Mule deer (Odocoilus hemionus) belong to the Cervidae family, hoofed mammals that have antlers, which also include elk (Cervus elaphus), moose (Alces alces), and caribou (Rangifer tarandus). There are numerous sub-classifications of deer, but the most distinctive include mule deer and white-tailed deer (O. virginianus). White-tailed deer are common throughout much of North America, and mule deer are common throughout western North America; both species are common throughout much of Colorado. The Latin term hemionus means “half mule,” referring to the relatively large ears characteristic of mule deer.
Mule deer evolved in North America and are well adapted to western landscapes. Colorado holds extensive mule deer habitat and supports some of the largest mule deer numbers when compared to other states and the Canadian provinces. Mule deer in Colorado and other western states have experienced large fluctuations in population size due to a variety of factors such as changes in habitat quality and quantity, severe weather, population management, predation, disease, and interactions with elk. Market hunting to feed miners and early settlers in the late nineteenth century initially reduced deer numbers in Colorado, and low deer numbers continued into the early 1900s as the state became more populated. Concern over low deer numbers during this period resulted in the first restrictions on deer hunting and also included predator control efforts to benefit deer populations.
Mule deer populations began to recover during the early 1930s and continued to increase for the next several decades, largely due to the advent and refinement of modern wildlife management techniques and changes in agricultural practices. Relatively high mule deer populations continued through the 1980s, but a population decline became evident during the 1990s, and the population has recently stabilized at lower numbers. Over the past forty years, mule deer populations in Colorado have ranged from above 600,000 to about 400,000 today. The exact reason for the most recent decline is uncertain, but it is likely related to habitat loss from human expansion and development, increased elk and predator populations, and changing weather patterns.
Characteristics and Habitat
Mule deer are mid-sized ruminants exhibiting a relatively low rumen to body-size ratio and a higher metabolic rate when compared with larger cervids. Mule deer are selective foragers, feeding on a variety of grasses, forbs, and shrubs. The animals prefer vegetated areas that not only provide them with ample foraging but also with both thermal and hiding cover. Thus, areas characterized by both vegetative and topographic diversity provide good mule deer habitat.
In Colorado mule deer typically breed during mid-to-late November and produce their young during June. Females commonly produce twins, with occasional singletons and rare triplets. Fawns are typically weaned by eight to ten weeks of age and become reproductively mature as yearlings. Only males (bucks) grow antlers, which shed and re-grow annually and increase in size as the animals mature. Males compete for the opportunity to breed with multiple females, ensuring the healthiest individuals pass along their genes to their offspring. Mule deer life span typically ranges from about twelve to fifteen years.
Because of western Colorado’s topographic diversity, many mule deer populations migrate from high-elevation summer ranges to low-elevation winter ranges. In Colorado higher elevations receive increased moisture during spring and summer and thus provide enhanced forage conditions for deer. As fall approaches and temperatures drop with intermittent snowstorms, plants become less palatable; when deeper snows make plants unavailable at higher elevations, mule deer seek lower-elevation winter ranges where plants (primarily shrubs) are more easily accessible. Mule deer migration typically occurs during April, May, and October. Mule deer in Colorado typically migrate twenty to thirty miles between summer and winter ranges. Because forage quantity and quality are reduced during winter, deer exhibit a negative energy balance and lose weight over winter. Winter is the most critical period for deer survival, and severe winters with prolonged deep snow and cold temperatures can result in high mortality rates, especially for the young.
Mule deer are a valued game species in Colorado, generating about $115 million annually from license fees, fuel, hotels, supplies, and other associated transactions. Hunters travel from all over the world to hunt mule deer in Colorado, making the animals an important revenue source for many Colorado towns during the fall hunting season. The world record mule deer buck came from Dolores County, Colorado in 1972.