| California mule deer | |
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| A young buck in Yosemite National Park | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Artiodactyla |
| Family: | Cervidae |
| Subfamily: | Capreolinae |
| Genus: | Odocoileus |
| Species: | |
| Subspecies: | O. h. californicus
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| Trinomial name | |
| Odocoileus hemionus californicus (Caton, 1876)
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The California mule deer (Odocoileus hemionus californicus) is a subspecies of mule deer whose range covers much of the state of California.[1]
A typical mature male mule deer stands at around 40 inches in height at the shoulder and measures approximately 57 inches in total length, weighing in around 150 pounds in females and 200 pounds in males. The name "California Mule Deer" comes from their large, mule-like ears which, along with their black-tipped tail, make them easily distinguishable from other deer species. With a lifespan that can reach up to 22 years, these deer are built for endurance and adaptability, showcasing an impressive ability to navigate the diverse terrains of California, from coastal prairies to the Sierra Nevada mountains.[2]
The coat color of the California mule deer undergoes seasonal changes, allowing it to adapt to different environmental conditions. In the warmer months, their coat tends to be a lighter, reddish-brown shade, which helps them blend into the dry, sunlit landscapes. Conversely, during the colder months, the coat becomes a darker, more muted brown, offering better camouflage in the denser, shadowed terrains and against the backdrop of leafless trees or snow. These seasonal changes in coat color are essential for evading predators and for overall survival. These color changes are a process called molting, driven by a complex interplay of hormonal changes, photoperiod (length of daylight), and temperature. In most deer species, the timing of coat changes is regulated by melatonin, a hormone whose production is influenced by the photoperiod. As the days lengthen in spring and shorten in fall, the pineal gland in the brain alters its melatonin production, signaling the body to initiate the molting process. In warmer months, a lighter, thinner coat that have shorter and more thinly spaced hairs allows deer to reflect the increased solar radiation and provides less thermal insulation, allowing the deer to dissipate heat more efficiently. This lighter, thinner coat is known as the "summer pelage," and it's often a reddish-brown color that blends in with the drier landscapes. Conversely, as the days shorten and temperatures drop, signaling the approach of winter, the deer molts into its "winter pelage." This coat is characterized by longer, denser, and darker hairs that have a hollow structure, trapping air and providing better thermal insulation against the cold. The darker brown coloration also aids in camouflage against the backdrop of leafless forests and snowy landscapes.[3]
One of the principal means of distinguishing the closely related black-tailed deer and white-tailed deer is the growth habit of the buck's antlers. In the case of the Black Tail and California mule deer, the antlers fork in an upward growth, whereas the other species' antlers grow in a forward direction. These antlers consist of two main beams that then each fork into two tines. Further forking is dependent upon age, genetics, and nutrition. They will shed these antlers around mid-February and regrow them throughout the Spring.
This subspecies, O. h. californicus, is widespread throughout northern and central California in the California coastal prairie, as well as inner coastal ranges and interior mountains, especially the Sierra Nevada. This deer is much less frequently found on the floor of the interior valleys, and then mostly frequently in riparian zones.[4] Additionally, mule deer are migratory mammals. In northern areas including Inyo and Mono counties, their summer ranges are typically located at higher elevations and are characterized by a higher plant density for foraging and fawning areas. They will migrate to lower elevations with more limited food during the winter as the higher elevations become covered with snow. In more southern areas of California including the San Bernardino mountains this migratory shift consists of seasonal elevations migrations rather than a complete shift in ranges.[5]
The California Mule deer's habitat generally consists of hilly terrain in oak woodlands.[6] They are abundant throughout Sequoia, King's Canyon, and Yosemite national parks as well as their surrounding areas.[7]
California Mule deer are herbivorous and often browse for food within a 1-2 mile radius of a water source (including rivers and lakes) which creates a feeding area that is often located in or around shaded grassy areas upon which they will make their beds.
