Endangered Species Recovery Program

5. Tulare Grasshopper Mouse (Onychomys torridus tularensis)

• Taxonomy
• Description
• Identification
• Historical Distribution
• Current Distribution
• Food and Foraging
• Reproduction and Demography
• Behavior and Species Interactions
• Activity Cycles
• Habitat and Community Associations
• Reasons for Decline
• Threats to Survival
• Conservation Efforts
• Conservation Strategy
• Conservation Actions

Taxonomy.-- The genus Onychomys was described by Baird (1858). The southern grasshopper mouse was described as Hesperomys (Onychomys) torridus by Coues (1874). The Tulare grasshopper mouse (O. torridus tularensis), one of 10 currently recognized subspecies, was described by Merriam (1904b) from the type specimen collected near Bakersfield, Kern County, California.

Description.-- In general, mice of the genus Onychomys have stout bodies with short, relatively thick tails (Figure 62). The pelage is sharply bicolored with the head, back, and upper sides pale-brown to grayish or pinkish cinnamon and the underparts white and distinctly different from the upper parts. The tail is usually bicolored with a white tip (Hall and Kelson 1959, McCarty 1975). Juvenile pelage is gray; adult pelage is buffy or tawny; and the pelage of older individuals may be gray, closely resembling subadults in color (Hall and Kelson 1959). Within-species variation in adult coat color may be a result of adaptation to local environmental conditions (McCarty 1975). The total body length of the southern grasshopper mouse ranges from 119 to 163 millimeters (4.69 to 6.42 inches); tail length, 33 to 62 millimeters (1.30 to 2.44 inches); hind foot length, 18 to 23 millimeters (0.71 to 0.91 inch); and ear length from notch, 11 to 18 millimeters (0.43 to 0.71 inch). Tail length is usually more than half the length of the body (48 to 56 percent) (Hall and Kelson 1959).

The southern grasshopper mouse has five tubercles (knob-like fleshy bumps) on the sole of each forefoot, and four on each hind foot. The soles of the feet are covered with fur from heel to the beginning of the tubercles (McCarty 1975).

Identification.-- The Tulare grasshopper mouse can be told externally from coexisting species of white-footed mice (Peromyscus spp.) by its relatively short, club-like tail and larger forefeet (McCarty 1975).

Figure 62. Illustration of a Tulare grasshopper mouse. Drawing by Wendy Stevens based on photos © by B. Moose Peterson.

Historical Distribution.-- The Tulare grasshopper mouse historically ranged from about western Merced and eastern San Benito Counties east to Madera County and south to the Tehachapi Mountains; on the east, they ranged from Madera County south (Figure 63) (Newman and Duncan 1973, Williams and Kilburn 1992).

Current Distribution.-- Currently, Tulare grasshopper mice are known to occur along the western margin of the Tulare Basin, including western Kern County, Carrizo Plain Natural Area, along the Cuyama Valley side of the Caliente Mountains, San Luis Obispo County, and the Ciervo-Panoche Region, in Fresno and San Benito Counties (Williams and Kilburn 1992, D.F. Williams unpubl. data). Though there has not been a comprehensive survey of existing potential habitat, there re several large blocks of historical habitat on the floor of the Tulare Basin where extensive trapping has occurred, but no Tulare grasshopper mice have been captured, such as Alkali Sink Ecological Reserve, Fresno County, and Pixley National Wildlife Refuge, Tulare County (Endangered Species Recovery Program unpubl. data). The only recent record is the capture of a grasshopper mouse in 1994 at Allensworth Ecological Reserve (CDFG in litt. 1998).

Figure 63. Distributional records for the Tulare grasshopper mouse (Onychomys torridus tularensis)

Food and Foraging.-- Southern grasshopper mice eat mostly small animals, with insects forming the bulk of their diets (Bailey and Sperry 1929, Chew and Chew 1970, Horner et al. 1964). Prey items include scorpions, beetles, grasshoppers, pocket mice, and western harvest mice. Other ingested animals include spiders, mites, ants, insect cocoons, caterpillars, lizards, and frogs (Rana sp.) (Horner et al. 1964). They also eat seeds. Captive grasshopper mice stored sunflower seeds in their nest boxes during the winter months. The cache was used only when no other food source was available (Bailey and Sperry 1929).

Reproduction and Demography.-- Specific information on the reproduction and the mating system of Tulare grasshopper mice is unknown. For southern grasshopper mice in general, breeding occurs throughout the year in laboratory settings, but is seasonal in natural populations (McCarty 1975). Gestation is between 27 and 32 days, with two to six young born per litter. In the wild, Tulare grasshopper mice may produce up to three litters per year. Most litters are born from May through July, with a sharp decline in August (Taylor 1968). Both male and female southern grasshopper mice care for the young (Horner 1961).

The reproductive efficiency of female grasshopper mice declines significantly following the first year. Taylor (1968) reported that only 17 percent (8 of 47) of females that bore young in the laboratory bred in their second year, and only 2 percent (1 of 47) continued into the third year. Female southern grasshopper mice rarely remain reproductively active in the laboratory after 2 years of age. The oldest female to successfully rear a litter was 24 months old. The oldest male to sire a litter was 31 months old (Pinter 1970). Southern grasshopper mice survived in the laboratory up to 3 years, but mice in the wild probably live less than 12 months (Horner and Taylor 1968).

Females appear to be sexually active for a single breeding season, with a rapid onset of reproductive senility following the first year. Females born early in the year (April) may produce two or three litters prior to the end of the breeding season. Females born later in the year would have the potential to produce up to six litters in the following breeding season, but seasonality of breeding probably reduces the actual number to one to three litters. Distinct lulls in the testicular activity of males during the breeding season also may contribute to low population densities (Taylor 1963).

