Chaetodipus intermedius (Merriam, 1889)

Don E. Wilson, Thomas E. Lacher, Jr & Russell A. Mittermeier, 2016, Heteromyidae, Handbook of the Mammals of the World – Volume 6 Lagomorphs and Rodents I, Barcelona: Lynx Edicions, pp. 170-233 : 217-218

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Rock Pocket Mouse

Chaetodipus intermedius

French: Souris-a-abajoues des rochers / German: Felsen-Rauhaartaschenmaus / Spanish: Raton de abazones de roca

Other common names: Intermediate Pocket Mouse

Taxonomy. Perognathus intermedius Merriam, 1889 ,

Mud Spring, Mohave Co., Arizona, USA.

Based on detailed sequencing of nDNA and mtDNA genes, C. intermedius is a member of the continental lineage of the “modern” clade of coarse-haired pocket mice, along with three smooth-haired species ( C. penicillatus , C. eremicus , and C. pernix ) and three coarse-haired species ( C. nelsoni —including C. lineatus —, C. goldman , and C. artus ). Chaetodipus intermedius is basal to the three smooth-haired species, apart from the other three coarse-haired species. Five of the populations of C. intermedius that occur on lava fields in the south-western USA have been recognized as subspecies based on their dark or black pelage (subspecies ater, beardi, rupestris, pinacate, and nigrimontis). Several of these have already been synonymized under other subspecies due to a lack of anything other than pelage color differences. In the two populations studied genetically, it has been shown that different mutations are responsible for color (i.e. it has evolved independently); that gene flow is uninhibited between the melanistic population on lava field and surrounding, normal colored populations; and that predation (selection) apparently is maintaining color polymorphism. Based on these findings, the subspecies ater, beardi, and rupestris should be re-examined and possibly synonymized under the nominate subspecies intermedius . Eight subspecies recognized.

Subspecies and Distribution. C.i.intermediusMerriam,1889—SWUSAandNMexico(SonoranDesertofWArizonaacrossSArizonaandNSonora,ChihuahuanDesertofC&SNewMexico,WTexas,andNChihuahua).

C.i.aterDice,1929—SWUSA(restrictedtotheCarrizozolavafieldofSCNewMexico).

C.i.beardiWeckerly,Gennaro&Best,1988—SWUSA(restrictedtothePedroArm-endarizlavafieldofSNewMexico).

C.i.eninitusBenson,1934—SWUSA(SCUtahandNArizona).

C.i.lithophilusHuey,1937NWMexico(NWSonora).

C.i.minimusBurt,1932—NWMexico(Turner[=Datil]I,GulfofCalifornia,Sonora).

C.i.phasmaGoldman,1918—SWUSAandNWMexico(restricteddistributioninextremeSWArizonaandextremeNWSonora).

C. i. rupestris Benson, 1932 — SW USA (restricted to the Afton lava field of S New Mexico).

Descriptive notes. Head—body 73-76 mm,tail 84-112 mm, ear mean 8 mm, hindfoot mean 21 mm; weight 11-20 g. There is no significant secondary sexual dimorphism. The Rock Pocket Mouse is a coarse-haired pocket mouse and medium-sized for the genus, with crested and tufted tail that is longer than head-body length and bicolored. Pelage is relatively coarse, with numerous conspicuous black-tipped spines on rump, and normally drab grayish brown, with pale orange-brown lateral stripe and white under parts. Dorsal pelage often closely matches color of the rocky or gravelly substrate, being more reddish in reddish soils and black in lava beds. Chromosomal complement has 2n = 46 and FN = 58. The Rock Pocket Mouse has a nearly complementary distribution in the Chihuahuan Desert with another coarse-haired, saxicolous species, Nelson’s Pocket Mouse ( C. nelsoni ). The two are narrowly sympatric in western Texas, and both have been collected at two localities just east of the Rio Grande. It is likely that the two species, which are not sister-taxa, competitively exclude each other along a front of contact because their habitat requirements appear to be very similar. The Rock Pocket Mouse is smaller, has fewer rump spines than Nelson's Pocket Mouse, and has white soles on hindfeet. Throughoutits distribution in the Sonoran and Chihuahuan deserts, the Rock Pocket Mouse may be found in the vicinity of four other species in the genus. Bailey’s Pocket Mouse (C. bailey:) and the Hispid Pocket Mouse (C. hispidus ) are larger and lack the conspicuous rump spines. Bailey's Pocket Mouse favors more open areas of pebbly soils that mark transitions from sandy flats to rocky alluvial slopes or “bajadas,” and the Hispid Pocket Mouse prefers open grassland situations on sandier soils. The other two species are both smooth-haired pocket mice restricted to sandy soils: the Desert Pocket Mouse in the Sonoran Desert and the Chihuahuan Desert Pocket Mouse in the Chihuahuan Desert.

