Chaetodipus fallax
San Diego pocket mouse

Chaetodipus fallax are a Nearctic species, inhabiting a region in southwestern California surrounding San Diego and extending south into northwestern Baja California, Mexico. (Anonymous, 2003; Brylski, 2003; Erickson and Patten, 1999; Hafner, 1998; Ingles, 1954; Jameson and Peeters, 1988; Lackey, 1996; Longland, 1999; McClenaghan, 1983; Stephens, 1906)

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. (Anonymous, 2003; Brylski, 2003; Erickson and Patten, 1999; Hafner, 1998; Ingles, 1954; Jameson and Peeters, 1988; Lackey, 1996; Longland, 1999; McClenaghan, 1983; Stephens, 1906)

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. (Anonymous, 2003; Erickson and Patten, 1999; Ingles, 1954; Jameson and Peeters, 1988; Lackey, 1996; Longland, 1999; McClenaghan, 1983; Stephens, 1906)

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. (Lackey, 1996; Longland, 1999; McClenaghan, 1983)

Low speed locomotion for San Diego pocket mice is quadrupedal. However, when moving at much greater speeds, they gallop with leaps of around one meter. During this gallop, both hind feet hit the ground at the same time, unlike their quadrupedal motion, in which the feet alternate hitting the ground. Their tails are used to keep balance. Moving along in this galloping style, C. fallax can reach speeds of nearly 13 km/hr. They spend most of the day underground in vast burrows, only coming out to forage at night when it is easier to avoid predators. Food is gathered in pouches inside their cheeks and brought back to the burrows where it is typically stored in separate chambers. These burrows are also their long-term homes during the winter when a decreased state of activity, but not true hibernation, occurs. San Diego pocket mice are a rather solitary species. As such, they are rather intolerant of the presence of both members of their species as well as the presence of other similar rodents; they are very protective of their territories. (Brylski, 2003; Jameson and Peeters, 1988; Lackey, 1996; Longland, 1999)

Home Range

The home range of Chaetodipus fallax varies slightly between males and females. The male range is around 3500 square meters while the female’s is a little smaller at 2500 square meters. This range is the same size as the general territory for San Diego pocket mice and is defended from all intruders, with the exception of members of the opposite sex during mating season. (Brylski, 2003; Lackey, 1996)

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. (Ingles, 1954)

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. (Anonymous, 2003; Brylski, 2003; Hulbert and MacMillan, 1988; Jameson and Peeters, 1988; Lackey, 1996; Longland, 1999; McClenaghan, 1983; Stephens, 1906)

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. (Brylski, 2003; Lackey, 1996; Longland, 1999; McClenaghan, 1983)

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. (Anonymous, 2003; Brylski, 2003; Eidemiller, 1980; Hulbert and MacMillan, 1988; Jameson and Peeters, 1988; Lackey, 1996; Longland, 1999; McClenaghan, 1983; Price and Waser, 1984; Stephens, 1906; Thompson, 1982)

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

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

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. (Hafner, 1998; Longland, 1999)

Chaetodipus fallax was known as Perognathus fallax until 1983 when the genus Chaetodipus was used to classify all spiny pocket mice. (Lackey, 1996)