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Peromyscus Genetic Stock Center

Peromyscus eremicus Cactus mouse

*Information adapted from Mammalian Species, No. 118, Rita Veal and William Caire, The American Society of Mammalogists, 1979.


Medium sized and pale gray above, Peromyscus eremicus lives in desert areas of the southwestern United States, Baja California, and north-central Mexico.


Pelage: Soft and silky; upper parts ochraceous-buff to cinnamon-buff washed with dusk; a lateral line of pure ochraceous-buff extending from along the sides to the outer surface of the front legs; sides and top of head slightly grayish; tail untufted, finely annulated and sparsely haired, dusky above, whitish below; under parts pure white to buffy, traces of pectoral spot sometimes present; sole of hind foot naked to heel, ears relatively large, almost naked (Osgood, 1909). Pelage coloration varies among the subspecies and even among different populations. Dark populations are known to occur in the lava areas of Arizona (Benson, 1933), New Mexico (Findley et al., 1975; Koschmann, 1974), and Sonora (Faire, 1978).


Size: Medium; tail distinctly longer than head and body, (Osgood, 1909).


Weight: Ranges from 18 to 40 g (most commonly found between 20 to 29g.).


Measurements: Total length is 160 to 211 mm; length of tail, 84 to 120; length of body, 72 to 100; hind foot, 18 to 22; length of ear, 13.4 to 20.0; greatest length of skull, 22.7 to 25.9; and zygomatic breadth, 11.2 to 13.5. Dice (1939) described female P. eremicus as being significantly larger than males in body length, ear length, length of mandible, and bullar width of skull. Females also average slightly more than males in weight (Davis, 1966).



Peromyscus eremicus occurs sympatrically with four other species of the subgenus Haplomylomys (P. californicus, P. crinitus, P. eva, and P. merriami) and with two members of the boylii species group (P. pectoralis and P. boylii). Because of broad overlap of species ranges and extensive geographic variation in most characters of P. eremicus and other species of Peromyscus, the diagnostic characters at one locality may not be diagnostic at another locality. To insure proper identification a combination of characters are needed. The best characters to distinguish males of P. eremicus from males of other sympatric Peromyscus are: baculum relatively short, broad (8.2 x 1.8 mm; Burt, 1960), and dorsally curved; base squarish and very small; cartilaginous tip diffuse; phallus small, broad, and with no lappets on glans; all male accessory reproductive glands present (Lawlor, 1971b). 

In California, P. eremicus is recognizably smaller than P. californicus, which has external measurements (mm) of: total length, 220 to 266; tail length, 117 to 148; hind foot, 25 to 29; ear, 20 to 25 (Hall and Kelson, 1959). Peromyscus californicus also differs in having a well haired tail and dark brown upper parts, and in lacking the ochraceous-buffy lateral line present in P. eremicus

Externally P. crinitus can usually be distinguished from P. eremicus by a well haired, tufted tail. Males of P. crinitus also differ in having a bacillus with a small, rounded cartilaginous tip; a slender phallus with two median lappets ventrally and two dorsal lappets separated by a shallow median cleft; and no preputial glands (Lawlor, 1971b). 

In south-central Baja California, P. eva is distinguishable from P. eremicus by pelage which is shorter and appears more finely textured because it is colored with a blend of rufous, buffy, and brown (P. eremicus has a strong admixture of dark brown or black). Peromyscus eremicus has a less grayish face. Peromyscus eva also differs in having a much longer tail; longer skull, greater zygomatic breadth, longer nasal bones, and longer rostrum; a molar toothrow averaging greater in length and width; a shallower zygomatic notch; a zygoma generally rounded laterally instead of slightly concave (figure 1); a greater number of tail vertebrae (36 in P. eva and 30 to 34 in P. eremicus); and a baculum with a narrower shaft, a rounded cartilaginous tip, and a small, rounded base (Lawlor, 1971b). The relationships of P. merriami and P. eremicus have been investigated by Commissaris (1960), Hoffmeister and Lee (1963), and Lawlor (1971b). Lawlor (1971b) characterized P. merriami as having a larger size; a more robust skull due to relatively greater zygomatic breadth; generally a deeper zygomatic notch in dorsal view a larger infraorbital canal; and a longer glans penis and a baculum with a narrow, straight (or ventrally curved) shaft and a rounded, narrow base. Other characters given by Hoffmeister and Lee (1963) are of less diagnostic value, especially on an interlocality basis. Where occurring sympatrically, P. merriami prefers shrubby, lowland areas of deep soil and P. eremicus prefers rocky areas (Commissaris, 1960; Lawlor, 1971b). 

