|The mule deer is a ruminant, even-toed ungulate named for its large, mule-like ears that allow it to have extremely acute hearing. This herbivore is abundant across most of western North America and can be found in nearly all types of habitats. This social species lives in groups that follow a dominance hierarchy.|
The mule deer is a medium-sized deer that closely resembles its closest relative, the white-tailed deer (Odocoileus virginianus) with a few noticeable differences, such as tail, body size, ear size, and the configuration of its antlers.
The easiest way to differentiate the mule deer from the white-tailed deer is to look at the tail. While both deer have mostly white tails, the white-tailed deer only has white along the underside of the tail while the mule deer’s tail is completely white, aside from the tip. The mule deer’s tail is also smaller, thinner, and more rope like and terminate in a tuft of black hairs, or less commonly in a thin tuft of white hairs. Lastly, mule deer do not flash their tails in alarm like the white-tailed deer.
On average, the mule deer is the larger of the two Odocoileus species. Growth in mule deer during the first year is roughly parallel in males and females, but afterward, males, in general, exceed females in carcass weight, chest girth, neck circumference, body length, head length, cranial breadth, shoulder height, hindfoot length, and hoof length. Unlike the white-tailed deer, the mule deer does not generally show marked size variation across its range. Environmental conditions can cause considerable weight fluctuations in any given population, however.
The mule deer’s most defining characteristic is its large ears, which are about three-fourths the length of its head, much larger than those of the white-tailed deer.
The mule deer’s antlers are bifurcated and “fork” as they grow, rather than branching from a single main beam or central branch, such as the white-tailed deer’s antlers.
All mule deer markings vary considerably among the species, but remain constant throughout the life of an individual. The pelage of the mule deer ranges from dark brown gray, dark and light ash-gray, to brown, and even reddish. In the summer, the deer tend to sport coats of tannish-brown which lighten to a brownish-gray in the winter. The mule deer’s rump patch may be white or yellow, while the throat patch is white. On some mule deer, a dark dorsal line runs from the back, down the top of the tail, to the black tail tip. Mule deer iconically possess a dark V-shaped mark extending from a point between the eyes upward and laterally. This mark is more conspicuous in males. This distinctive black forehead, or mask, contrasts with the light gray face.
Bevy, Gang, Herd, Leash, Mob, Parcel, Rangale
Calf, Fawn, Kid
O. h. californicus (California)
O. h. cerrosensis (Cedros Island Black-Tailed, Cerros)
O. h. columbianus (Columbian Black-Tailed)
O. h. eremicus (Desert, Burro)
O. h. fuliginatus (Southern)
O. h. hemionus (Rocky Mountain)
O. h. inyoensis (Inyo)
O. h. peninsulae (Peninsular)
O. h. sheldoni (Tiburon Island)
O. h. sitkensis (Sitka Black-Tailed)
The mule deer is named for its ears, which are large like those of the mule.
It is the larger of the two extant species in the Odocoileus genus, the other being the white-tailed deer (Odocoileus virginianus). The name, Odocoileus, is sometimes spelled Odocoeleus, a contraction of the roots odonto– and coelus meaning hollow-tooth.
Ten subspecies of the mule deer have been identified, two of which, the Columbian black-tailed deer (O. h. columbianus and the Sitka deer O. h. sitkensis,) are classified as black-tailed deer. The black-tailed deer was at one time treated as a separate species, but is now mostly recognized as conspecific with the mule deer.
Some theories suggest that mule deer evolved from the black-tailed deer, but the mtDNA of the white-tailed deer (Odocoileus virginianus) and the mule deer are similar and differ from that of the black-tailed deer. This may be the result of introgression.
The Sitka deer (O. h. sitkensis), one of the black-tailed deer subspecies, is markedly smaller than other mule deer with an average weight of 54.5 kilograms (120 pounds) in males and 36 kilograms (79 pounds) in females.
The validity of the Inyo mule deer (O. h. inyoensis) is questionable.
