Perognathus merriami, J. A. Allen, 1892

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 : 205-206

21.

Merriam’s Pocket Mouse

Perognathus merriami

French: Souris-a-abajoues de Merriam / German: Merriam-Seidentaschenmaus / Spanish: Ratén de abazones de Merriam

Taxonomy. Perognathus merriami J. A. Allen, 1892 ,

Brownsville, Cameron Co., Texas, USA.

Based on detailed sequencing of nDNA and mtDNA genes, P. merriami is a member of the flavus species group of silky pocket mice along with P. flavus , from which it is distinguished with great difficulty. In Chihuahua, where the two are widely sympatric, P. flavus has a slightly more lax pelage, of pinkish (rather than orange) hue, and tends to have a mid-dorsal dark stripe. The post-auricular spots are larger and more conspicuous in P.flavus , which also has a shorter tail and larger ears. Discriminant functions analysis of multiple cranial characters was able to correctly assign 88% of specimens from New Mexico and Texas to species. Elsewhere, it has been said that the two can be distinguished only by biochemical analysis. Until recently the two were considered conspecific based on morphological and behavioral similarity. Electrophoretic analysis of allozymes among populations of both species in Texas and New Mexico revealed distinct complements in several zones of sympatry, with only a few probable hybrids from one locality in south-eastern New Mexico. In their 2010 analysis of nDNA and mtDNA sequences of samples from throughout the range of both species, S. A. Neiswenter and B. R. Riddle supported distinction of the two species, expanded the known area of distributional overlap, extended the distribution of P. merriami farther north in the Great Plains, and revealed deeper phylogenetic structure in both the flavus species group and within P. merriami . Four major lineages were recovered that diverged in the late Miocene, 6-10 million years ago: two widespread and overlapping species ( P. flavus and P. merriami ), and two relictual, geographically restricted forms of equally ancient age in the central and southern Chihuahuan Desert. It is likely that future studies will demonstrate that these two geographically restricted but relatively ancient lineages are distinct but morphologically cryptic species. One of these lineages (the “Southern Chihuahuan Desert” lineage) is somewhat more widespread across southern Coahuila, edging into eastern Chihuahua and eastern Durango, in the general vicinity of the Desierto de Mayran. Most of the specimens from other localities in the vicinity have been assigned to P. flavus pallescens, but northern and eastern populations are close to or within the range of P. merriami . Within P. merriami (other than this cryptic lineage), there are three basal clades that diverged an estimated 3-9—4-8 million years ago, in the Pliocene: populations in the Tamaulipan Mezquital ecoregion, south of the Balcones Escarpment along the Edwards Plateau of Texas and into the Tamaulipan Plain of Tamaulipas and Nuevo Leon; the Central and Southern Mixed Grasslands ecoregions of the central Great Plains; and the widespread Chihuahuan Desert lineage. This latter lineage is further divided into eastern and western lineages. Two subspecies recognized.

Subspecies and Distribution.

P. m. merriamiJ. A. Allen, 1892 — SWUSAandNEMexico (NCTexasStoECoahuila, CNuevoLeon, andCTamaulipas).

P.m. gilvus Osgood, 1900 — SW USA and N Mexico (SE New Mexico, W Texas, E Chihuahua, and NW Coahuila).

Descriptive notes. Head—body 53-60 mm, tail 42-61 mm, ear mean 6 mm, hindfoot mean 17 mm; weight 5-9 g. There is no significant secondary sexual dimorphism. Pelage of Merriam’s Pocket Mouse is soft and fine, with no hint of spines or stiff bristles; posterior one-half of sole of hindfoot has a sparse covering of short hairs; and tail is short and not tufted. Merriam’s Pocket Mouse has short, rounded ears without lobed antitragus. It is small-sized for the genus. Dorsal pelageis yellowish or yellowish-orange and finely lined with black hairs. It has a small buffy post-auricular patch, indistinctlateral line, and white or creamy under parts. Tail is shorter than head-body length and moderately bicolored. Chromosomal complement has 2n = 50 and FN = 86.