Peromyscus eremicus (as most other Haplomylomys) can usually be separated from P. pectoralis and P. boylii by its simple molar teeth which usually lack accessory styles or loph between the first and second main cusps of the upper molars. 



P. eremicus inhabits southern California, Baja California, and several islands in the Gulf of California. Range extends eastward to western Texas; southward toward San Luis Potosi, Mexico; and north along the Colorado River to extreme southwestern Utah  and southern tip of Nevada. Found from elevations of 35 m (Cockrum, 1960) to elevations of 2130 m (Hall, 1946).


The cactus mouse inhabits various desert environments. For instance, P. eremicus has been found in areas of mesquite grass, malpais lava, and the desert mountain ranges of New Mexico (Blair, 1943; Findley et al., 1975); the rock hill, Tamarix, and desert plain of Arizona (Lewis, 1972; Cahalane, 1939); the coastal sage scrub and desert slopes of the San Gabriel Mountains in California (Vaughn, 1954); the creosote and desert shrub of southern Nevada (Bradley and Mauer, 1973); the thorn and short tree forests, and desert shrub of Sonora, Mexico (Faire, 1978); the desert shrub of the TransPecos area in Texas (Davis, 1966); riparian associations of Utah (Long, 1940); and the desert shrub areas of Durango and San Luis Potosi, Mexico (Baker and Greer, 1962; Dalquest, 1953). Cactus mice also inhabit rocky areas and outcroppings at some localities (Davis, 1966; Vaughan, 1954; Blair, 1943; Lewis, 1972; MacMillen, 1964; Cahalane, 1939), however, a preference for sandy substrates and loamy soils has been noted (Dalquest, 1953; Cahalane, 1939; Baker and Greer, 1962; MacMillen, 1964) (Lewis, 1972; Long, 1940). Findley et al. (1975) found P. eremicus to be restricted in varying degrees to the south facing slopes of mountains during the winter months in New Mexico.

Ogston (1974) found the home ranges of P. eremicus averaged 0.3 hectares. Females showed almost no overlap of home ranges, while the home ranges of males overlapped considerably (MacMillen, 1964).

In areas where P. crinitus and P. eremicus occur sympatrically, isenberg (1963) found P. eremicus generally inhabits the brushy, flat desert floors, while P. crinitus generally inhabits the rocky areas.


In captivity, Peromyscus eremicus is typically described as docile, quiet, and shy, yet excitable; they seldom attempt to bite when handled (Svihla, 1932; Brand and Ryckman, 1968; Murie, 1961).

P. eremicus is known for its arboreal activity on certain coastal shrubs, it has also been observed foraging in mesquite and hackberry trees (Meserve, 1977; Davis, 1966).

Cactus mice nest in rock heaps (Lewis, 1972) and stone walls (Dalquest, 1953), in burrows around the base of mesquites (Cahalane, 1939), and in the abandoned burrows of gophers and kangaroo rats (Caire, 1978). The nests of P. eremicus found in the brush fences near Sonoyta and Quitobaquito were of grass, feathers, stems, and leaves (Faire, 1978). In the laboratory, P. eremicus built fluffy, globular, cotton nests (Brand and Ryckman, 1968).

Water requirements, body temperature, and metabolism of P. eremicus have been studied in relation to adaptations to desert living by Lindeborg (1952), Murie (1961), McNab and Morrison (1963), MacMillen (1964, 1965), and Morhardt and Hudson (1966). Murie (1961) reported P. eremicus to have a 10 to 20% lower metabolic rate and to resort to saliva spreading for evaporative cooling at high temperatures less readily than P. maniculatus.

Diet consists of fruit and flowers of shrubs; seeds of various desert annuals including mesquite, and hackberry; insects; and green vegetation. (Bradley and Mauer, 1973; Meserve, 1976; Dalquest, 1953; Davis, 1966; Reichman, 1975; Reichman and Van de Graff, 1973a).

In cactus mice, torpor is mainly circadian (torpid by day, active by night); (MacMillen, 1972) and can be employed anytime their energy supplies become limited (Morhardt and Hudson, 1966). MacMillen (1965, 1972) distinguished between winter (circadian) torpor, induced only by food restriction, and summer torpor, which may be circadian or may last two to three months. Summer torpor was induced by food restriction or by imposing a negative water balance. According to MacMillen (1964, 1965), cactus mice aestivate during the summer to conserve water and prolong food reserves.

Owings and Lockard (1971) found P. eremicus to be most active on moonlit nights and P. californicus to be most active on moonless nights, these results suggest a temporal dimension of competitive exclusion for these sympatric species.

