Peromyscus fraterculus ( Miller, 1892 )

Cornejo-Latorre, Cristian, Cortés-Calva, Patricia & Álvarez-Castañeda, Sergio Ticul, 2021, Peromyscus fraterculus (Rodentia: Cricetidae), Mammalian Species 53 (1008), pp. 112-124 : 112-117

Peromyscus fraterculus ( Miller, 1892)

Northern Baja Deermouse

Vesperimus fraterculus Miller, 1892:261 . Type locality “Dulzura, San Diego Co[unty]., Cal[ifornia].”

Sitomys fraterculus: Bryant, 1892:213 . Name combination.

Sitomys herronii Rhoads, 1893:832 . Type locality “San Bernardino Valley, Cal[ifornia].”

Sitomys eremicus fraterculus: Rhoads, 1893:833 . Name combination.

Sitomys herronii nigellus Rhoads, 1894:257 . Type locality “West Cajon Pass, San Bernardino Mts., California.”

[ Peromyscus (Peromyscus) ] fraterculus: Trouessart, 1897:515 . First use of current name combination.

[ Peromyscus (Peromyscus) ] Herroni Trouessart, 1897:516 . Incorrect subsequent spelling of Peromyscus herronii ( Rhoads, 1893) .

[ Peromyscus (Peromyscus) Herroni ] nigellus: Trouessart, 1897:516. Name combination.

Peromyscus eremicus propinquus Allen, 1898:154 . Type locality “San Pablo Point (San Pablo Bay), Lower California [= Baja California Sur],” Mexico.

P [eromyscus]. texanus propinquus: Allen, 1898:154. Lapsus for P. e. propinquus.

P [eromyscus]. texanus fraterculus: Allen, 1898:154 . Lapsus for P. e. fraterculus .

Peromyscus cedrosensis Allen, 1898:154 . Type locality “Cerros Island, Lower California [= Baja California],” Mexico.

P [eromyscus]. eremicus fraterculus: Allen, 1898:154 . Name combination.

[ Peromyscus ] herroni Elliot, 1901:137 . Incorrect subsequent spelling of Peromyscus herronii ( Rhoads, 1893) .

[ Peromyscus herroni ] nigellus: Elliot, 1901:137. Name combination.

Peromyscus herronii: Miller and Rehn, 1901:75 . Name combination.

Peromyscus herroni nigellus: Miller and Rehn, 1901:75 . Name combination.

Peromyscus homochroia Elliot, 1903:158 . Type locality “San Quentin, Lower California [= Baja California], Mexico.”

P [eromyscus]. propinquus: Elliot, 1904:172. Name combination.

P [eromyscus]. cedrosensis: Elliot, 1904:173 . Name combination.

Peromyscus [( Haplomylomys )] eremicus insulicola Osgood, 1909:246. Type locality “Espiritu Santo Island, off east coast of southern Lower California [= Baja California Sur], Mexico.”

Peromyscus [( Haplomylomys )] eremicus polypolius Osgood, 1909:248. Type locality “Margarita Island, off west coast of southern Lower California [= Baja California Sur],” Mexico.

Peromyscus eremicus cedroscensis Townsend, 1912:125 . Incorrect subsequent spelling of Peromyscus cedrosensis Allen, 1898 .

Peromyscus pseudocrinitus Burt, 1932:173 . Type locality “Coronados Island (latitude 26° 06’ N., longitude 111° 18′ W.), Gulf of California, Lower California [= Baja] California Sur], Mexico.”

P [eromyscus]. fraterculus insulicola: Hafner et al., 2001:781. Name combination.

Peromyscus fraterculus cedrosensis: Ceballos, 2014:355 . Name combination.

P [eromyscus ( Haplomylomys )]. f [raterculus]. polypolius: Cornejo-Latorre et al. 2017:1632. Name combination.