The desert mule deer, also known as the burro mule deer, is sometimes referred to as O. h. crooki, but O. h. eremicus is considered the correct name. The specimen type of this subspecies is a hybrid of the mule deer and the white-tailed deer (Odocoileus virginianus). These hybrids are rare in the wild, but are more common locally in west Texas. Many claims of observations of wild hybrids are not legitimate, as identification based on external features is complicated. The hybrid survival rate is low, even in captivity.
Although most of the mule deer’s subspecies are not threatened, the Cedro Island subspecies (O. h. cerrocensis) is considered to be Vulnerable on the International Union for Conservation of Nature (IUCN) Red List of Threatened Species, as of 1988. This subspecies is in danger of becoming extinct because its densities are very low on the island where it occurs and poaching and predation by feral dogs are high.
The mule deer is indigenous to most of western North America, west of the Missouri River, or the 100th meridian from 23 degrees to 60 degrees North.
Unlike the related white-tailed deer (Odocoileus virginianus), which is found throughout most of North America east of the Rockies Mountains and in the valleys of the Rocky Mountains from Idaho and Wyoming northward, mule deer are only found on the western Great Plains, in the Rocky Mountains, in the United States southwest, and on the West Coast of North America.
In western North America, the mule deer occurs from Alaska and western Canada through the Rocky Mountains and western plains states of the United States, south to the Peninsula of Baja California, Cedros Island, Tiburon Island and Northwestern Mexico.
The eastern edge of the usual range extends from southwestern Saskatchewan through central North and South Dakota, Nebraska, Kansas, and western Texas.
The southernmost distribution reaches central Mexico, but the historical boundary is not very clear.
Major gaps in geographic distribution are in southern Nevada, southeastern California, southwestern Arizona, and the Great Salt Lake desert region. Isolated occurrences have been reported from Minnesota, Iowa, and Missouri.
Apart from these gaps, mule deer occur in all of the biomes of western North America north of central Mexico, except the Arctic tundra.
Mule deer have also been introduced to Argentina and Kauai, Hawaii.
The mule deer is remarkably adaptable and is well adapted to a variety of habitats.
The mule deer can be found in temperate forests, savannas, shrublands, scrub habitats, open ranges, grasslands, fields, wetlands, mountainous areas, deserts, semideserts, intertidal shorelines, artificial terrestrial and aquatic habitats, and even habitats with introduced vegetation.
In western North America, the mule deer occupies a wide range of habitat province, regions of land containing particular vegetation types. These habitat provinces include the California woodland chaparral, the Mojave Sonoran desert, the Interior semidesert shrub woodland, the Great Plains, the Colorado Plateau shrubland and forest, the Great Basin, the Sagebrush steepe, the Northern mountain, and the Canadian boreal forest.
Of at least sixty types of natural vegetation west of the 100th meridian in the United States, all but two or three have been occupied by the deer. Several additional vegetation types are inhabited in Canada and Mexico as well. The vegetation types in Mexico are similar to the types occurring in the United States, however, the tropical deciduous vegetation at the tip of Baja California is unique. In Canada, the mule deer occupies five boreal forest types that do not occur in the United States.
Mule deer thrive in habitats that have a combination of early-stage plant growth, mixed-species plant communities, and diverse and extensive shrub growth. A mixture of plant communities provides better forage for the deer than any single species. Plants that are young and emerging are more nutritious than mature trees and shrubs.
