Mule deer
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.[1][5][6][7][8][9]
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 only found 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.[5]
Seasonal behaviors[edit]
In addition to movements related to available shelter and food, the breeding cycle is important in understanding deer behavior. The rut or mating season usually begins in the fall as does go into estrus for a period of a few days, and males become more aggressive, competing for mates. Does may mate with more than one buck and go back into estrus within a month if they did not become pregnant. The gestation period is about 190–200 days, with fawns born in the spring.[17] The survival rate of the fawns during labor is about 50%.[18] Fawns stay with their mothers during the summer and are weaned in the fall after about 60–75 days. Mule deer females usually give birth to two fawns, although if it is their first time having a fawn, they often have just one.[17]
A buck's antlers fall off during the winter, then grow again in preparation for the next season's rut. The annual cycle of antler growth is regulated by changes in the length of the day.[17][19]
The size of mule deer groups follows a marked seasonal pattern. Groups are smallest during fawning season (June and July in Saskatchewan and Alberta) and largest in early gestation (winter; February and March in Saskatchewan and Alberta).[19]
Besides humans, the three leading predators of mule deer are coyotes, wolves, and cougars. Bobcats, Canada lynx, wolverines, American black bears, and grizzly bears may prey upon adult deer but most often attack only fawns or infirm specimens, or they may eat a deer after it has died naturally. Bears and small carnivores are typically opportunistic feeders and pose little threat to a strong, healthy mule deer.[13]
In 99 studies of mule deer diets, some 788 species of plants were eaten by mule deer, and their diets vary greatly depending on the season, geographic region, year, and elevation.[20] The studies[21] gave these data for Rocky Mountain mule deer diets:[22]
The diets of mule deer are very similar to those of white-tailed deer in areas where they coexist.[23][20] 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.[20][22]
Mule deer readily adapt to agricultural products and landscape plantings.[24][25] In the Sierra Nevada range, mule deer depend on the lichen Bryoria fremontii as a winter food source.[26]
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.[27] Where available, mule deer also eat a variety of wild mushrooms, which are most abundant in late summer and fall in the southern Rocky Mountains; mushrooms provide moisture, protein, phosphorus, and potassium.[20][27]
Humans sometimes engage in supplemental feeding efforts in severe winters in an attempt to help mule deer avoid starvation. Wildlife agencies discourage such efforts, which cause harm to mule deer populations by spreading disease (such as tuberculosis and chronic wasting disease) when deer congregate for feed, disrupting migratory patterns, causing overpopulation of local mule deer populations, and causing habitat destruction from overbrowsing of shrubs and forbs. 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.[28]
Mule deer are variably gregarious, with a large proportion of solitary individuals (35 to 64%) and small groups (groups with ≤5 deer, 50 to 78%).[29][30] Reported mean group size measurements are three to five and typical group size (i.e., crowding) is about seven.[19][31]
Nutrition[edit]
Mule deer are ruminants, meaning they employ a nutrient acquisition strategy of fermenting plant material before digesting it. Deer consuming high-fiber, low-starch diets require less food than those consuming high-starch, low-fiber diets. Rumination time also increases when deer consume high-fiber, low-starch diets, which allows for increased nutrient acquisition due to greater length of fermentation.[32] Because some of the subspecies of mule deer are migratory, they encounter variable habitats and forage quality throughout the year.[33] Forages consumed in the summer are higher in digestible components (i.e. proteins, starches, sugars, and hemicellulose) than those consumed in the winter. The average gross energy content of the consumed forage material is 4.5 kcal/g.[34]
Due to fluctuations in forage quality and availability, mule deer fat storage varies throughout the year, with the most fat stored in October, which is depleted throughout the winter to the lowest levels of fat storage in March. Changes in hormone levels are indications of physiological adjustments to the changes in the habitat. Total body fat is a measure of the individual's energy reserves, while thyroid hormone concentrations are a metric to determine the deer's ability to use the fat reserves. Triiodothyronine (T3) hormone is directly involved with basal metabolic rate and thermoregulation.[35]
Disease[edit]
Wildlife officials in Utah announced that a November–December 2021 field study had detected the first case of SARS-CoV-2 in mule deer. Several deer possessed apparent SARS-CoV-2 antibodies, however a female deer in Morgan County had an active Delta variant infection.[60] White-tailed deer, which are able to hybridize with mule deer and which have shown high rates of SARS-CoV-2 infection, have migrated into Morgan County and other traditional mule deer habitats since at least the early 2000s.[61][62]