Habitat. Rocky and gravelly soils and rocky slopes in rocky gulches and canyons. Desert scrub vegetation in these areas is dominated by creosote bush ( Larrea , Zygophyllaceae ), saltbush ( Atriplex , Amaranthaceae ), mesquite ( Prosopis , Fabaceae ), and lechugilla ( Agave , Asparagaceae ). Burrows of Rock Pocket Mice are inconspicuous and are usually located near or under rocks. One study demonstrated that a sympatric smoothhaired species, the Chihuahuan Desert Pocket Mouse, actively excluded the Rock Pocket Mouse from burrow habitats in sandy soils that provide a microhabitat in which the sand-obligate species can survive and that the Rock Pocket Mouse has a broader tolerance for more extreme environments.

Food and Feeding. Diet of the Rock Pocket Mouse consists largely of seeds of shrubs, annuals, and grasses, but it also includes smaller amounts of green vegetation and insects. Food items are collected in external, fur-lined cheek pouches and transported back to burrows, where they are stored in burrow caches. It does not need to drink water, subsisting entirely on water from its food and water produced as a byproduct of metabolism.

Breeding. Breeding season in the Rock Pocket Mouse occurs in February—July, with a peak in June. Average litter size is four young (range 1-7), and gestation is ¢.30 days. Young females of the year are able to breed later in the same season.

Activity patterns. The Rock Pocket Mouse is nocturnal and terrestrial, and it is probably active throughout the year in the southern part ofits distribution, although it is torpid in November-December in its northern distribution. Individuals can remain active for as long as two hours in temperatures below freezing. It rarely strays from the shelter of bushes or rocks while foraging.

Movements, Home range and Social organization. Estimates of densities of the Rock Pocket Mouse are 10-58 ind/ha.

Status and Conservation. Classified as Least Concern on The IUCN Red List.

Bibliography. Aquino & Neiswenter (2014), Best (1993a), Ceballos & Oliva (2005), Hoffmeister (1986), Hoover et al. (1977), Linzey, Timm, Alvarez-Castaneda, Castro-Arellano & Lacher (2008n), Rogers (1999), Williams etal. (1993).

Figures

27. Hispid Pocket Mouse (Chaetodipus hispidus), 28. Long-tailed Pocket Mouse (Chacetodipus formosus), 29. Bailey's Pocket Mouse (Chaetodipus bailey), 30. Peninsular Pocket Mouse (Chaetodipus rudinoris), 31. Spiny Pocket Mouse (Chaetodipus spinatus), 32. California Pocket Mouse (Chaetodipus californicus), 33. San Diego Pocket Mouse (Chaetodipus fallax), 34. Little Desert Pocket Mouse (Chaetodipus arenarius), 35. Dalquest’s Pocket Mouse (Chaetodipus ammophilus), 36. Cerralvo Island Pocket Mouse (Chaetodipus siccus), 37. Desert Pocket Mouse (Chaetodipus penicillatus), 38. Chihuahuan Desert Pocket Mouse (Chaetodipus eremicus), 39. Sinaloan Pocket Mouse (Chaetodipus pernix), 40. Rock Pocket Mouse (Chaetodipus intermedius), 41. Nelson’s Pocket Mouse (Chaetodipus nelsoni), 42. Lined Pocket Mouse (Chaetodipus lineatus), 43. Goldman’s Pocket Mouse (Chaetodipus goldmani), 44. Narrow-skulled Pocket Mouse (Chaetodipus artus)

Subspecies and Distribution. C.i.intermediusMerriam,1889—SWUSAandNMexico(SonoranDesertofWArizonaacrossSArizonaandNSonora,ChihuahuanDesertofC&SNewMexico,WTexas,andNChihuahua). C.i.aterDice,1929—SWUSA(restrictedtotheCarrizozolavafieldofSCNewMexico). C.i.beardiWeckerly,Gennaro&Best,1988—SWUSA(restrictedtothePedroArm-endarizlavafieldofSNewMexico). C.i.eninitusBenson,1934—SWUSA(SCUtahandNArizona). C.i.lithophilusHuey,1937NWMexico(NWSonora). C.i.minimusBurt,1932—NWMexico(Turner[=Datil]I,GulfofCalifornia,Sonora). C.i.phasmaGoldman,1918—SWUSAandNWMexico(restricteddistributioninextremeSWArizonaandextremeNWSonora). C. i. rupestris Benson, 1932 — SW USA (restricted to the Afton lava field of S New Mexico).