Canada: Alberta, British Columbia, Manitoba, Northwest Territories, Yukon
Mexico: Baja California, Chihuahua, Coahuila, Nuevo León, Sonora, Tamaulipas
United States of America: Alaska, Arizona, California, Colorado, Hawaiian Islands (Introduced), Idaho, Iowa, Kansas, Montana, Nebraska, Nevada, New Mexico, North Dakota, Oklahoma, Oregon, South Dakota, Texas, Utah, Washington, Wyoming
Forest: Subarctic, Temperate, Subtropical / Tropical Dry, Subtropical / Tropical Moist Lowland, Subtropical / Tropical Mangrove Vegetation Above High Tide Level, Subtropical / Tropical Swamp, Subtropical / Tropical Swamp
Savanna: Dry, Moist
Shrubland: Subarctic, Temperate, Subtropical / Tropical Dry, Subtropical / Tropical Moist, Subtropical / Tropical High Altitude, Mediterranean-Type Shrubby Vegetation
Grassland: Subarctic, Temperate Subtropical / Tropical Dry, Subtropical / Tropical Seasonally Wet/Flooded, Subtropical / Tropical High Altitude
Wetlands: Permanent Rivers / Streams / Creeks / Waterfalls, Seasonal / Intermittent/Irregular Rivers / Streams / Creeks, Shrub Dominated Wetlands, Bogs, Marshes, Swamps, Permanent Freshwater Lakes, Seasonal / Intermittent Freshwater Lakes, Permanent Freshwater Marshes/Pools, Seasonal / Intermittent Freshwater Marshes / Pools, Freshwater Springs & Oases, Geothermal Wetlands, Permanent Inland Deltas, Seasonal / Intermittent Saline, Brackish, or Alkaline Lakes & Flats
Desert: Hot, Temperate, Cold
Marine Intertidal: Salt Marshes, Emergent Grasses
Artificial/Terrestrial: Arable Land, Pastureland, Plantations, Rural Gardens, Urban Areas, Subtropical / Tropical Heavily Degraded Former Forest
Artificial/Aquatic & Marine: Seasonally Flooded Agricultural Land
The mule deer has excellent binocular vision. While unable to detect motionless objects, it’s extraordinarily sensitive to moving objects. Males can quickly detect and visually track another animal from as far as 600 meters away. Because of its large ears, the mule deer’s sense of hearing is also extremely acute. Because of their acute hearing and excellent binocular vision, mule deer specialize in detecting danger at a very long range.
Mule deer have several distinct strategies for avoiding predators. Once danger is detected, a mule deer may choose to hide, or move into cover and cautiously outmaneuver the predator. The most common strategy is to depart while the predator is still a long way off and move several miles to another area. At an unacceptably high cost per unit time of locomotion, a mule deer may also choose to rapidly bound uphill, imposing on pursuing predators. In yet another strategy, the mule deer may bound off and then trot away, stopping frequently to gain information on the disturbance. This initial bounding, combined with release of metatarsal scent that inhibits feeding, is highly advantageous in that, by alarming others, it causes other mule deer to bound off as well, reducing the conspicuousness of the deer who bounded off first. This strategy would also trigger group formation. Finally, when the predator closes in, the deer initiates evasive maneuvers based on sudden unpredictable changes in direction and on placing obstacles between itself and the predator. This strategy, however, does not work against group-hunting predators. Lastly, mule deer are excellent swimmers, but water is rarely used as a means of escaping predators.
The social system of mule deer consists of clans of females related by maternal descent. These clans are the facultative resource defenders. Males disperse as individuals or aggregate in groups of unrelated individuals. During winter and spring, the stability of female clans and male groups is maintained with dominance hierarchies. Increases in strife and alarm behavior and decreases in play among fawns occur as population density increases. The frequency of aggressive behavior between the sexes remains low year round.
Communication among mule deer is facilitated by the sebaceous and sudoriferous secretory cells of five integumentary glands. The cells of each gland produce specific scents, or pheromones, that elicit specific reactions in conspecifics. The metatarsal gland produces an alarm pheromone, the tarsal gland aids in mutual recognition, the interdigital gland leaves a scent trail, and the function of the tail gland is unknown. Urine has a pheromone function at all ages and for both sexes. It is deposited on tufts of hair surrounding the tarsal glands. In fawns, it functions as a distress signal, while in adults, it functions as a threat signal.
Individuals of mule deer tend to confine their daily movements to discrete home ranges. Most mule deer with established home ranges use the same winter and summer home ranges in consecutive years. Dispersal involves movements beyond the home range to distances of up to 8 kilometers. This movement results in the establishment of a new home range.
Seasonal movements involving migrations from higher elevations in summer ranges to lower winter ranges are associated, in part, with decreasing temperatures, severe snowstorms, and snow depths that reduce mobility and food supply. Deep snows ultimately limit useable range to a fraction of the total. Mule deer in the arid southwest may migrate in response to rainfall patterns.