They will participate in a behavior where they create beds to sleep on. These beds are roughly 2 meters in diameter and often appear as flatted grassy areas. However frequently used and re-used beds may appear significantly more level and decomposed compared to their surrounding environment.[6] This behavior is often observed among bucks during the later fall season as temperatures drop. They will choose a sunny spot, remove any rocks or branches, and take a nap. These beds are often located on grasses or dry leaves under a tree or a rock, or on flattened areas of snow if there is nothing else available.[7]
Mule deer are often opportunistic and will consume a large variety of vegetation mainly consisting of whatever is available and easily digestible including stalks, flowers, fruits, seeds of grasses, forbs, buds, seeds (particularly acorns), stems, leaves, the bark of trees, shrubs, fungi, lichens, algae, mosses, and ferns. Because of their opportunistic nature, they will adjust their diet in accordance to what is available given various factors including snow and fire. In summer, California mule deer mainly browse on leaves of small trees, shrubs, and herbaceous plants, but also consume many types of berries (including blackberry, huckleberry, salal, and thimbleberry). In winter, they may expand their forage to conifers (particularly twigs of Douglas fir), aspen, willow, dogwood, juniper, and sage. Year-round, they feed on acorns; grasses are a secondary food source. Where humans have encroached on historic deer habitat by suburban development or orchards, California mule deer diversify their diet with garden plant material, tree fruit, and occasionally, pet food.[6][8]
Fawns and does tend to browse together in familial groupings, while bucks tend to travel singly or with other bucks. California mule deer browse most actively near dawn and dusk, but also forage at night in open agricultural areas or when experiencing hunting pressure.[6]
Mule deer will often form family groups made up of 2 or more generations of females and their fawns. Grown bucks leave these groups and will occasionally form groups with other adult bucks but are more often observed living solitarily. Females will spend their summers isolated in fawning areas after their young have sufficiently grown, where they will remain until the breeding season begins again in the fall.[8]
Mule deer have been classified as occasionally polygamous and occasionally polygynous. The average lifespan of the California Mule Deer is a robust twenty-two years, making it one of the longer-lived deer species.The males have sometimes been observed wandering extensively, seeking out females, and occasionally does will seek out a dominant buck who will tend to them until breeding. In either system, larger bucks with large antlers tend to be the most dominant individuals in a community. Rutting season occurs in autumn starting as early as September and lasting occasionally until March. During this time, the does will come into estrus for a period lasting only several days. Males exhibit aggressive behavior in competing for mates. Does begin estrus again if they do not become pregnant in a process that will be repeated up to 5 times with intervals of 22–29 days between estrous periods. The gestation period for these deer is about 200 days, culminating in the birth of fawns during the spring season.[9]
Reproductively, mothers usually give birth to one to two fawns per season. Notably, first-time mothers or those in their 2nd year of birthing tend to have singleton births, contributing to an average litter size of approximately 1.5.[10]
After birth in the spring, the young fawns are highly dependent on their mothers, staying close to them throughout the summer months. They gradually become more independent as they are weaned in the autumn, usually at the age of about six months. The familial structure during this period is typically a mother-and-fawn unit, while mature bucks generally keep to themselves or form small bachelor groups. Since young fawns are more susceptible to predators like mountain lions, bobcats, and coyotes, does care for the fawns by teaching them how to forage and avoid dangers, key survival skills that they will carry into their adulthood.[11]
The buck's antlers fall off in the winter, and commence growing once more in spring in anticipation of next autumn's rut.[9]
Bobcats, mountain lions, coyotes, and black bears are all common predators of California Mule Deer. The largest predator of the California Mule deer is the Mountain Lion. Occasionally, these predators will hunt large healthy deer; however, these predators most often prey on weak, sick, or young deer or scavenge remains of dead deer.[6]
The California mule deer often falls victim to the woodtick, which is extremely common throughout its habitat. Additionally lungworms and the nasal botfly will enter and parasitize the deer's lungs and nasal passageways. Parasitic eye worms of the genus Thelisa or Thelazia have also been found parasitizing the deer.[7]
Since prehistoric times, the Native American indigenous peoples of California are known to have hunted California mule deer. Thus, since about 12,000 BCE, Gage suggests that human populations have served as a control to the numbers of California mule deer.[12]
In the modern era, since European colonists and Euro-Americans settled in California, hunting pressure intensified as the human population expanded and hunting became an activity not just associated with food supply. In addition, human population growth (through urban development) in California has consumed large amounts of natural habitat of the California mule deer starting in the late 19th century and continuing through the present.
