The San Joaquin antelope squirrel is found in the San Joaquin Valley, including slopes and ridge tops along the western edge of the valley. It is endemic to the region, and is found in a much smaller range today than it originally inhabited. Since the San Joaquin Valley fell under heavy agricultural cultivation, habitat loss combined with rodenticide use has reduced the squirrels numbers enough that it is now listed as a threatened species.
Most of today's remaining San Joaquin antelope squirrels can be found in the Carrizo Plain, where their original habitat remains undisturbed. The squirrels live in small underground familial colonies on sandy, easily excavated grasslands in isolated locations in San Luis Obispo and Kern Counties. Common vegetation associated with the squirrel includes Atriplex and Ephedra, and some junipers. The binomial of this species commemorates the American naturalist Edward William Nelson.
The San Joaquin antelope squirrel is dull yellowish-brown or buffy-clay in color on upper body and outer surfaces of the legs with a white belly and a white streak down each side of its body in the fashion of other antelope squirrels. The underside of the tail is a buffy white with black edges. Males are approximately 9.8 inches and females are approximately 9.4 inches in length.
Studies by Hawbecker provide abundant information on breeding and the life cycle of Nelson's antelope squirrel. They breed in late winter to early spring and have nearly all their young in March. Once pregnant, gestation lasts a little less than a month. The young do not emerge from their dens until approximately the first week of April. Nelson's antelope squirrel has only one breeding season, which is timed appropriately so that the young are born during the time of year when green vegetation is the most abundant.
Weaning is thought to start or be completed even before the young emerge. Once above ground, young are seen foraging for food independently. During the weaning period, the mother feeds alone and ignores any attempt of the young trying to nuzzle or nurse from her. A mother will sometimes spend the night in a different den if necessary. By early to mid-May the young squirrels have had their juvenile pelage for some time and begin to show the changes into adult pelage. By summer, the adult pelage is present. Once an individual has reached adulthood it is difficult to tell differences in age. Nelson's antelope squirrel is a short-lived species that often does not survive to a year. However, several individuals have been observed to live more than four years in the wild.
Colonies have about six or eight individuals, however these individuals are not distributed evenly across their range. There is usually about 1 per hectare. Nelson's antelope squirrel prefers deep, rich soil types since they are easy to dig through in both winter and summer temperatures. Although these squirrels may dig for food, they do not make their own burrows. Instead they claim abandoned Dipodomys (kangaroo rats) burrows as their own. Both males and females have the same size home range of about 4.4 hectare. Of course, there are areas of concentration within this range where the squirrels spend the majority of their time.
It is omnivorous, feeding on seeds, green vegetation, insects, and dried animal matter. It occasionally caches food. Redstem fialree (Erodium cicutarium) and brome grass (Bromus rubens) are important food items for the squirrels. However, their diet may differ depending on the time of day or time of year. Green vegetation is the most common diet type from December to mid-April because it is the most abundant during this period. Likewise, insects make up more than 90% of the squirrel's diet from mid-April to December because they are more abundant. Although seeds are available for most of the year, it is not the preferable diet of the squirrels. They will choose insects or green vegetation when available over seeds, even if the seeds are more abundant and easier to access. Some speculate that this could be due to the higher amount of water in insects and green vegetation, which would be necessary for the species to survive in such a hot, dry climate. Unfortunately for the Nelson's antelope squirrel, there is not an abundant water source nearby. Under laboratory conditions, the squirrels readily accept water. However, they can also survive at least 7 months in the shade without water. At the end of 7 months they appeared relatively healthy and not at all emaciated.
Nelson's antelope squirrels are social animals. When individually taken out of their home range and released in an unknown area, they seem helpless and confused. They do not expend much energy throughout the day because of the extreme temperatures in their environment. In fact, when in the direct sunlight, a temperature of 31-32 °C can kill them. Therefore, there is little activity from the squirrels during the heat of the day. Although there is no evidence of hibernation, the squirrels are not bothered by the cold and can survive temperatures below freezing, in their burrows. They are not early risers and are usually not seen until after sunrise, however it does forage in the morning and evening, avoiding the midday heat. Around noon the squirrels disappear into their burrows and are not seen again until about 2 pm at the earliest. On moderate days, the squirrels will take their time foraging, in contrast to bringing as much food back to their burrows as quickly as possible on hot or cold days. The squirrels are also known to fully stretch out and roll over in the dust on the ground. These dust baths appear to be very enjoyable activities for the squirrels and may also be used to prevent infestation of parasites.