P [eromyscus ( Haplomylomys )]. f [raterculus]. pseudocrinitus: Cornejo-Latorre et al. 2017:1632 . Name combination.

CONTEXT AND CONTENT. Order Rodentia , family Cricetidae , subfamily Neotominae , tribe Reithrodontomyini , genus Peromyscus , subgenus Haplomylomys . P. fraterculus is considered a polytypic species (P. f. cedrosensis , P. f. insulicola, P. f. polypolius, and P. f. pseudocrinitus ), by Cornejo-Latorre et al. (2017); however, some of these subspecies are considered as full species or subspecies of P. eremicus ( Ramírez-Pulido et al. 2005, 2014; Bradley et al. 2017) and others as subspecies of P. fraterculus ( Álvarez-Castañeda et al. 2015, 2017; CortésCalva et al. 2016; Cornejo-Latorre et al. 2017). Much of the literature containing information on P. fraterculus was published prior to its elevation to species level and within that literature it may be referred to as P. eremicus . To ensure that we are presenting information that pertains specifically to P. fraterculus we have evaluated the literature and included only information pertaining to specimens collected west of the Colorado River within the known distribution of P. fraterculus .

The following subspecies are recognized by Cornejo-Latorre et al. (2017):

P. f. cedrosensis Allen, 1898:154 . See above.

P. f. fraterculus ( Miller, 1892:261) . See above.

P. f. insulicola Osgood, 1909:246. See above.

P. f. polypolius Osgood, 1909:248. See above.

P. f. pseudocrinitus Burt, 1932:173 . See above.

NOMENCLATURAL NOTES. Peromyscus fraterculus is included in the subgenus Haplomylomys within the eremicus species group based on its simple molar patterns that lack accessory cusps evident in transverse or longitudinal views ( Osgood 1904, 1909; Musser and Carleton 2005). Other studies support this assignment on the basis of variation in dentition ( Hooper 1957) and phallus morphology (Hooper and Musser 1964). Originally, P. fraterculus was described as a species separate from P. eremicus , although both are regarded as very similar morphologically ( Vesperimus fraterculus Miller, 1892 ). Later, P. fraterculus was subsumed as a subspecies of P. eremicus by Allen (1898:154), who considered it a “dark coastal form of eremicus .” It was therefore regarded as a geographic race in the revision of the genus Peromyscus by Osgood (1909), and then also by other authors ( Lawlor 1971a; Hall 1981; Álvarez-Castañeda and Cortés-Calva 1999). However, in a phylogenetic analysis of the eremicus species complex based on mitochondrial cytochrome- c oxidase subunit III, P. fraterculus and P. eremicus were formally classified as separate species ( Riddle et al. 2000). The process of speciation of P. fraterculus from P. eremicus occurred during the late Neogene period as a consequence of the geomorphological evolution of the western North American deserts ( Riddle et al. 2000). The taxonomic distinction of P. fraterculus at the species level is also supported by studies based on allozymes ( Avise et al. 1974) and morphometric analyses ( Legg 1978). Currently, it is considered that populations of P. fraterculus are distributed to the west of the Colorado River, whereas P. eremicus occurs on the east side of the river ( Riddle et al. 2000; Hafner et al. 2001; Ramírez-Pulido et al. 2005, 2014; Álvarez-Castañeda et al. 2015; CornejoLatorre et al. 2017). The evolutionary relationships between mainland and insular populations of P. fraterculus from the Gulf of California and the Pacific Ocean areas have been elucidated by Lawlor (1971a, 1983), Avise et al. (1974), and Hafner et al. (2001). The current consensus is that all island populations of P. fraterculus show a high degree of genetic similarity with the source mainland populations due to their recent isolation as a result of climatic fluctuations during the late Pleistocene epoch ( Hafner et al. 2001; Lawlor et al. 2002; Cornejo-Latorre et al. 2017). The name Peromyscus is derived from a combination of three Greek words: pera = small or small bag, mys = mouse, and iskos, a diminutive suffix, which means “small mouse” (Álvarez-Castañeda and Álvarez 1997). The specific epithet name fraterculus comes from the Latin word frater or fratr, it means “little brother.” The common name in Spanish is ratón de las californias (Álvarez-Castañeda and González-Ruiz 2018).