The mule deer is a small ruminant with limited ability to digest highly fibrous roughage. Based on its stomach structure and its diet of woody and herbaceous forage in approximate equal proportions, the mule deer is classified as an intermediate feeder.
Mule deer frequently browse leaves and twigs of trees and shrubs. Green leaves are very succulent and, except for epidermal tissue and structural ribs, consist largely of easily digestible cell contents. Dead and weathered leaves have little protein and high cell-wall values. As a result, they are of very low digestibility. Mule deer also eat acorns, legume seeds, and fleshy fruits, including berries and drupes that have moderate cell-wall levels and are easily digested.
Optimum growth and productivity of individuals and populations are dependent upon adequate supplies of highly digestible, succulent forage. Diets consisting primarily of woody twigs cannot meet the maintenance requirements of the mule deer.
Because nutritious forage is in poor supply for much of the year, the mule deer has an annual cycle of metabolic rates. A higher energy flux and food intake in the summer enables the mule deer to capitalize on abundant high-quality forage for growth and fat storage. A lower energy flux in the winter permits the mule deer to survive on a lower intake of poor-quality forage while minimizing the catabolism of stored fat for body functions.
The estimated rate of food intake is about 22 grams per kilogram of body weight per day. In adult males, food intake drops abruptly with the onset of rut.
The mule deer is a polygynous species, having a tending-bond type breeding system. Courtship and mating occur within the group and the breeding peak occurs mainly from late November through mid-December. Among male mule deer, testicular mass and volume are maximal during November and minimal during April and May. The breeding period is mediated by decreasing day length through the action of gonadotropins on Leydig cells, thus producing androgens.
A dominant male mule deer tends an estrus female until mating or displacement by another male occurs. Dominance is largely a function of size, with the largest males, which possess the largest antlers, performing most of the copulations.
The annual cycle of antler growth in the mule deer is initiated and controlled by changes in day length acting on several cell types of the anterior pituitary. These cell types secrete growth-stimulating hormones that act mainly on the antlers and incidentally on the testes.
Antler hardening, shedding, and even the breeding period are mediated by decreasing day length through the action of gonadotropins on Leydig cells, thus producing androgens. Androgens induce secondary ossification, accelerate maturation, induce behavioral changes that result in shedding antler velvet, and aid in the maintenance of osteoblasts and osteocytes to maintain antlers in hard bone condition. Withdrawal of androgens at the end of the breeding season permits resorption of bone at the pedicel-antler junction and antler shedding.
Most female mule deer conceive during their second year and only rarely during their first year. Ovulation occurs about 12 to 14 hours after estrus terminates. Approximately 27 to 29 days elapse between conception and implantation.
The average gestation length for a mule deer is 204 days and the peak birth period is estimated to be from June 16th to July 8th, though the time of birth will vary according to the environment. A 305-meter rise in elevation has been associated with a 7-day delay in the birth period.
The common litter size of the mule deer is two, with mothers in their first or second breeding year most frequently producing singletons.
At birth, mule deer have a mass from two to five kilograms, affected by litter size and sex, with males being heavier than females.
Weaning begins at about five weeks of age and usually is completed at age 16 weeks. Full development of most skeletal attributes occurs at about 49 months of age in males and 37 months of age in females. However, gains in carcass mass are continuous until an age of 120 months in males and 96 months in females.
Male neonates predominate when poor nutrition prevails about 6 weeks before and during the breeding period.
The mule deer is mainly used as a game animal for trophy hunting and is of tremendous interest to hunters. Populations of deer that are large enough to support hunting during two or three weeks in autumn offer countless recreational opportunities for the public. This desire to hunt generates revenue for the economy. In Mexico, mule deer hunting mainly occurs in northwestern states as Baja California, Sonora, and Chihuahua.
Douglas fir and Ponderosa pine are of major economic importance for commercial timber, however, these trees are browsed heavily by mule deer. Browsing of other trees is seldom considered an economic problem. In the Douglas fir region, the mule deer browses on trees during both the dormant and growing seasons. Practices that encourage the growth of mule deer populations can therefore also encourage damage. Douglas fir is harvested mainly by clearcutting and is regenerated by planting with nursery-grown stock. The mule deer is attracted to clear-cuts, and Douglas fir is an acceptable and sometimes preferred forage species. This situation invites browsing of sufficient intensity to influence forest regeneration in many areas.