Nelson's antelope squirrels are cautious when emerging from their burrows. They have a specific route that they follow when foraging for food. If danger seems near, they will run into a burrow along their foraging route to get to safety. They move quickly and do not spend much time in one place. They are particular about what they choose to eat and very rarely even waste time to pick up food they are not interested in. There are other features in addition to their quick movements that help keep them from danger. They whitish color of the underside of their tail can be seen when they run. The squirrels will curl their tail forward over their back and flick and twitch it back and forth as it runs. This movement can present the illusion of thistledown fluttering in the wind, which could be ignored by any potential predators.
To further help prevent predation, the Nelson's antelope squirrel has an alarm call. These alarm calls are not loud, but associated with convulsive body movements. Horned larks and the white-crowned sparrow also aid in predator detection. Squirrels will listen to alarm calls given by these two birds. The badger (Taxidea taxus) is a main predator of Nelson's antelope squirrel and will destroy burrows to get its meal. Coyotes (Canis latrans) and San Joaquin Valley Kit Fox (Vulpes macrotis mutica) are also known to consume the squirrels, but they are not a main part of their diet.
Increasing agriculture and urban development is an increasing problem for Nelson's antelope squirrel. This species will not colonize cultivated land. Therefore, an increase in agriculture land is taking away their habitat and leaving them with no alternative. Grazing livestock further destroys what habitat may be left, and exotic plants are able to take over native grasses that the squirrel forages upon and relies on for shade and cover. Also, pesticide drift from nearby agricultural fields encroaches in on the existing squirrel habitat. Not only are these practices affecting the population of the Nelson's antelope squirrel, but they are also causing problems for other native animal and plant species in the San Joaquin Valley. Native plant species such as the kern mallow, San Joaquin woolly threads, California jewelflower, and Bakersfield cactus are all federally endangered plant species that are being outcompeted by invasive plant species. Many invasive plants grow in very dense patches. These dense patches are not adequate habitats for Nelson's antelope squirrel and many other San Joaquin Valley Species.
There have been attempts to manage the invasive species and other anthropogenic causes to species decline in the San Joaquin Valley. Prescribed burns are one option to control invasive plant species, however this method can cause native species to also be killed and can be expensive. Studies determining the effects of cattle grazing on the land are also being done so that plans can be developed to reduce the impact on the land. Suggestions of using prescribed grazing to help reduce the growth of non-native species in the valley.
Other control efforts include chemical and mechanical treatments, however these too can be time consuming and expensive, especially for large areas. Also, the use of herbicides could potentially negatively affect species in the San Joaquin Valley if there are significant winds that spread the chemicals. The U.S. Fish and Wildlife Service (USFWS) has a recovery plan dated 1998 that includes the ideas of using Safe Harbor Agreements (SHA's) under Section 10 of the Endangered Species Act. This could potentially begin a relationship between the USFWS and the farm landowners to help determine the best compromise in order to manage the endangered species of the valley.
A multispecies approach to conservation is important because of the increasing number of native species becoming threatened and endangered in the San Joaquin Valley. Management on an ecosystem level would allow the role of all the species to be taken into account. Also, an increase in awareness and education of the public around the Valley may further help increase funding for conservation and management plans. Monitoring and studying of the species is needed to determine just how threatened the species is and what needs to be done to reestablish stable populations in the valley.
Unfortunately, most information found on the Nelson's antelope squirrel discuss the problems and reasons of declines, but do not give much insight on the potential recovery of the species. Even in 1918, Grinnell and Dixon believed it to only be a matter of time before the species faces extinction.
The distribution of Ammospermophilus nelsoni is restricted to the floor of the southern San Joaquin Valley, California, the Cuyama and Panoche valleys in San Luis Obispo County, and the Carrizo and Elkhorn Plains of the United States. The species is Nearctic and endemic to the above range.
Biogeographic Regions: nearctic (Native )
Nelson's antelope squirrels have fusiform bodies, small, rounded ears, short legs, and short tails. The dorsal head and neck and the outer surfaces of the legs are a dull yellowish-brown or buffy-tan. The tail has thick fringes of hair and the underside is light grey to white. A distinctive light-colored stripe runs along the side of the body from behind the shoulder to the rump. Males are slightly larger than females, with total length of males ranging from 234 to 267 mm (average 249 mm). Female length ranges from 230 to 256 mm (average 238 mm). Nelson's antelope squirrels have distinct summer and winter pelages, with the autumn or winter pelage being darker than the summer pelage.