DIAGNOSIS

Based on external morphology, Peromyscus fraterculus ( Fig. 1 ) is most similar to P. eremicus (cactus mouse— Lawlor 1971a); however, both taxa have an allopatric distribution ( Riddle et al. 2000). P. fraterculus can be distinguished from P. eremicus by its smaller skull (greatest length of skull, 24.53 mm, range 22.3–25.0 mm; basilar length, 19.83 mm; nasal length, 8.50 mm; length of palatal bridge, 3.55 mm [mean of 161 adults with sexes combined]— Legg 1978; Álvarez-Castañeda et al. 2015) compared to P. eremicus (greatest length of skull, 25.23 mm, range 24.0– 26.5 mm; basilar length, 20.36 mm; nasal length, 8.63 mm; length of palatal bridge, 3.65 mm [mean of 216 adults with sexes combined]— Osgood 1909; Legg 1978; Álvarez-Castañeda et al. 2015). P. fraterculus has dorsal pelage cinnamon rufous and richly sprinkled with black, middle of dorsum darker, including flanks, never with orange shades; lateral line when present thin and light yellow ( Álvarez-Castañeda et al. 2017). The pelage of P. fraterculus is reddish brown and much darker than that of P. eremicus .

Peromyscus fraterculus is distributed sympatrically on the Baja California Peninsula along with two other species of the subgenus Haplomylomys : P. californicus (California mouse) in the northern part of the peninsula and southern California, and P. eva (Baja California Sur deermouse) in the central and southern portion of the peninsula. Morphologically P. fraterculus can be distinguished from P. eva by the number of caudal vertebrae ( P. fraterculus ranges from 30 to 34, whereas P. eva has 36— Lawlor 1971a); the pelage of P. fraterculus is darker (admixture of brown and black pigment) than that of P. eva that presents pelage containing ocherous buff, mixed conspicuously with uniformly distributed fine, dusky, dorsal lines. On the lower sides and the ventral region, P. fraterculus is whitish with one yellowish reddish pectoral spot, whereas the ventrum of P. eva is pure creamy white without a pectoral spot ( Osgood 1909; Lawlor 1971a; Álvarez-Castañeda and Cortés-Calva 2003; Álvarez-Castañeda et al. 2015). The morphology of the skull of P. fraterculus ( Fig. 2 ) is similar to that of P. eva , but the skull of P. fraterculus is less robust and shorter; P. eva has a greater zygomatic breadth, longer nasal bones and rostrum, and the molar toothrow is longer and wider ( Osgood 1909; Lawlor 1971a). The premaxillaries of P. fraterculus extend beyond the level of the nasals, whereas in P. eva the premaxillaries are variable in shape, they can be about equal in length to or extend beyond the nasal margin ( Lawlor 1971a). In P. fraterculus the baculum is narrower (7.61 by 1.76 mm) with a squarish base; for P. eva it is 7.96 by 0.79 mm, with a rounded base ( Lawlor 1971a).

Peromyscus fraterculus is noticeably smaller than P. californicus ; the cranial length is generally less than 23.0 mm (22.3–25.0 mm), and the total length ranges from 185.0 to 218.0 mm in P. fraterculus ( Álvarez-Castañeda et al. 2015) . In P. californicus the cranial length may be greater than 28.0 mm (28.1–32.1 mm), and the total length ranges from 220 to 285 mm ( Merrit 1978; Álvarez-Castañeda et al. 2015). The measurements (mean) for distinguishing P. fraterculus from P. merriami are: total length (189.4 versus 196.7); tail length (105.7 versus 107.0); length of skull (24.6 versus 25.2); mastoid breadth (10.8 versus 11.5); zygomatic breadth (12.2 versus 12.8), in all cases P. merriami has larger means and a more robust skull ( Lawlor 1971a:9, 10, 12; figures 5–7). Hoffmeister and Lee (1963) found the following characters to be most useful in distinguishing P. fraterculus from P. merriami : ear length, total skull length, palatal bridge length, incisive foramina length, interorbital length, hind foot length, and mastoid breadth.

