Chaetodipus fallax are a Nearctic species, inhabiting a region in southwestern California surrounding San Diego and extending south into northwestern Baja California, Mexico.

Biogeographic Regions: nearctic (Native )

occurs (regularly, as a native taxon) in multiple nations

United States

Origin: Native

Regularity: Regularly occurring

Currently: Present

Confidence: Confident

Type of Residency: Year-round

Global Range: Restricted to southwestern California (north to southeastern Los Angeles County (Erickson and Patten 1999) and the southern edge of the Mohave Desert in Riverside County, east to Twentynine Palms and to near Mountain Springs, Imperial County) and the western portion of northern Baja California, Mexico. Elevational range in the U.S. is about 138-1835 m (Lackey 1996).

San Diego pocket mice are moderately sized pocket mice, ranging in length from 170 to 200 mm and weighing from 17 to 22 grams. There is very little difference in size between males and females. Both are colored a dark brown on top and white underneath, with spines that are black on the rump and white on the hips. The tail length is shorter than the body by about 20 mm, it has a darkly colored dorsal crest and is and light below. This appearance is very similar to Chaetodipus californicus that also occupy the same general habitat except that C. fallax has shorter ears, usually shorter than 9 mm. San Diego pocket mice are homeothermic endotherms with hypsodont and lophodont teeth.

Range mass: 17 to 22 g.

Average mass: 20 g.

Range length: 170 to 200 mm.

Other Physical Features: endothermic ; homoiothermic; bilateral symmetry

Sexual Dimorphism: sexes alike

Average basal metabolic rate: 0.15 W.

Length: 20 cm

Differs from Chaetodipus californicus by having shorter, more rounded ears and smaller hind feet (Erickson and Patten 1999). See Lackey (1996).

• Terrestrial

San Diego pocket mice are found terrestrially in a wide variety of temperate habitats ranging from chaparral and grasslands to scrub forests and deserts. This area includes a vast range of elevations, extending from sea level along the Pacific coast to around 1400 m in the mountains of southwest California and Baja California. Rarely found in cities, the major habitat requirement for C. fallax is the presence of low growing vegetation or rocky outcroppings, as well as sandy soil in which they dig burrows.

Range elevation: 0 to 1400 m.

Habitat Regions: temperate ; terrestrial

Terrestrial Biomes: desert or dune ; savanna or grassland ; chaparral ; scrub forest

Comments: Open, sandy, areas in low desert and foothills; weed grown areas. In the Lower and Upper Sonoran life zones. Sleeping and birthing occur in underground burrows.

Non-Migrant: Yes. At least some populations of this species do not make significant seasonal migrations. Juvenile dispersal is not considered a migration.

Locally Migrant: No. No populations of this species make local extended movements (generally less than 200 km) at particular times of the year (e.g., to breeding or wintering grounds, to hibernation sites).

Locally Migrant: No. No populations of this species make annual migrations of over 200 km.

San Diego pocket mice are predominantly granivores, eating the seeds from a variety of shrubs and grasses found in their habitat. Their cheeks have fur-lined pouches in which they can store these seeds to carry them back to their burrows where they are stored. These pouches help keep the seeds isolated from the moisture of the mouth, helping C. fallax to avoid losing water to the seeds. In fact, they are so good at conserving water that they can survive on just seeds alone, getting all necessary water from either food or the byproducts of metabolic processes. As ambient temperature increases, San Diego pocket mice tend to more exclusively eat seeds that contain high amounts or moisture to compensate for the additional losses of moisture due to the heat. In times of seed shortage, San Diego pocket mice may also eat leaves, stems, and even insects.

Animal Foods: insects

Plant Foods: leaves; wood, bark, or stems; seeds, grains, and nuts

Foraging Behavior: stores or caches food

Primary Diet: herbivore (Granivore )

Comments: Feeds mainly on seeds. Stores food in storage chamber within burrow system.

In addition to providing food for a number of predators and eating many kinds of plants, San Diego pocket mice play several roles in their ecosystem. Through burrowing, they help to aerate the soil. The manner in which they store their seeds helps the plants distribute their offspring. They are also the host for a number of mites and fleas. In addition, they live in constant competition with other small rodents found in the same region. In order to reduce competition over food, Chaetodipus fallax rely more on seeds with high amounts of moisture, leaving the drier seeds for rodents that obtain more of their needed water through drinking. The competition between species is also seen in habitat use. San Diego pocket mice typically inhabit the rockier microhabitats in the region, while other soil types contain other rodents. As changes in the microhabitats occur, however, so too does the community of the rodent inhabitants. An example of this has been seen in regions that have increased plant density, providing more protection for C. fallax. As populations of Chaetodipus fallax have increased in these regions, Dipodomys merriami, a competing species, have decreased.