Nelson's antelope squirrels can be distinguished from white-tailed antelope squirrels by its larger size and grey coloring of the pelage. Nelson's antelope squirrels have wider zygomatic arches, more inflated auditory bullae, and larger nasal bones. They also have larger upper incisors and first upper molars.
Range mass: 142 to 179 g.
Average mass: 155 g.
Range length: 230 to 267 mm.
Average length: 249 mm.
Average basal metabolic rate: 0.8 cm3.O2/g/hr.
Other Physical Features: endothermic ; homoiothermic; bilateral symmetry
Sexual Dimorphism: male larger
Nelson's antelope squirrels are found in hot deserts that comprise the Lower Sonoran life zone. Lower Sonoran deserts of North America include areas in Texas, New Mexico, Arizona, Nevada, and California. Nelson's antelope squirrels are found in arid grasslands and shrub lands. They have been recorded in areas where shrub cover ranges from light to medium density and ranges from xerophytic, alkali desert scrub and annual grassland receiving less than 15 cm annual precipitation, to halophytic, alkali desert scrub and annual grassland receiving 18 to 23 cm of annual precipitation. Nelson's antelope squirrels prefer alkaline, loamy soils from 50 to 1100 meters elevation. Nelson’s antelope squirrels depend on kangaroo rat burrows, so areas they inhabit may be limited to areas with kangaroo rat popuations.
Range elevation: 50 to 1100 m.
Habitat Regions: temperate ; terrestrial
Terrestrial Biomes: desert or dune
Nelson’s antelope squirrels are omnivorous and active foragers. They feed primarily on insects, green vegetation, and seeds, but will sometimes also feed on small vertebrates. With regards to the latter, the San Joaquin antelope squirrel consumes rodents, lizards and members of its own species as carrion. Their diet preferences are largely dependent on the amount of moist vegetation available. With respect to green material, red-stemmed filaree (Erodium cicutarium) and the red brome (Bromus rubens) are preferred. Other food sources include the ephedra, a species of clover and locoweed. When moist food becomes very scarce turpine weed is eaten. A preference for a particular type of insect has not been documented. The San Joaquin squirrel eats seeds only when easily obtainable green material and insects are not available.
Nelson’s antelope squirrel assumes a distinctive posture while feeding. It squats on its rear limbs with the tail cocked behind the back and holds its food in the forepaws. Its enlarged incisors and abrasive cheek teeth help in breaking down food.
The Nelson’s ground squirrel does not live near a source of water and so water does not constitute a great part of its diet. It can be inferred that its principal source of moisture is from the vegetation eaten. Although it accepts large amounts of water in laboratory conditions, it can survive for seven months in the shade, without water.
Animal Foods: reptiles; carrion ; insects
Plant Foods: leaves; seeds, grains, and nuts
Foraging Behavior: stores or caches food
Primary Diet: carnivore (Eats terrestrial vertebrates, Insectivore , Scavenger ); herbivore (Folivore , Granivore ); omnivore
Nelson's antelope squirrels are preyed upon by mid-sized predators, and serve as host to varius endoparasites and ectoparasites. They are hosts to cestodes (Hymenopelis citelli), nematodess (Spirura infundibuliformis and Physaloptera spinicauda), and also acanthocephalans (Moniliformes dubius). The ectoparasites they are host to include fleas (Siphonaptera) and ticks (Ixodes). Nelson's antelope squirrels have a symbiotric relationship with kangaroo rats Dipodomys deserti, taking refuge in their burrows. They further impact their habitat by continuing to burrow. Because they are seed collectors, they also disperse seeds in their environment.
Ecosystem Impact: disperses seeds; creates habitat
Nelson's antelope squirrels employ a variety of techniques to guard against predators. The complex burrow systems serve as protection against predators. When emerging from a burrow, they exercise extreme caution and do not exit quickly. They rely on olfaction and use this capability while foraging and fleeing danger. They use characteristic alarm calls to communicate danger and may also rely on the warning calls of birds like horned larks and white-crowned sparrows for detection of predators. Nelson's antelope squirrels use their surroundings while evading predators. They use shrubs and burrows of kangaroo rats as sites of refuge. Their buffy or tan pelage make them difficult to see in their arid habitats. When foraging, these squirrels move close to the ground in a very distinctive manner, by a series of short, rapid jumps. American badgers are the most important predators of these squirrels. They dig into the burrows to prey on young and adults. They are also eaten by coyotes (Canis latrans) and (kit foxes Vulpes macrotis).