There is no information on demography or dispersal of Tulare grasshopper mice. Generally, southern grasshopper mice exist at relatively low density and have home ranges much larger than similarly-sized rodents such as white-footed mice (McCarty 1975).

Behavior and Species Interactions.-- The most consistent social unit is reported to be a male-female pair with offspring in a burrow system within a wide home range (McCarty 1975). Blair (1943) reported the home range size of male southern grasshopper mice was 3.2 hectares (7.8 acres), and that of females was 2.4 hectares (5.9 acres). The nest of the southern grasshopper mouse is typically located in a burrow system that may have been abandoned by another small mammal (Bailey and Sperry 1929, Hall and Kelson 1959).

Adult males are highly territorial and frequently vocalize during nocturnal activity. Adult males emit a high-pitched call, lasting several seconds, while standing on the hind legs with head raised and mouth open. Calls are less frequently given by females. Calling appears to function as a territorial and spacing mechanism (McCarty 1975).

Small mammals associated with Tulare grasshopper ice include giant kangaroo rats, San Joaquin kangaroo rats (all three subspecies), Heermanns kangaroo rats, California ground squirrels, San Joaquin antelope squirrels, San Joaquin pocket mice, California pocket mice, deer mice, harvest mice, and house mice (Hawbecker 1951, D.F. Williams unpubl. data).

Predators of the Tulare grasshopper mouse are known to include American badgers, San Joaquin kit foxes, coyotes, and barn owls (Hawbecker 1951).

Activity Cycles.-- Tulare grasshopper mice are nocturnal and active year round. They probably do not become dormant, at least not for long periods, though in captivity individuals have exhibited short episodes of torpor (D.F. Williams unpubl. observ.). Other aspects of activity of Tulare grasshopper mice are unknown.

Habitat and Community Associations.-- Tulare grasshopper mice typically inhabit arid shrubland communities in hot, arid grassland and shrubland associations (Williams and Kilburn 1992). There is little information about the habitat requirements of the Tulare subspecies. Habitats recorded in the literature include Blue Oak Woodland at 450 meters (1,476 feet) where it is very rare (Newman and Duncan 1973), and Upper Sonoran Subshrub Scrub (Hawbecker 1951). Other reported habitats are alkali sink, dominated by one or more saltbush species, iodine bush, seepweed, and pale-leaf goldenbush; mesquite associations on the Valley floor; saltbush scrub; Upper Sonoran shrub associations dominated by California ephedra/Anderson desert thorn; and grassland associations (primarily Arabian grass and red brome) on the sloping margins of the San Joaquin Valley and the Carrizo Plain region (Williams and Tordoff 1988).

Reasons for Decline.-- The habitat reduction, fragmentation, and degradation accompanying settlement and development of the Valley for agriculture are the principal causes of decline of Tulare grasshopper mice. Random catastrophic events (e.g. floods, drought combined with their low reproductive rate and other demographic factors probably are the most significant factors in elimination of fragmented populations. However, use of insecticides (first DDT and others, now mainly malathion) on natural lands to control beet leafhoppers could have contributed to the disappearance of grasshopper mice from fragmented islands of natural land on the Valley floor, both from direct and indirect poisoning, and reduction of their staple food, insects. Rodenticides targeted for ground squirrels and insecticide drift from adjacent farmland may also have been a factor in elimination of grasshopper mice from fragmented parcels on the Valley floor.

Threats to Survival.-- Habitat fragmentation and loss to cultivation, and, perhaps, inappropriate land management, are the most serious threats to Tulare grasshopper mice. The naturally low reproductive rate, low population density, and large home range characteristic of southern grasshopper mice (McCarty 1975) make this subspecies particularly vulnerable to loss and fragmentation of habitat (Williams and Kilburn 1992). There are no current overall estimates of population size for this subspecies.

Conservation Efforts.-- The Tulare grasshopper mouse is not a candidate for Federal listing, but is considered a species of concern (USFWS 1996).

Conservation Strategy.-- The Tulare grasshopper mouse lives in the same communities as the listed kangaroo rats, blunt-nosed leopard lizard, and San Joaquin kit fox. Its habitat needs, then, are essentially the same as those of other members of this arid grassland and shrubland community assemblage. Protecting habitat for the listed members of this assemblage should simultaneously protect habitat for Tulare grasshopper mice. Of greatest concern, however, is the apparent elimination of populations on the Valley floor. This loss, if substantiated, suggests relatively high vulnerability to extinction by random catastrophic events (e.g., drought, flooding, fire) or use of pesticides on even relatively large habitat areas. Effort needs to be directed at reaching an understanding of the environmental factors of islands where extinction hasoccurred. Knowledge gained can be used in refining a strategy for ensuring that the same processes do not result in further eliminations and eventual extinction of the entire metapopulation.

Conservation Actions.-- Habitat protection needs for Tulare grasshopper mice are essentially the same as those for San Joaquin antelope squirrels and the three subspecies of the San Joaquin kangaroo rat. Additional measures of highest priority for conservation of the Tulare grasshopper mice are:

1. Determine the current distribution and population status of Tulare grasshopper mice on isolated blocks of historical habitat on the Valley floor of the Tulare Basin.
2. Analyze the environmental features of inhabited and uninhabited fragmented islands of natural land on the Valley floor to determine factors, including pesticide use, that might be associated with survival and elimination.
3. Establish a range-wide monitoring program at sites representative of the range of occupied communities and areas.
4. As (if) habitat areas on the Valley floor are increased in size by retirement of agricultural land, restore habitat and reintroduce Tulare grasshopper mice.
5. Include Tulare grasshopper mice in studies of management and land uses on habitat of other species of the same community associations.
6. Reevaluate the status of the Tulare grasshopper mouse within 3 years of recovery plan approval.