Photo Credit: Brian Wolitski
Mule Deer [Odocoileus
Geographic Range: Odocoileus hemionus occurs over most of North America west of the 100th meridian from 23 degrees to 60 degrees N. The eastern edge of the usual range extends from southwestern Saskatchewan through central North and South Dakota, Nebraska, Kansas, and western Texas. Isolated occurrences have been reported from Minnesota, Iowa, and Missouri. Major gaps in geographic distribution are in southern Nevada, southeastern California, southwestern Arizona, and the Great Salt Lake desert region. Apart from these gaps, O. hemionus occurs in all of the biomes of western North America north of central Mexico, except the Arctic tundra (Anderson 1984).
Physical Characteristics: Mass: 43 to 150 kg. The pelage of Odocoileus hemionus ranges from dark brown gray, dark and light ash-gray to brown and even reddish. The rump patch may be white or yellow, while the throat patch is white (Geist 1981). The white tails of most mule deer terminate in a tuft of black hairs, or less commonly in a thin tuft of white hairs. On some mule deer, a dark dorsal line runs from the back, down the top of the tail, to the black tail tip. All markings vary considerably among O. hemionus , but remain constant throughout the life of an individual. O. hemionus possess a dark V-shaped mark, extending from a point between the eyes upward and laterally. This mark is more conspicuous in males. Growth in O. hemionus during the first year is roughly parallel in males and females. Thereafter, 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 (Anderson 1984). Carcass weight ranges from 45 to 150 kg in males, and 43 to 75 kg in females. Chest girth ranges from 80 to 117 cm in males, and 78 to 97 cm in females. Neck circumference ranges from 30 to 65 cm in males, and 26 to 38 cm in females. Body length ranges from 126 to 168 cm in males, and 125 to 156 cm in females. Head length ranges from 28 to 35 cm in males, and 27 to 33 cm in females. Cranial breadth ranges from 11 to 16 cm in males, and 10 to 14 cm in females. Shoulder height ranges from 84 to 106 cm in males, and 80 to 100 cm in females (Wallmo 1981).
Food Habits: Odocoileus hemionus is a small ruminant with limited ability to digest highly fibrous roughage (Short 1981). 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 O. hemionus . Based on its stomach structure and its diet of woody and herbaceous forage in approximate equal proportions, O. hemionus is classified as an intermediate feeder. Because nutritious forage is in poor supply for much of the year, O hemionus has an annual cycle of metabolic rates. A higher energy flux and food intake in the summer enables O. hemionus to capitalize on abundant high-quality forage for growth and fat storage. A lower energy flux in the winter permits O. hemionus 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 g/kg body weight/day. In adult males, food intake drops abruptly with the onset of rut (Anderson 1984). O. hemionus frequently browses 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. O. hemionus also eats acorns, legume seeds, and fleshy fruits, including berries and drupes, that have moderate cell-wall levels and are easily digested (Short 1981).
Reproduction: Odocoileus hemionus is a polygynous species, having a tending-bond type breeding system. Courtship and mating occur within the group (Geist 1981). A dominant male 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 (Kucera 1978). Most O. hemionus females conceive during their second year and only rarely during their first year. The breeding peak in O. hemionus occurs mainly from late November through mid-December. The average gestation length is 204 days. The peak birth period in O. hemionus is estimated to be from June 16th to July 6th, with most births occurring in June. The time of birth varies according to the environment. Robinette (1977) calculated that a 305-m rise in elevation is associated with a 7-day delay in the birth period. The mass at birth of O. hemionus ranges from 2 to 5 kg. Mass at birth is affected by litter size and sex, with males being heavier. The common liter size is two, with mothers in their first or second breeding year most frequently producing singletons. Weaning begins at about 5 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. In O. hemionus , male neonates predominate when poor nutrition prevails about 6 weeks before, and during, the breeding period. Ovulation in female O. hemionus occurs about 12 to 14 hours after estrus terminates. Approximately 27 to 29 days elapse between conception and implantation in female O. hemionus . Among male O. hemionus , testicular mass and volume are maximal during November and minimal during April and May (Anderson 1984).