GENERAL CHARACTERS

Peromyscus fraterculus shows a seasonal variation in fur coloration, being reddish brown in summer and more blackish in winter, while the ventral part of the body maintains a creamy or buff shade ( Osgood 1909). There is also a slight variation in the coloration of P. fraterculus in relation to the type of substrate present and the sex of the individual (females are slightly paler in pelage color than males— Dice 1939; Hall 1981), and environmental factors such as humidity and altitude can influence pelage color ( Dice 1939). Usually, the tone of the pelage in P. fraterculus matches the color of the soil in its habitat (Dice and Blossom 1937; Dice 1939). In the highlands, the pelage of P. fraterculus is darker owing to a greater admixture of brown and black pigment in the hair ( Lawlor 1971a). Leucism has been reported in individuals of P. fraterculus in the San Quintín area, Baja California, México, likely associated with a low genetic diversity within this population due to habitat fragmentation ( Camargo et al. 2016).

Females are slightly larger in every dimension of the body and skeleton (with statistically significant differences in body length, mandible length, and bullar width), and are heavier than males ( Dice 1939). Despite seasonal differences in pelage color, geographically P. fraterculus exhibits a uniform pattern of morphological variation, and in general the northern populations are only slightly larger than those inhabiting the Baja California Peninsula ( Lawlor 1971a; Legg 1978). Information obtained from specimens distributed in 19 localities of the peninsula of Baja California indicates that the zygomatic notch in P. fraterculus is deeper, slightly concave, and gradually convergent anteriorly, whereas it is robust, laterally rounded, and sharply convergent in P. eva ( Lawlor 1971a; Álvarez-Castañeda and Cortés-Calva 2003). Mean craniodental measurements (mm; with parenthetical range; sexes combined, means calculated by us from the data of Miller 1892:263) for four males and three females from the type locality (Dulzura, San Diego County, California) of P. fraterculus were: basilar length, 20.22 (19.0–21.4); basilar length of Hensel, 17.88 (16.4–19.0); greatest nasal length, 8.68 (8.4–9.6); interorbital constriction, 3.88 (3.6–4.0); length of mandibular toothrow, 3.80 (3.6–4.0); length of maxillary teeth, 3.62 (3.4–3.8); zygomatic breadth, 12.06 (11.4–12.6; n = 6). Mean cranial measurements (mm; ± SD; with parenthetical range and n; sexes combined—Brand and Ryckman 1969) from a series of specimens collected at Reche Canyon, 4.82 km north of Sunnymead, Riverside County, California, were: incisor width, 2.04 ± 0.01 (1.90–2.25, 43); interparietal length, 3.38 ± 0.04 (2.90–4.00, 40); interparietal width, 9.35 ± 0.04 (8.85–10.00, 39); length of maxillary toothrow, 3.73 ± 0.02 (3.50–4.00, 43); nasal length, 9.15 ± 0.05 (8.40–9.95, 43); rostral width, 2.47 ± 0.02 (2.30–2.75, 43); rostral–incisor height, 6.14 ± 0.04 (5.35–6.80, 43); skull length, 24.24 ± 0.09 (23.70– 25.90, 34); zygomatic breadth, 12.03 ± 0.05 (11.80–13.50, 38). Mean cranial measurements (mm; sexes combined— Legg 1978) of 161 adult specimens from the vicinity of San Diego, San Diego County, California, were: basilar length, 19.83; greatest skull length, 24.53; incisive foramen length, 4.24; interorbital constriction, 3.68; mastoidal breadth, 10.90; length of maxillary toothrow, 3.52; nasal length, 8.50; palatal bridge, 3.55; postpalatal length, 8.26; rostral length, 8.82; skull depth, 8.82; width across third upper molar, 4.01; zygomatic breadth, 12.19. Mean craniodental measurements (mm; ± SD; with parenthetical range and sexes combined—Carleton and Lawlor 2005) of 23 specimens from California were: braincase breadth, 11.6 ± 0.3 (10.8–12.0); rostrum breadth, 4.0 ± 0.1 (3.8–4.3); breadth of zygomatic plate, 2.0 ± 0.1 (1.9–2.4); coronal length of maxillary toothrow, 3.62 ± 0.12 (3.35–3.82); greatest breadth of zygomatic, 12.3 ± 0.3 (11.8–12.9); interorbital breadth, 3.8 ± 0.1 (3.5–4.0); length of auditory bulla, 4.5 ± 0.1 (4.3–4.7); length of bony palate, 3.7 ± 0.1 (3.5–4.0); diastema length, 5.9 ± 0.2 (5.6–6.3); length of incisive foramen, 4.9 ± 0.2 (4.6–5.4); rostrum length, 8.0 ± 0.2 (7.6–8.6); occipitonasal length, 24.4 ± 0.5 (23.4–25.5); postpalatal length, 8.8 ± 0.3 (8.3–9.7); width of the first upper molar, 1.12 ± 0.04 (1.05–1.24).

