Speciation and Evolution of the Pygmy Mice, Genus Baiomys.

by Robert L. Packard.

INTRODUCTION

Pygmy mice (_Genus Baiomys_) are the smallest cricetine rodents in North America. They occur from Nicaragua in Central America into the southwestern United States. The princ.i.p.al part of the geographic range of the pygmy mice lies in the Republic of Mexico. They are notably common in central Mexico, but are only locally common to the north and to the south, and then only in certain seasons.

Pygmy mice were first brought to the attention of biologists in 1887 when Oldfield Thomas described a diminutive species of cricetine rodent, _Hesperomys_ (_Vesperimus_) _taylori_. The description was based on a specimen obtained by William Taylor from San Diego, Duval County, Texas.

C. Hart Merriam (1892:70) described _Sitomys musculus_ on the basis of specimens from Colima [City of], Colima, Mexico. Merriam (_loc. cit._) mentioned that the two kinds of mice, _Hesperomys taylori_ and _Sitomys musculus_, "in general appearance look almost precisely like the common house mouse (_Mus musculus_) but are still smaller and have shorter tails." He placed the two species in the genus _Sitomys_. Frederick W.

True in 1894 regarded them as composing a distinct subgenus of _Sitomys, Baiomys_. According to True (1894:758), _S. taylori_ and _S. musculus_ possessed a different combination of characters (ascending ramus of mandible short and erect, condyle terminal, coronoid process well-developed, uncinate, and near the condyle, size small, tail short, plantar tubercles six, soles hairy) than either _Vesperimus_, or _Onychomys_ (which had been considered as a subgenus of _Hesperomys_ until 1889). In 1907, E. A. Mearns accorded _Baiomys_ generic rank.

Osgood (1909:252) treated _Baiomys_ us a subgenus of _Peromyscus_, whereas, Miller, in 1912, regarded _Baiomys_ as a distinct genus. Most recent students of North American mammals have followed Miller, but usually with reservations. Ellerman (1941:402) emphasized that the taxonomic position of the genus was uncertain, and wrote that _Baiomys_ "... seems to be considerably distinct from _Peromyscus_, and may perhaps be a northern representative of _Hesperomys_ or one of the small South American genera."

Only two comprehensive a.n.a.lyses of geographic variation and interspecific taxonomic relationships have been made; the first was by Osgood (1909) who had fewer than a fourth of the specimens of _Baiomys_ available to me; the second was by Hooper (1952a:90-97) who contributed importantly to understanding the relationships of the two living species in central Mexico. No attempts heretofore have been made to correlate and understand the relationships of the five fossil species to one another and to the living species a.s.signed to the genus.

Six objectives of the following report are to: (1) list characters taxonomically useful in recognizing species and subspecies; (2) record amount of variation within and between populations; (3) correlate observed variations with known biological principles; (4) show geographic ranges of the two living species; (5) indicate relationships between fossil and living species of the genus; and (6) clarify the systematic position of the genus.

MATERIALS, METHODS AND ACKNOWLEDGMENTS

This report is based on the study of approximately 3,520 museum study skins, skulls, complete skeletons, and entire animals preserved in liquid. Most specimens examined were accompanied by an attached label bearing data on locality and date of capture, name of collector, external measurements, and s.e.x. In addition, 49 fossil specimens referable to _Baiomys_ were studied. Nearly two-thirds of the specimens were a.s.sembled at the University of Kansas Museum of Natural History; the remainder were examined in other inst.i.tutions.

Specimens studied were grouped by geographic origin, s.e.x, age, and season of capture. Individual variation was then measured in several of the larger samples of each living species and in measurable fossil material. External measurements used were those recorded by the collectors on the labels attached to the skins. Twenty cranial measurements employed in the past in the study of _Baiomys_ and closely related cricetine rodents were statistically a.n.a.lyzed. The coefficient of variation was calculated for each of the 20 measurements in order to determine which varied least. In general, measurements having the least coefficient of variation were used in comparing samples from different geographic areas. Figure 1 shows the points between which measurements were taken.

