Tamiasciurus mearnsi ( Townsend, 1897 )

Koprowski, John L., Steele, Michael A. & Ramos-Lara, Nicolás, 2016, Tamiasciurus mearnsi (Rodentia: Sciuridae), Mammalian Species 48 (936), pp. 66-72 : 66-68

Tamiasciurus mearnsi ( Townsend, 1897)

Mearns’s Squirrel

Sciurus hudsonius mearnsi Townsend, 1897:146 . Type locality “ San Pedro Martir Mountains, Lower California (altitude about 7,000 feet).”

Sciurus mearnsi Allen, 1898:286 . Elevation to specific status. Sciurus douglasi mearnsi Elliot, 1903:210 . Replacement name for Sciurus hudsonius mearnsi Townsend, 1897 . Tamiasciurus douglasii mearnsi Hayman and Holt in Ellerman, 1940:348. Replacement name for Sciurus hudsonius mearnsi Townsend, 1897 (= Sciurus douglasi mearnsi Elliot, 1903 ). Tamiasciurus mearnsi: Lindsay, 1981:680 . Elevation to specific status; first use of current name combination.

(red squirrel) from Arizona and New Mexico. Both univariate (12 of 17 cranial features) and multivariate statistics separated CONTENT AND CONTEXT. Order Rodentia , suborder Sciuromorpha , family Sciuridae , subfamily Sciurinae , tribe Sciurini , genus Tamiasciurus . Tamiasciurus mearnsi is monotypic (Wilson and Reeder 2005).

NOMENCLATURAL NOTES. Lindsay (1981) recommended that Tamiasciurus mearnsi be elevated to species status based on the comparison of skull characteristics (of 32 specimens) with similar samples of T. douglasii albolimbatus (Douglas’s squirrel) from south central California and T. hudsonicus mogollonensis the 3 species equally ( Lindsay 1981). The specific epithet is in honor of Dr. Edgar A. Mearns, Naturalist of the United States- Mexican Boundary Commission of 1892–1894 ( Townsend 1897). The common name of Mearns’s squirrel is used as recommended by Wilson and Cole (2000). Other common names are Mearns squirrel, ardilla roja, Mearns Chickaree ( Mearns 1907), and Sierra San Pedro Martir Chickaree ( Minnich et al. 1997).

DIAGNOSIS

No native populations of sympatric species of arboreal squirrels exist within the range of Tamiasciurus mearnsi (Yensen and Valdés-Alarcón 1999; Thorington et al. 2012). However, in 1946, 2 populations of eastern gray squirrels ( Sciurus carolinensis ) were introduced separately in the same mountains where T. mearnsi occurs ( Huey 1964). Eastern gray squirrels have a slate to steel gray dorsum (not pale gray to brown as in T. mearnsi ) occasionally suffused with buff or brown (not reddish-yellow to peach); venter is white and tail is gray (not black) frosted with white to buff ( Koprowski 1994); eastern gray squirrels lack the distinctive lateral stripe present in T. mearnsi (Yensen and Valdés-Alarcón 1999) . Eastern gray squirrels (550 g) are about twice the body size of T. mearnsi (271.6 g — Koprowski 1994; Ramos-Lara 2012). The dental formula of T. mearnsi is i 1/1, c 0/0, p 1/1, m 3/3, total 20 (Yensen and Valdés-Alarcón 1999 and that of the eastern gray squirrel is i 1/1, c 0/0, p 2/1, m 3/3, total 22, with the first upper premolar peg-like and usually present ( Koprowski 1994).

Tamiasciurus mearnsi differs from T. h. mogollonensis and T. d. albolimbatus in being paler gray dorsally and having a dorsal band of reddish-yellow. T. d. albolimbatus is darker gray to almost black dorsally, more yellow on the venter, and has a less distinct lateral stripe. The tail is more red dorsally with some white ventrally; ears are slightly tufted. T. h. mogollonensis is a uniform reddish-brown dorsally without a central band, white venter, less distinct lateral stripe; legs more reddish-brown than yellow, white beneath; tail reddish toward base; ears not tufted ( Lindsay 1981). T. mearnsi is also on average heavier than T. hudsonicus (male = 194.0 g, female = 213.0 g) and T. douglasii (male = 206.9 g, female = 199.3 g —Ramos-Lara 2012; Thorington et al. 2012).

GENERAL CHARACTERS

Tamiasciurus mearnsi is a diurnal tree squirrel of small size ( Fig. 1 ). Pelage is soft and dense with thick long underfur beneath lengthy guard hairs. The body is dorsally pale gray-brown with reddish-yellow middorsal band; venter is white with some scattered yellow spots, hairs are gray at base; a distinct black lateral stripe separates the dorsal and ventral pelage. The forelegs are reddish-yellow to peach above and whitish beneath; hind legs are gray-brown above, and yellowish on sides and underside. The tail is black, short, and bushy with slight outer white fringe and dorsal yellow fringe toward base. Top of head is light gray. Ears are blackish and yellowish toward base, with slight tuft. Nose and mouth area are light yellowish with gray-brown above nose. Ring around the eye is whitish ( Townsend 1897; Nelson 1899; Lindsay 1981; Thorington et al. 2012).