Habitat. Semiarid to arid grasslands on sandy and rocky soils in a variety of plant communitiesin its broad distribution that extends from the Tamaulipan Gulf coastal plains to the eastern and northern Chihuahuan Desert and up into the central Great Plains. Merriam’s Pocket Mouse is common in short-grass prairie, desert scrub, and open,arid brushland, where there is short, sparse ground cover. This includes grazed and overgrazed pastures and grasslands. Merriam’s Pocket Mouse may use abandoned burrows of pocket gophers ( Geomyidae ) and maintains multiple burrows within the home range. Males have 6-7 burrows, and females have c.5 burrows. Home burrow is more complex, with multiple openings, and females build more elaborate nest chambers. Refuge burrows of Merriam’s Pocket Mice are short and simple, sometimes with seed caches that are stored along the side of a widened tunnel rather than in large chambers. Most burrows are located at bases of clumps of grass or other vegetation, where root systems add stability to burrow entrances. During the day, the small (2-3 cm) entrance to the occupied burrow is plugged from within; unoccupied burrows usually are plugged (from the outside) level with the ground surface.

Food and Feeding. Diet of Merriam’s Pocket Mouse consists largely of seeds of shrubs, annuals, and grasses, but it also includes smaller amounts of green vegetation and insects. It prefers smaller seeds, but burrows in Texas contained juniper seeds or hulls from which the seeds had been eaten out through a small hole in one end, and old moldy corn. Food items are collected in external, fur-lined cheek pouches and transported back to burrows, where they are stored in burrow caches. Like all small silky pocket mice, Merriam’s Pocket Mouse tends to husk seeds or separate seeds from seed-heads before stuffing them in its cheek pouches. 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 of Merriam’s Pocket Mouse appears to occur in April-November. Breeding begins soon after emerging from winter periods of torpor in burrows. Reproduction typically occurs in spring and summer, and females usually have two litters of 3-6 young each year. Population turnoveris high (75-84%), and life span is 22-33 months. Captive individuals have lived for up to four years.

Activity patterns. Merriam’s Pocket Mouse is nocturnal and terrestrial, and it likely enters torpor for long periods in winter, particularly in the northern parts of its distribution. Activity is reduced in winter, and individuals may not appear aboveground on particularly wet or cold nights. When inactive aboveground, they awake periodically from torpor to feed on seed caches.

Movements, Home range and Social organization. Estimates of home range sizes of Merriam’s Pocket Mouse are 0-18-2-2 ha. One female traveled a maximum distance of 57 m in one night. Males move greater distances than females, and subadult males travel farther than other sex and age categories. Subadults of both sexes travel greater average distances between capture sites than do adults, and they may more readily disperse into marginal areas. Adults are solitary, and there is no evidence of social grouping. Density estimates of 2:7-10-2 ind/ha vary considerably with season, being lowest in spring following high mortality (69-80%) in July-December. Densities are highest in summer and autumn with the addition ofjuveniles. When juveniles and transient mice are removed from these estimates, density is c.5 ind/ha.

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

Bibliography. Aquino & Neiswenter (2014), Best & Skupski (1994b), Engstrom (1999), Linzey, Timm, Alvarez-Castaneda, Castro-Arellano & Lacher (2008g), Neiswenter & Riddle (2010), Williams (1978a), Williams et al. (1993).

Figures

1. Salvin’s Spiny Pocket Mouse (Heteromys salvini), 2. Panamanian Spiny Pocket Mouse (Heteromys adspersus), 3. Mexican Spiny Pocket Mouse (Heteromys irroratus), 4. Jaliscan Spiny Pocket Mouse (Heteromys spectabilis), 5. Painted Spiny Pocket Mouse (Heteromys pictus), 6. Nelson's Spiny Pocket Mouse (Heteromys nelsoni), 7. Trinidad Spiny Pocket Mouse (Heteromys anomalus), 8. Overlook Spiny Pocket Mouse (Heteromys catopterius), 9. Paraguana Spiny Pocket Mouse (Heteromys oasicus), 10. Southern Spiny Pocket Mouse (Heteromys australis), 11. Ecuadorean Spiny Pocket Mouse (Heteromys teleus), 12. Gaumer’s Spiny Pocket Mouse (Heteromys gaumeri), 13. Desmarest’s Spiny Pocket Mouse (Heteromys desmarestianus), 14. Goldman’s Spiny Pocket Mouse (Heteromys goldmani), 15. Cloud-dwelling Spiny Pocket Mouse (Heteromys nubicolens), 16. Mountain Spiny Pocket Mouse (Heteromys oresterus)

Subspecies and Distribution. P. m. merriamiJ. A. Allen, 1892 — SWUSAandNEMexico (NCTexasStoECoahuila, CNuevoLeon, andCTamaulipas). P.m. gilvus Osgood, 1900 — SW USA and N Mexico (SE New Mexico, W Texas, E Chihuahua, and NW Coahuila).