_Occipitonasal length._--From anteriormost projection of nasal bones to posteriormost projection of supraoccipital bone. _A_ to _A'_

_Zygomatic breadth._--Greatest distance across zygomatic arches of cranium at right angles to long axis of skull. _B_ to _B'_

_Postpalatal length._--From posterior margin of hard palate to anterior margin of foramen magnum. _C_ to _C'_

_Least interorbital breadth._--Least distance across top of skull between orbits. _D_ to _D'_

_Length of incisive foramina._--From anteriormost point to posteriormost point of incisive foramina. _E_ to _E'_

_Length of rostrum._--The distance in a straight line from the notch that lies lateral to the lacrimal to the tip of the nasal on the same side. _F_ to _F'_

_Breadth of braincase._--Greatest distance across braincase, taken at right angles to long axis of skull. _G_ to _G'_

_Depth of cranium._--The distance from the dorsalmost part of the braincase to a flat plane touching tips of incisors and ventral border of each auditory bulla. A gla.s.s slide one millimeter thick was placed on the ventral side of the skull. One jaw of the caliper was on the lower surface of the slide and the other jaw on the dorsalmost part of the braincase. The depth of the slide was subtracted from the total reading.

_H_ to _H'_

_Alveolar length of maxillary tooth-row._--From anterior border of alveolus of M1 to posterior alveolus of M3. _I_ to _I'_

[Ill.u.s.tration: FIG. 1. Three views of the skull to show points between which measurements were taken. Based on _B. m. pullus_, adult, female, No. 71611 KU, 8 mi. S Condega, Esteli, Nicaragua.

1-1/3.]

Capitalized color-terms refer to Ridgway (1912). Color terms without initial letters capitalized do not refer to any one standard.

The names of the cusps and ridges of the teeth (see Figure 2) are those suggested by Wood and Wilson (1936:389-390). Terminology of the enamel grooves and folds is that of Hershkovitz (1944:17) and Hooper (1952b:20-21).

Because secondary s.e.xual variation was not significant (see page 597), both males and females of like age and pelage were used in comparisons of samples designed to reveal geographic variation.

The species are arranged from less to more progressive; the subspecies are arranged alphabetically.

In the synonymy of each subspecies, the plan has been to cite: (1) the name first proposed; (2) the first usage of the name combination employed by me; (3) all other name combinations in chronological order that have been applied to the subspecies concerned.

The localities of specimens examined are listed by country from north to south. Within a country, the listing is by state, beginning with the northwesternmost state and proceeding by tiers (west to east) to the southeasternmost state. Within a state of the United States, the listing is by counties in the same geographic order as described for states.

Within any county in the United States, within any state in Mexico, and within any country in Central America, the listing of localities is from north to south. When more than one locality is on the same line of lat.i.tude, the westernmost locality is listed first. Marginal localities for each subspecies are listed in a paragraph at the end of each account. Each marginal locality is mapped by means of a circle. The circles are listed in clockwise order, beginning with the northernmost.

When more than one of these localities lies on the same line of lat.i.tude, the westernmost is cited first. Localities not represented on the distribution maps, so as to avoid undue crowding of symbols, are italicized in the lists of specimens examined.

[Ill.u.s.tration: FIG. 2. Occlusal views of molars. 13.

A. _B. taylori a.n.a.logous_, subadult, female, No. 28102 KU, 4 km.

ENE Tlalma.n.a.lco, 2290 meters, Estado de Mexico. Right, upper molars.

B. _B. musculus musculus_, subadult, male, No. 45456 USNM, Colima, Colima, Mexico. Left, upper molars.

A'. _B. taylori a.n.a.logous_, subadult, female, No. 28102 KU 4 km.

ENE Tlalma.n.a.lco, 2290 meters, Estado de Mexico. Left, lower molars.

B'. _B. musculus musculus_, subadult, male, No. 45456 USNM, Colima, Colima, Mexico. Right, lower molars.]

The largest single collection of pygmy mice is in the University of Kansas Museum of Natural History, and, unless otherwise indicated, specimens cited in the taxonomic accounts beyond are there.

I am indebted to the following named inst.i.tutions and persons for making specimens available for study:

American Museum of Natural History, G. G. Goodwin and R. G. VanGelder.

Carnegie Museum, J. K. Doutt.

California Academy of Sciences, Robert T. Orr.

Chicago Natural History Museum, Phillip H. Hershkovitz.

Cleveland Museum of Natural History (Collection now a part of Museum of Zoology, University of Michigan, W. H. Burt, E. T. Hooper).

Louisiana State University, Museum of Natural History, George H. Lowery, Jr.

Los Angeles County Museum, Charles A. McLaughlin.

United States National Museum (Biological Survey Collections), David A.

Johnson, and Viola S. Schantz.

United States National Museum, Division of Vertebrate Paleontology, C. Lewis Gazin.