Moor (1968) suggested that P. eremicus is probably reproductively active throughout the year in the lower desert areas but that reproduction is curtailed during the hot dry periods of summer. March through April appear to be the peak breeding season (Lewis, 1972). In the laboratory, Drickamer and Vestal (1973) noted a seasonal breeding pattern of spring through autumn with a peak occurring during the months of increased day lengths. MacMillen (1964) proposed a seasonal reproductive cycle for males of P. eremicus. In October and November only males with atrophied testes were trapped, but caught males with increasing testicular size from December through February, and found males with the most fully developed gonads from March through September. MacMillen (1964) suggested that females that become pregnant while lactating have a postpartum estrus and a prolongation of gestation; this would explain the occurrence of pregnant and lactating females during the nonfecund period of males.

Average litter size has been reported to vary from 2.2 to 2.8 (Brand and Ryckman, 1968; Davis and Davis, 1947; Drickamer and Vestal, 1973). Hall and Kelson (1959) refer to Asdell's (1946:237) recorded extremes of 1 to 4.

Barn owls (Tyto alba) (Bradshaw and Hayward, 1960), the screech owl (Otus asio) (Miller and Stebbins, 1964), and king snakes are likely predators (Dice and Blossom, 1937).


P. eremicus has a diploid number of 48 chromosomes. The autosomes are all biarmed, with the large ones being submetacentric or subtelocentric, and many being morphologically similar. The smallest pair is nearly metacentric. The X-chromosome is large with nearly equal arms while the Y-chromosome is medium sized and submetacentric.


Order Rodentia, Suborder Myomorpha, Family Muridae, Subfamily Sigmodontinae, Tribe Peromyscini, Genus Peromyscus, Subgenus Haplomylomys, eremicus-species group.


Fifteen subspecies of P. eremicus are presently recognized. Eight are known only from their type localities and are indicated below with an asterisk (*) (Hall and Kelson, 1959; Anderson, 1972; Lawlor, 1971a, b). Additional study is needed to clarify the status of many of these subspecies.

P. e. eremicus (Baird, 1858:479), Type locality Fort Yuma, California, Imperial County, on Colorado River, opposite Yuma, Arizona. (arenarius Mearns a synonym).

P. e. anthonyi (Merriam, 1887:5), Type locality Camp Apache, Big Hachita Mountains, Hidalgo County, New Mexico.

P. e. fraterculus (Miller, 1892:261), Type locality Dulzura, San Diego County, California. (herronii Rhoads, nigellus Rhoads, and homochroia Elliot are synonyms).

*P. e. tiburonensis (Mearns, 1897:720 Type locality Tiburon Island, Gulf of California, Sonora.

*P. e. cedrosensis (J. A. Allen, 1898:154), Type locality Cerros (=Cedros) Island, Baja California.

P. e. phaeurus (Osgood, 1904:75). Type locality Hacienda la Parada, San Luis Potosi.

*P. e. avius (Osgood, 1909:247). Type locality Ceralbo Island (=Cerralvo Island), Gulf of California Baja California.

*P. e. insulicola (Osgood, 1909:246). Type locality Espiritu Santo Island, Gulf of California, Baja California.

*P. e. polypolius (Osgood, 1909:248). Type locality Margarita Island, off the west coast of southern Baja California.

P. e. papagensis (Goldman, 1917:110).Type locality Sierra Pinacate, Sonora.

*P. e. cinereus (Hall, 1931:87). Type locality SW end San Jose Island, latitude 25°N, Baja California.

*P. e. collatus (Burt, 1932:172). Type locality Turners Island, latitude 28°19'W, Gulf of California, Sonora.

*P. e. pullus (Blossom, 1933:3). Type locality Black Mountain, 10 mi. S Tucson, Pima County, Arizona.

P. e. alcorni (Anderson, 1972:341). Type locality 11 mi. NNW San Buenaventura, Chihuahua, Mexico.

P. e. sinaloensis (Anderson, 1972:342). Type locality 26 mi. NE Choix, Sinaloa, Mexico.


Hesperomys eremicus Baird, 1858:479. Type locality "Fort Yuma, California," Imperial County, on Colorado River, opposite Yuma, Arizona.

Hesperomys (Vesperimus) anthonyi Merriam, 1887:5. Type locality Camp Apache, Big Hachita Mountains, Hidalgo County, New Mexico.

Vesperimus fraterculus Miller, 1892:261. Type locality Dulzura, San Diego County, California.

Sitomys herronii Rhoads, 1893:832. Type locality Reche Canyon, San Bernardino County, California.

Peromyscus tiburonensis Mearns, 1897:720. Type locality Tiburon Island, Gulf of California, Sonora.

Peromyscus cedrosensis J. A. Allen, 1898:154. Type locality Cerros (=Cedros) Island, Baja California.

Peromyscus homochroia Elliot, 1903:158. Type locality San Quintin, Baja California.


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