Mean external measurements (mm; ± SD; with parenthetical range; sexes combined—Brand and Ryckman 1969) of 41 specimens were: total length, 189.02 ± 1.34 (171.00–211.00); tail length, 103.51 ± 1.02 (96.00–120.00); head–body length, 85.12 ± 0.73 (72.00–96.00). Mean external measurements (mm; sexes combined— Legg 1978) of 161 adult specimens were: total length, 188.7; tail length, 104.8; length of hind foot, 20.2; ear length, 16.6. Additionally, mean external measurements (mm ± SD and range; sexes combined—Carleton and Lawlor 2005) were transcribed from skin tags as given by the collector: total length, 190.6 ± 7.1, 177–204; tail length, 106.8 ± 6.8, 93–119; length of hind foot, 20.0 ± 0.9, 17–21.

DISTRIBUTION

Peromyscus fraterculus is distributed in an area west of the Colorado River, from Ojai (Ventura County) in southern California, southward across Baja California to south of La Paz Isthmus, in Mexico ( Fig. 3 ; Legg 1978; Hall 1981; Riddle et al. 2000; Álvarez-Castañeda et al. 2010a). In the State of Baja California Sur, it occurs in the midlands and highlands of the Sierra de La Giganta ( Álvarez-Castañeda et al. 2010b) with a disjunct population at Sierra de Las Cruces, south of La Paz ( Lawlor 1971a); it inhabits an elevational range from sea level to about 1,175 m ( Álvarez-Castañeda et al. 2010b). Its distribution is typically associated with mountains or canyons where hard soil with large numbers of medium-size stones prevails (ÁlvarezCastañeda et al. 2010b). Four subspecies of P. fraterculus are distributed in the islands surrounding Baja California, which have an area between 8.5 and 348 km 2 ( Hall 1981; ÁlvarezCastañeda and Cortés-Calva 1999; Hafner et al. 2001; Carreño and Helenes 2002; Álvarez-Castañeda and Ortega-Rubio 2003). P. f. cedrosensis occurs on Cedros Island (ca. 348 km 2) and P. f. polypolius at Margarita Island (ca. 200 km 2) in the Pacific Ocean, whereas P. f. insulicola is on Espíritu Santo Island (ca. 112 km 2), and P. f. pseudocrinitus at Coronados Island (ca. 8.5 km 2) in the Gulf of California (Álvarez-Castañeda and CortésCalva 1999; Hafner et al. 2001; Cornejo-Latorre et al. 2017). No fossils of P. fraterculus are known.

