Chaetodipus baileyi (Merriam, 1894)

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 : 211-212

29.

Bailey's Pocket Mouse

Chaetodipus baileyi

French: Souris-a-abajoues de Bailey / German: Bailey-Rauhaartaschenmaus / Spanish: Raton de abazones de Bailey

Taxonomy. Perognathus baileyi Merriam, 1894 ,

Magdalena, Sonora, Mexico.

Based on detailed sequencing of nDNA and mtDNA genes, C. baileyi is a close relative of C. rudinoris (formerly considered conspecific), and the two species make up one of the three “ancient” clades of coarse-haired pocket mice. Chaetodipus rudinoris has a chromosomal complement with a higher FN (66), lacks any supernumerary chromosomes, and is differentiated based on allozymes and mtDNA sequence. Two subspecies recognized.

Subspecies and Distribution.

C.b.baileysMerriam,1894—SWUSAandNWMexicoWofColoradoRiver(SArizona,SWNewMexico,Sonora,andN Sinaloa).

C. b. insularis Townsend, 1912 — NW Mexico (Tiburon I, Gulf of California , Sonora).

Descriptive notes. Head-body 85-99 mm, tail 103-124 mm, ear 7-11 mm, hindfoot 25-27 mm; weight 24-28 g. Male Bailey's Pocket Mice are slightly larger than females. It is large-sized for the genus; hindfoot is usually longer than 25 mm, with strongly crested and tufted tail that is longer than head-body length. Dorsal pelage is relatively smooth, lacks spines on rump, and is grayish, washed to a varying degree with yellow, and under parts are whitish. Basic chromosomal complement of Bailey’s Pocket Mouse has 2n = 46 and FN = 64, but there is extensive diploid number variation within and among populations due to supernumerary (or “B”) chromosomes and variation in FN due to fixed differences: populations from Tiburén Island (insularis) and adjacent mainland ( Bahia Kino , Sonora) have FN = 62. Known from only 18 other species of rodents, the B-chromosome system in Bailey's Pocket Mouse is unique among the family. Supernumerary chromosomes are usually (but not always) entirely heterochromatic, are by definition lacking in some (usually most) individuals in a population, and while not necessary for the life of the individual, do generally have some expression and so are not usually “silent.” In Bailey's Pocket Mouse, supernumerary chromosomes increase from zero to two in the west along the Colorado River from two to twelve in south-eastern Sonora, with significant differences among populations in mean and range of supernumeraries. In populations studied more intensively, males and females do not differ in mean number of supernumeraries, and number per individual is randomly distributed around the population mean; mean numbers remained stable over a ten-year period; and young individuals tended to have higher numbers of supernumeraries. This suggests that natural selection is acting to offset the meiotic tendency to increase number of supernumeraries and that they have some expression on the phenotype. Bailey’s Pocket Mouse is parapatric with its close relative, the Peninsular Pocket Mouse ( C. rudinoris ), on the opposite side of the Colorado River. Bailey’s Pocket Mouse on the east side of the Colorado River has darker pelage and wider rostrum than the Peninsular Pocket Mouse on the west side of the river. In its Sonoran Desert distribution, Bailey's Pocket Mouse co-occurs with seven other species of coarse-haired pocket mice. Bailey's Pocket Mouse is larger than all others except perhaps the Hispid Pocket Mouse (C. hispidus ), which has a shorter, non-crested, and non-tufted tail; browner and hispid pelage color; and a conspicuous buffy-to-ocherous lateral stripe. In addition to being larger than the othersix species with which it can be found, Bailey's Pocket Mouse favors more open areas of pebbly soils that mark transitions from sandyflats to rocky alluvial slopes or “bajadas,” apart from the sand-obligate smooth-haired species in the same vicinity (the Desert Pocket Mouse, C. penicillatus ; the Sinaloan Pocket Mouse, C. pernix ; and the Chihuahuan Desert Pocket Mouse, C. eremicus ) and apart from the saxicolous (rock-dwelling), coarse-haired species in the vicinity (the Rock Pocket Mouse, C. intermedius ; Goldman's Pocket Mouse, C. goldmani , and the Narrow-skulled Pocket Mouse, C. artus ).

Habitat. Areas of medium to large bushes and trees, often in ecotones between rocky hillsides and desert flats. Bailey's Pocket Mouse also occurs in low-desert flats, small sandy washes, and rocky hillsides. It occurs most frequently with the Desert Pocket Mouse, sometimes using areas with coarser, more gravelly soils. In some areas,it is associated with jojoba ( Simmondsia chinensis, Simmondsiaceae ), and combined distribution of Bailey's Pocket Mouse and its close relative, the Peninsular Pocket Mouse, closely matches distribution ofjojoba. Burrows are often found at the base ofjojoba bushes, and in one study, Bailey's Pocket Mouse was found on all study plots with jojoba, and the Desert Pocket Mouse on all plots without jojoba. Bailey's Pocket Mouse does occur in the absence ofjojoba (and vice versa) but reachesits highest densities in dense stands of jojoba. In long-term study plots in south-eastern Arizona, Bailey’s Pocket Mouse established new colonies following greater than average winter precipitation that increased shrub cover. In south-western New Mexico, Bailey’s Pocket Mouse might have expanded its distribution slightly to the north and east during the past 30 years.

Food and Feeding. Diet of Bailey's Pocket Mouse consists largely of seeds of shrubs, annuals, and grasses, but it also includes smaller amounts of green vegetation and insects. In laboratory experiments, it selected larger seeds than other heteromyids tested. Bailey’s Pocket Mouse apparently has some detoxification mechanism that allows it to eat seeds ofjojoba, which contain toxic cyanogenic glucosides and are rejected by the Desert Pocket Mouse, the Rock Pocket Mouse, and Merriam’s Kangaroo Rat ( Dipodomys merriami ) in laboratory tests. Other common foods of Bailey's Pocket Mouse are cholla ( Opuntia , Cactaceae ) and ocotillo ( Fouquieria , Fouquieriaceae ); grass seeds are less frequently taken. Food items are collected in external, fur-lined cheek pouches and transported back to burrows, where they are stored in burrow caches. Volume of cheek pouches (1-6 cm?) is sufficient to carry daily energy requirements of an individual in a single seed load. Bailey’s Pocket Mouse does not need to drink water, subsisting entirely on water from its food and water produced as a byproduct of metabolism.

Breeding. Male Bailey's Pocket Mice are reproductively active in all months exceptJanuary; females were not reproductively active until March or June. Reproductive peaks occur in spring following winter rains and early autumn following summer monsoons. Young-of-the-year breed in late summer. Litter size averages 3-5 young.

Activity patterns. Bailey’s Pocket Mouse is nocturnal and terrestrial, and it is active throughout the year, with peak activity in autumn and reduced winter activity after December.

Movements, Home range and Social organization. Density estimates of Bailey's Pocket Mouse are 2-2-86-1 ind/ha. At one site, they varied between 17-1 ind/ha and 52-4 ind/ha over several years.

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

Bibliography. Aquino & Neiswenter (2014), Ceballos & Oliva (2005), Geluso (2009), Linzey, Timm, Alvarez-Castaneda, Castro-Arellano & Lacher (2008h), Patton (1972), Patton & Rogers (1993), Patton et al. (1981), Paulson (1988a), Price (1999b), Riddle, Hafner & Alexander (2000), Williams et al. (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.b.baileysMerriam,1894—SWUSAandNWMexicoWofColoradoRiver(SArizona,SWNewMexico,Sonora,andN Sinaloa). C. b. insularis Townsend, 1912 — NW Mexico (Tiburon I, Gulf of California , Sonora).