Behavior: Individuals of Odocoileus hemionus 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 in O. hemionus involves movements beyond the home range to distances of up to 8 km. This movement results in the establishment of a new home range. Seasonal movements involving migrations from higher elevations (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. Common predators of O. hemionus include pumas, coyotes, bobcats, golden eagles, feral dogs, and black bears. The social system of O. hemionus 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 in O. hemionus . Communication among O. hemionus is facilitated by the sebaceous and sudoriferous secretory cells of five integumentary glands. The cells of each gland produce specific scents (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 (Anderson 1984). O. hemionus has several distinct strategies for avoiding predators. O. hemionus specializes in detecting danger at a very long range by means of large ears and excellent vision. Males can quickly detect and visually track another animal as far as 600 meters. Once danger is detected, O. hemionus may choose to hide, or move into cover and cautiously outmaneuver the predator. Another strategy is to depart while the predator is still a long way off and move several miles to another area. O. hemionus , instead, may bound rapidly uphill, imposing on pursuing predators an unacceptably high cost per unit time of locomotion. In yet another strategy, O. hemionus 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 a predator closes in, O. hemionus 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. O. hemionus is an excellent swimmer, but water is rarely used as a means of escaping predators (Geist).
Habitat: Odocoileus hemionus is remarkably adaptable. Of at least sixty types of natural vegetation west of the 100th meridian in the United States, all but two or three are or once were occupied by O. hemionus . 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, O. hemionus occupies five boreal forest types that do not occur in the United States. O. hemionus occupies a wide range of habitat provinces (regions of land containing particular vegetation types) in western North America. 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 (Wallmo 1981). Biomes: tropical deciduous forest, temperate forest & rainforest, temperate grassland, chaparral, mountains, desert
Economic Importance for Humans
Positive: Odocoileus hemionus is of tremendous interest to hunters. Populations of O. hemionus 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 (Wallmo 1981).
Negative: Douglas fir and Ponderosa pine are of major economic importance for commercial timber. However, these trees are browsed heavily by O. hemionus . Browsing of other trees is seldom considered an economic problem. In the Douglas fir region, O. hemionus browses on trees during both the dormant and growing seasons. Practices that encourage the growth of O. hemionus populations can therefore also encourage damage. Douglas fir is harvested mainly by clearcutting and is regenerated by planting with nursery-grown stock. O. hemionus 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 (Wallmo 1981).
Conservation Status: no special status All federal, state, and provincial land and wildlife management agencies recognize the fundamental need to maintain O. hemionus ranges and keep them habitable. 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 O. hemionus . But, due to political opposition to government acquisition of privately owned lands, plus a scarcity of funds for this purpose, only a small fraction of O. hemionus ranges has been acquired by the government. The effects of reduced O. hemionus ranges can be mitigated by better management of the remaining lands to maximize their productiviy for O. hemionus . Various habitat management programs include the manipulation of livestock grazing, the manipulation of cultivative communities, and the manipulation of vegetative communities. For O. hemionus , the optimal successional stages are subclimax plant communities that can be perpetuated only through the influence of humans. Since O. hemionus production is not the primary management goal on most private or public lands in western North America, O. hemionus habitat improvement programs typically involve a complex process of coordination among bureaucracies with missions that are usually not compatible (Wallmo 1981).
Other Comments: The annual cycle of antler growth in O. hemionus 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 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. O. hemionus has excellent binocular vision. While unable to detect motionless objects, O. hemionus is extraordinarily sensitive to moving objects. The sense of hearing is also extremely acute. O. hemionus is a target for various viral, bacterial, and parasitic diseases. For example, heavy amounts of gastrointestinal nematodes may cause death in O. hemionus . 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 O. hemionus . Livestock may transmit viral diseases to O. hemionus as seen in foot-and-mouth disease. This infection is characterized by blisters in the mouth, above the hooves, and between the digits (Anderson 1984).