FORM AND FUNCTION

Form. —The dental formula of Peromyscus fraterculus , similar to most other species in the superfamily Muroidea , is i 1/1, c 0/0, p 0/0, m 3/3, total 16 ( Hooper 1957). The premaxillaries extend beyond the level of the nasals, the zygoma is usually slight, gradually convergent anteriorly, and the molars show a low frequency of styles and lophs, the incidence of which seems to vary geographically ( Hooper 1957; Lawlor 1971a). “The teeth of P. fraterculus are simple in pattern, and typically uncomplicated by secondary folds” ( Hooper 1957:17). The second tooth in each jaw tends to be simpler than the first with respect to entostyles and ectostylids ( Hooper 1957). The phallus of P. fraterculus resembles that of P. californicus but is smaller. Hooper (1958) describes the glans as vase-shaped with the body covered with stubby spines. It has a simple outline, lappets are not present, and the tip is nonconical. The distal edge of the glans body is notched middorsally and midventrally, the tip is not protractile. The most distal part of the glans is rounded and covers the baculum (which is not extensible). The baculum is thick throughout its entire length, broad and flat at the base and large in diameter distally; its average length is 50% of the length of the hind foot ( Hooper 1958). Weights of reproductive tissue for P. fraterculus are not known, but the following means (mg; ± SE) are for P. eremicus a closely related species: testes, 271 (± 60) and 225 (± 39); epididymis, 68 (± 11) and 73 (± 10); epididymal fat pad, 175 (± 66) and 141 (± 28); body mass-adjusted testes, 24 (± 1) and 22 (± 0.8— Trainor et al. 2006).

Function. — Peromyscus fraterculus has the ability to withstand prolonged periods of inactivity (e.g., estivation) during the summer months as an adaptive strategy toward aridity under conditions of moderate ambient temperatures (T A) ranging from 15°C to 30°C ( MacMillen 1965; Caire 1999), but not under conditions of low T A (<15°C— MacMillen 1965). If food and water are freely available, P. fraterculus is a good thermoregulator within a T A range of 5–35°C ( MacMillen 1965). Within this range of increasing T A there is a gradual increase in body temperature (T B), with a mean T B of 36.1°C at T A = 5°C, and 37.9°C at T A = 35°C (n = 10 individuals— MacMillen 1965). At T A = 38°C the animals became hyperthermic, with a mean T B of 39.9°C ( MacMillen 1965). Torpor is accompanied by a marked depression in T B and oxygen consumption ( MacMillen 1965). In winter, torpor may be induced only by food restriction; in summer, torpor is induced by food restriction or by imposing a negative water balance state ( MacMillen 1965). P. fraterculus dies in torpor at T A = 5°C, is essentially moribund at T A = 10°C, and cannot arouse from nor survive a T B below 16°C ( MacMillen 1965). The thermal neutral zone of active animals is narrow, spanning from 28°C to 35°C ( MacMillen 1965). Mean basal metabolic rate (1.56 [range 1.48–1.60] cm 3 O 2 /g/h—McNab and Morrison 1963; MacMillen 1965) is reduced to 88.6% of the predicted value relating metabolic rate to body size ( MacMillen 1965).

ONTOGENY AND REPRODUCTION

At birth, the dorsal region of Peromyscus fraterculus is darkly pigmented due to the presence of pigment granules in the developing hair ( Svihla 1932). Hair growth is rapid, and when the eyes open at 15–17 days after birth the young are covered with short hair, the juvenile pelage is retained until the young is about 40 days of age ( Dice 1939).