High predator populations threaten the mule deer. Common predators of mule deer include pumas (Puma concolor), coyotes (Canis latrans), bobcats (Lynx rufus), golden eagles (Aquila chrysaetos), feral dogs (Canis lupus familiaris), Canada lynx (Lynx canadensis), American black bears (Ursus americanus), and brown bears (Ursus arctos).
The mule deer, as a species, is considered to be Least Concern on the International Union for Conservation of Nature (IUCN) Red List of Threatened Species in light of its adaptability to a wide range of habitats, large populations, occurrence in numerous protected areas, and populations that seem to be relatively stable.
Although the entirety of the mule deer species is considered Least Concern, there are subspecies that are in danger of becoming extinct. Most of the mule deer’s subspecies are not threatened, but the Cedro Island subspecies (O. h. cerrocensis) is considered to be Vulnerable as of 1988. This subspecies is in danger of becoming extinct because its densities are very low on the island where it occurs and predation by feral dogs and poaching are high. Other subspecies that live on islands are also considered endangered.
Additionally, in Mexico, some data show local extinction of some populations in the Chihuahuan desert region of Coahuila and Nuevo León Mexico, and in some populations, evidence of metapopulation dynamics for the species have been found.
Today the most urgent threat to wild mule deer is the spread of Chronic Wasting Disease (CWD), a transmissible spongiform encephalopathy (TSE). Currently, CWD is more prominent at the local or regional level. CWD has currently been diagnosed in mule deer in the Rocky Mountains region of the United States and other mid-western states.
The mule deer is also a target for various viral, bacterial, and parasitic diseases. For example, heavy amounts of gastrointestinal nematodes may cause death in mule deer. This parasitic disease is usually indicative of such predisposing factors as high mule deer density and malnutrition. Infection by the parasitic meningeal worm can cause fatal neurologic disease in mule deer. Livestock may transmit viral diseases to mule deer as seen in foot-and-mouth disease. This infection is characterized by blisters in the mouth, above the hooves, and between the digits.
Other threats include: high predator populations (including feral dogs), competition with livestock grazing, human habitat alterations, and other anthropogenic forces.
Where they occur, mule deer populations are typically managed by federal, state, and provincial agencies that monitor abundance and trends in order to set species management objectives. The species also occurs in several protected areas across its distribution. As a result, mule deer remain abundant throughout much of their native range and are not currently in urgent need of further conservation action, but some evidence in the United States and Canada has shown declines in some populations.
All federal, state, and provincial land and wildlife management agencies recognize the fundamental need to maintain mule deer ranges and keep them habitable. Where they occur, mule deer populations are typically managed by organizations that monitor abundance and trends in order to set species management objectives.
To counter the trend of agricultural development, rangeland conversion, mining, road and highway construction, and the development of housing tracts, many states and provinces have purchased critical areas, especially winter ranges, to maintain the various habitats of the mule deer. Due to political opposition to government acquisition of privately owned lands, plus a scarcity of funds for this purpose, only a small fraction of mule deer ranges has been acquired by the government.
Unfortunately, one obstacle involved in evaluating mule deer populations trend is inconsistent and incomplete data collections. This problem results from the wide variety of methodologies and approaches used by states and provinces for mule deer management. There is a real need to standardize many of the methodologies to obtain data collections and establish protocols to obtain those measurements about mule deer populations.
The effects of reduced mule deer ranges can be mitigated by better management of the remaining lands to maximize their productivity for the deer.
Various habitat management programs include the manipulation of livestock grazing, the manipulation of cultivative communities, and the manipulation of vegetative communities. For mule deer, the optimal successional stages are subclimax plant communities that can be perpetuated only through the influence of humans.
Since mule deer production is not the primary management goal on most private or public lands in western North America, mule deer habitat improvement programs typically involve a complex process of coordination among bureaucracies with missions that are usually not compatible.