Anti-predator Adaptations: cryptic
The main senses used by the Nelson's antelope squirrels are hearing and olfaction, but the species also relies on vision and will scan its environment before making the decision to fully exit its burrow. Olfaction is used to detect danger, to find food and to communicate. Nelson's antelope squirrels also twitch their tails rapidly, in fore and aft movements when frightened or excited.
The Nelson’s antelope squirrel gives out alarm calls, which are indicative of altruistic behavior as it compromises the safety of the individual. These calls are in the form of trills that are low-pitched and are characteristic of antelope squirrels that live in closed habitats. In fact, the call is accomplished by a convulsive motion of its body rather than by vocal stress. The female uses this call to communicate with her family when she is weaning.
Communication Channels: visual ; acoustic ; chemical
Perception Channels: tactile ; acoustic ; chemical
Nelson's antelope squirrels usually live for less than a year in the wild because of high infant and young mortality rates. They have been documented to live for about 4 years in captivity, with the longest captive lifespan being 5.7 years. Adult mortality rate is 80% and the average lifespan is 8 months.
Status: captivity: 5.7 (high) years.
Status: wild: 8 months.
Status: captivity: 4 years.
Nelson's antelope squirrels have a promiscuous mating system in which females and males mate with multiple partners. Not much is known about specific mating behaviors, but an interesting observation was made regarding mate searching behavior. Mates are typically found within the home range, but there have been instances of females travelling up to 1 km from their home range in search of a mate. Mate guarding and mate defense have not been observed in Nelson's antelope squirrels.
Mating System: polygynandrous (promiscuous)
The breeding season of Nelson's antelope squirrels extends from late winter to early spring and this species breeds once annually. Males achieve reproductive maturity earlier than females. Brood size varies from 6 to 11 young; with the average being 9. The average gestation period is 26 days. Females give birth to offspring in burrows. The young come above ground approximately on the 30th day after birth. Weaning can start or be completed before the young emerge from their burrow, but this is highly variable. Females wean the young by refusing to nurse and visiting the burrow less frequently.
Breeding interval: Nelson's antelope squirrels breed once annually.
Breeding season: Nelson's antelope squirrels breed from late winter to early spring.
Range number of offspring: 6 to 11.
Average number of offspring: 9.
Range gestation period: 25 to 30 days.
Average gestation period: 26 days.
Range weaning age: 30 (high) days.
Average weaning age: 15 days.
Range time to independence: 3 to 4 weeks.
Average age at sexual or reproductive maturity (female): 377 days.
Average age at sexual or reproductive maturity (male): 377 days.
Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; viviparous
Nelson's antelope squirrel young are born in an altricial state. Males do not play a large role in caring for the young, as females perform all activities of nursing and weaning. When weaning, the female distances herself from the young and does not respond even when young make attempts to nurse. The female maintains contact with the young by visiting them sometimes or by just using calls to communicate. In captivity, at times when foraging opportunities were limited, instances of cannibalism have been observed.
Parental Investment: altricial ; female parental care ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Female)
Nelson’s, or San Joaquin, antelope squirrels are classified as endangered on the IUCN Red List. According to the California Department of Fish and Game, the biggest threat to Nelson’s antelope squirrels is habitat destruction due to crop cultivation and urbanization. Excessive grazing by livestock affects these squirrels and also causes soil erosion. Uses of rodenticides and insecticides, which destroy prey populations, negatively impact Nelson’s antelope squirrels. The Endangered Species Recovery Program (2006) of California has suggested a list of conservation measures that stress the importance of land use practices and management strategies. A detailed conservation program for San Joaquin antelope squirrels includes determining habitat management, protecting additional habitat in surrounding areas of the squirrel’s range, habitat enhancement in areas like Kern County, and reintroduction to protected areas, such as Pixley National Wildlife Refuge.
US Federal List: no special status
CITES: no special status
State of Michigan List: no special status
In laboratory conditions, Nelson's antelope squirrels have been positive for western equine encephalomyelitis, St. Louis encephalitis, Powassan virus, and Modoc virus. However, there is no documented instance their role in spreading disease to humans or domestic animals.
Nelson’s antelope squirrels do not generally live close to human settlements. They may contribute to controlling insect populations by eating them and they may help to disperse the seeds they gather and store.
The genus Ammospermophilus has a rich fossil record. Its divergence can be dated to the Miocene as post-Pleistocene fossils resembling Nelson’s antelope squirrels have been found near Kern County, California. They are thought to have invaded the area over the passes of southern Sierra Nevada and Tehachapi mountains. Due to the change in the moisture levels to less arid conditions in the corridor between Mojave Desert and San Joaquin valley, Nelson’s antelope squirrels became restricted to the San Joaquin valley.