breeding cycle is important in understanding deer behavior.

Bears and small carnivores are typically opportunistic feeders and pose little threat to a strong, healthy mule deer.

[35][32] Mule deer are intermediate feeders rather than pure browsers or grazers; they predominantly browse but also eat forb vegetation, small amounts of grass and, where available, tree or shrub fruits such as beans, pods, nuts (including acorns), and berries.

[36][37] In the Sierra Nevada range, mule deer depend on the lichen Bryoria fremontii as a winter food source.

[38] The most common plant species consumed by mule deer are the following: Mule deer have also been known to eat ricegrass, gramagrass, and needlegrass, as well as bearberry, bitter cherry, black oak, California buckeye, ceanothus, cedar, cliffrose, cottonwood, creek dogwood, creeping barberry, dogwood, Douglas fir, elderberry, Fendlera species, goldeneye, holly-leaf buckthorn, jack pine, knotweed, Kohleria species, manzanita, mesquite, pine, rabbitbrush, ragweed, redberry, scrub oak, serviceberry (including Pacific serviceberry), Sierra juniper, silktassel, snowberry, stonecrop, sunflower, tesota, thimbleberry, turbinella oak, velvet elder, western chokecherry, wild cherry, and wild oats.

[32][39] Humans sometimes engage in supplemental feeding efforts in severe winters in an attempt to help mule deer avoid starvation.

Supplemental feeding efforts might be appropriate when carefully conducted under limited circumstances, but to be successful, the feeding must begin early in the severe winter (before poor range conditions and severe weather cause malnourishment or starvation) and must be continued until range conditions can support the herd.

[31][43] Mule deer are ruminants, meaning they employ a nutrient acquisition strategy of fermenting plant material before digesting it.

[50][51][52][53][excessive citations] 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.

[56][57] Human activities such as natural resource extraction, highways, fencing, and urban development all have an impact on mule deer populations and migrations through habitat degradation and fragmentation.

[58][59][60][61][excessive citations] Natural gas extraction has been found to have varying negative effects on mule deer behavior and can even cause them to avoid areas they use to migrate.

[62] As traffic volumes increase, the more mule deer tend to avoid those areas and abandon their typical migration routes.

[63] In addition, urban development has replaced mule deer habitat with subdivisions, and human activity has increased.

One thing everyone can do is help slow the increase in climate change by using greener energy sources and reducing the amount of waste in our households.

[67] Approaches to mitigating the impact of drilling and mining operations include regulating the time of year when active drilling and heavy traffic to sites are taking place, and using well-informed planning to protect critical deer habitat and using barriers to mitigate the activity, noise, light at the extraction sites.