Mean measurements (± SD mm) for 32 adults ( Lindsay 1981) were: head and body length, 201.00 ± 13.22; hind foot length, 51.39 ± 5.15; greatest length of skull, 49.17± 0.84; greatest height of skull, 17.23 ± 0.33; zygomatic width, 27.65 ±0.56; nasal length, 13.18 ± 0.34; interorbital breadth, 15.00 ± 0.49; braincase breadth, 20.49 ± 0.30; length of maxillary toothrow, 8.17± 0.17; temporal fossa diagonal, 16.97 ± 0.33; diastema length, 12.91 ± 0.37; 2nd molar width, 2.59 ± 0.06; premolar width, 2.11± 0.06; breadth at 3rd molars, 6.27± 0.21; pterygoid width, 4.02± 0.15; foramen magnum height, 6.31 ± 0.15; foramen magnum width, 7.30 ± 0.18; nasal width, 7.86± 0.30; and infraorbital foramen breadth, 6.93 ±0.42 ( Fig. 2 ). Mean body mass (± SD g; range) for 20 males and 18 females, respectively, were: 271.6 (± 18.1; 220–300) and 271.7 (± 31.1; 220–360), with females weighing slightly more than males (Ramos-Lara 2012). However, Lindsay (1981) did not find sexual dimorphism in cranial and skin characters.

DISTRIBUTION

Tamiasciurus mearnsi is endemic to the Sierra de San Pedro Mártir in Baja California, Mexico ( Lindsay 1981; Mellink-Bijtel 1991; Fig. 3 ). T. mearnsi is separated from the nearest population of its congener T. douglasii by about 560 km of mostly nonforested lowlands ( Taylor 1999; Yensen and Valdés-Alarcón 1999). Anecdotal accounts of its presence in the Sierra de la Laguna, to the south of Sierra de San Pedro Mártir, in Baja California Sur, Mexico ( Leopold 1959) and northward into the Cuyamaca and Laguna Mountains of southern California, United States ( Mearns 1907), have never been substantiated; it is believed endemic to a single montane island (Yensen and Valdés-Alarcón 1999; Thorington et al. 2012). Within the Sierra de San Pedro Mártir, T. mearnsi is known from only 3 sites <10 km apart: La Grulla (30°53 ′ N, 115°28 ′ W —Minnich and Vizcaino 1998), Vallecitos (31°00 ′ N, 115°31 ′ W —Minnich and Vizcaino 1998), and 6.3 km west of Vallecitos by road (Yensen and Valdés-Alarcón 1999). To the east of the Sierra de San Pedro Mártir, the San Felipe desert provides a formidable barrier between T. mearnsi and T. hudsonicus . To the north and west of the Sierra de San Pedro Mártir, Californian chaparral is a barrier between T. mearnsi and T. douglasii in the Sierra Nevada (Yensen and Valdés-Alarcón 1999).

FOSSIL RECORD

No occurrence of Tamiasciurus mearnsi in the fossil record is known. Lindsay (1981) and Arbogast et al. (2001) suggest that much of the geographic variation in Tamiasciurus , including that of T. mearnsi , likely resulted from periodic isolation during the Pleistocene. Ancestors of T. mearnsi could have invaded the Peninsula of Baja California during 1 or more mesic cycles, perhaps as early as 700,000 years ago (Hafner and Riddle 1997). Yensen and Valdés-Alarcón (1999) believed that Mearns’s squirrel was present in the Sierra de Juárez Mountains of Baja California during the Wisconsin glaciation of more northerly portions of North America.

FORM AND FUNCTION

One of 4 individuals collected in April and May had initiated molt into summer pelage ( Allen 1893; Townsend 1897; Mearns 1907). During the summer rains, Tamiasciurus mearnsi may consume water from temporary creeks (RamosLara 2012). Communal nesting occurs among adult individuals during the summer months; however, the physiological significance is still unknown (Ramos-Lara and Koprowski 2012).

ONTOGENY AND REPRODUCTION

Tamiasciurus mearnsi may reach sexual maturity between 10 and 12 months of age when ≥ 240 g (Ramos-Lara 2012). An estrous female with an enlarged pink vulva was reported in May during a mating chase involving 2– 3 males ( Koprowski et al. 2006). Males may possess scrotal testes during 5–7 months, with mating season starting in February or March and ending in July or August. Pregnant females have been recorded in April and May. Litter size ranges from 2 to 5 young/litter (Ramos-Lara 2012; Fig. 4 ). Female T. mearnsi may produce 1 or 2 litters/year with weather and food supply apparently influencing reproduction (Ramos-Lara 2012).

Figures

Fig. 1.—An adult female Tamiasciurus mearnsi from the Sierra de San Pedro Mártir, Baja California, Mexico. Photograph taken at Vallecitos on 10 August 2005 by N. Ramos-Lara.

Fig. 2.—Dorsal, ventral, and lateral views of skull and lateral view of mandible of an adult female Tamiasciurus mearnsi (The Field Museum of Natural History [FMNH] #10576) from Vallecitos, Sierra de San Pedro Mártir, Baja California, Mexico. Photographs taken by B. Patterson used with permission. Greatest length of skull is 51.1mm.

Fig. 3.—Geographic distribution of Tamiasciurus mearnsi. Created by M. J. Merrick.

Fig. 4.—A litter of 5 Tamiasciurus mearnsi placed on the ground by their mother during a move to a new cavity nest in the Sierra de San Pedro Mártir, Baja California, Mexico. Photograph taken at Vallecitos on 25 May 2006 by N. Ramos-Lara.