Ecosystem Impact: disperses seeds; soil aeration

Commensal/Parasitic Species:

• Androlaelaps sinuosa and other mites 
• Meringis dipodomys and other fleas 

Chaetodipus fallax are preyed on by a variety of foxes, coyotes, badgers, owls, and snakes. In order to avoid predation, they have dark pelage to help camouflage at night, when they are most active. They also have an erratic hopping style when being pursued by a predator, making it difficult to anticipate their path. The burrows in which they live also provide some safety from these predators, and the young remain in the shelter of the burrows until they are able to survive on their own.

Known Predators:

• foxes (Vulpes)
• coyotes (Canis latrans)
• American badgers (Taxidea taxus)
• owls (Strigiformes)
• snakes (Serpentes)

Anti-predator Adaptations: cryptic

Primarily solitary.

Due to their solitary nature, very little is known about communication between San Diego pocket mice. When in danger, they have been observed to squeak, suggesting that some form of vocal communication may be used. They use their large ears and eyes, as well as their good sense of touch, to perceive their environment.

Communication Channels: acoustic

Perception Channels: visual ; acoustic

Comments: Probably active throughout the year.

The lifespan of C. fallax ranges from about 4 months to nearly 2 years in the wild. Mortallity is largely due to predation, which occurs most heavily during the spring months. At this time, San Diego pocket mice are most active foraging, reproducing and defending their territories. In captivity, without the presence of predators, C. fallax can live up to 6 years.

Typical lifespan

Status: wild: 4 to 18 months.

Average lifespan

Status: wild: 5 months.

Typical lifespan

Status: captivity: 5 to 6 years.

Due to the solitary nature of C. fallax not much is known about their mating habits. In addition, they rarely breed in captivity, making analysis of their habits even more difficult. San Diego pocket mice are known to mate throughout the year, however mating appears to be concentrated during the spring.

San Diego pocket mice typically reproduce once a year in the spring, however, some have been known to reproduce year round and as much as three times a year. It appears that reproductive patterns are strongly related to rainfall. Each litter typically contains between 2 to 6 young that mature in 5 to 6 months. The gestation period for C. fallax is about 24 to 26 days. Not many details are known about the rearing patterns of San Diego pocket mice.

Breeding interval: San Diego pocket mice can produce 1 to 3 litters per year.

Breeding season: Most San Diego pocket mice breed in the spring but they do breed year round in the right conditions

Range number of offspring: 2 to 6.

Range gestation period: 24 to 26 days.

Range age at sexual or reproductive maturity (female): 5 to 6 months.

Range age at sexual or reproductive maturity (male): 5 to 6 months.

Key Reproductive Features: iteroparous ; year-round breeding ; gonochoric/gonochoristic/dioecious (sexes separate); fertilization ; viviparous

Average number of offspring: 3.

There is very limited information related to the parental investment of San Diego pocket mice. Females are the exclusive caregivers, birthing, nursing, and protecting their young inside their burrows. The specifics pertaining to the types of care given and the duration of this care are not known.

Parental Investment: precocial ; female parental care ; pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Female, Protecting: Female); pre-independence (Provisioning: Female, Protecting: Female)

Breeding probably occurs mainly in March and April. A pregnant female was captured on June 27; adult females without embryos have been captured on May 2, July 5, August 1 and September 1 (Miller and Stebbins 1964).

San Diego pocket mice are common in their habitat, and seem to be under no immediate threat. However, due to the expansion of human cities, like San Diego, there is a decrease in possible habitat. The severity of this is not well known but does not appear to suggest any immediate threat for C. fallax.

US Federal List: no special status

CITES: no special status

IUCN Red List of Threatened Species: least concern

United States

Rounded National Status Rank: N3 - Vulnerable

Rounded Global Status Rank: G5 - Secure

There are no known adverse effects of Chaetodipus fallax on humans.

There are no known positive effects of Chaetodipus fallax on humans.

Comments: This species formerly was included in the genus Perognathus. Subgenus Chaetodipus was elevated to full genus status by Hafner and Hafner (1983); this treatment was supported by a phylogenetic analysis of Heteromyidae based on myology (Ryan 1989). Chaetodipus was accepted as a full genus by Jones et al. (1992), Patton (in Wilson and Reeder 1993, 2005), and most other authors subsequent to Hafner and Hafner (1983). In a phylogeny based on molecular data, Riddle (1995) found support for the monophyly of Chaetodipus, including C. formosus, relative to Perognathus.

Includes C. anthonyi, which was recognized as a distinct species by Hall (1981).

Six subspecies were listed by Lackey (1996).