Peromyscus fraterculus is a facultative monogamous species that displays parental care ( Kleiman 1977; Wolff 1989; Glasper and DeVries 2005; Kalcounis-Rüppel and Ribble 2007). The monogamous features of P. fraterculus include a high tolerance of young and an increased social contact with mates, since pairs can remain together during the rearing phase without fighting ( Eisenberg 1963; Ribble 2003). In the field, the home range of heterosexual pairs overlaps, and the occurrence of family groups has been reported, but the associations are not necessarily long-lasting ( Eisenberg 1963; Kleiman 1977). P. fraterculus seems to have a loose social structure, and pairing appears to be transient, although males, females, and litters may remain associated in nature for longer periods ( Eisenberg 1963). The spacing characteristics differ between sexes: solitary in females and roving in males ( Ribble 2003; Waterman 2007).

It has been suggested that in xeric environments, the genus Peromyscus can breed year-round, but hot and dry conditions tend to inhibit reproduction ( Abbott 1969 [not seen cited in Trainor et al. 2006]); it is still unclear whether P. fraterculus is an aseasonal breeder ( Trainor et al. 2006). In the Baja California Peninsula, the pregnancy period for P. fraterculus has been observed from February to August. February and March were when the highest number of pregnant females were registered (80%, while the lactating females accounted for the remaining 20%). Females have an average embryo size of 18 mm (range 7–25 mm — Álvarez-Castañeda et al. 2008; P. Cortés-Calva, in litt.). Males show an increase in testicular size between February and May, with an average testicular size of 11 mm (range 6–17 mm), decreasing in November with an average testicular size of 4 mm ( Álvarez-Castañeda et al. 2008; P. Cortés-Calva, in litt.). Females attain sexual maturity at 61 days and the earliest estrum is at 28 days of age, whereas males can be fertile at 40 days of age ( Clark 1938). The average age at first estrum (first cornified cell stage) is 39.21 days (± 1.54— Clark 1938). Litter size might average four young (range 3–8— Álvarez-Castañeda et al. 2008). Under laboratory conditions, P. fraterculus can reproduce year-round, with the births dates of 8 litters distributed as follows: January, 1 litter; February, 2 litters; June, 1 litter; July, 1 litter; October, 1 litter; and December, 2 litters (Brand and Ryckman 1968). In the same study, the maximum litter size was three, with an average of 2.22 (n = 14 litters), including 1 litter of 1 pup, 9 litters of 2, and 4 litters of 3 (Brand and Ryckman 1968). Additionally, two females had litters in consecutive months at intervals of 35 days in each instance (Brand and Ryckman 1968). Reproductive data obtained during 1 year from 61 captive females were: number of breeding females, 33; maximum number of litters produced by one female, 16; maximum number of litters in 1 year, 12; minimum interval between litters (in days), 27; fertility ratio of females producing 1 or more litters, 1 litter/52.9 days; total litters born and percentage of litter mortality, 153 and 54%, respectively; percentage of litters born between 0900 and 1700 h, 64%; mean days between date of mating and first litter, 46.3 (range 28–91); mean litter size, 2.55 (range 1–5— Rood 1966). In captivity, P. fraterculus has an average of 1–3 litters per year ( Bradley et al. 2017).

Figures

Fig.1.—Adult(male)Peromyscusfraterculus fromBahíadelosÁngeles, Baja California. Photograph by Sergio Ticul Álvarez Castañeda.

Fig. 2.—Dorsal, ventral, and lateral views of the skull, and lateral view of the mandible of an adult male of Peromyscus fraterculus fraterculus (CIB [Centro de Investigaciones Biológicas mammal collection] #8849) from 23 km N, 36 km E of San Ignacio, Baja California Sur, México. The occipitonasal length is 23.5 mm.

Fig. 3.—Geographic distribution of Peromyscus fraterculus. The subspecies are: 1, P. f. cedrosensis; 2, P. f. fraterculus; 3, P. f. insulicola; 4, P. f. polypolius; 5, P. f.pseudocrinitus. Map redrawn from Hall (1981) and Álvarez-Castañeda et al. (2010b) with modifications.