Robert L. Packard

“ Morphological characters of the parental stock of the two living species, musculus and taylori, may have been intermediate between those of B. minimus and those of B. musculus. The principal part of the range of Baiomys today is in México, and probably was there through much of Pleistocene time. Extension northward of the species and retreat southward of those northern populations of pygmy mice would not only have left isolated populations in the north, but would have allowed the mice that retreated south to share a common gene pool. ”

“ My studies reveal a grayish-brown population in central Honduras near to and including the type locality. This population appears to grade into a slightly paler, particularly as concerns color of hind foot and tail, group of Guatemalan mice from 1 mi. S Rabinal, from 1/2 mi. N, 1 mi. E Salama, and from Lake Atescatempa. Specimens from western Guatemala at Nentón and Jacaltenango, on the other hand, are darker brownish-black, more nearly like the paratypical series of nigrescens from the Valley of Comitán, Chiapas, Republic of México. ”

“ Specimens from the central and western parts of Chiapas (see Figure 10 for localities) are difficult to assign to either pallidus or nigrescens. Equal justification exists for assignment to either subspecies. I have assigned the specimens to nigrescens because they are geographically closer to the type locality of nigrescens. Specimens from Reforma, Oaxaca (assigned by Hooper, 1952a:93-94, to nigrescens) , are nearly identical in size and color to paratypes of pallidus. I assign the Reforma specimens to pallidus. ”

“ If, after the postjuvenal molt, a distinct adult pelage is acquired it is difficult to separate it from the annual replacement of pelage in adults at the beginning of the rainy season. Adults of both species have been found in molt in all months of the year. To the north, in Texas, the pelage of winter-taken specimens is denser and slightly more reddish than that of specimens taken in spring and summer. In the two last mentioned seasons, the pelage is more uniformly gray. To the south, in México, the pelage is heavy and long in most specimens taken in the rainy season. ”

“ Although all bones of the skull grow in the subadult and early adult stages (see table 1) , the above-named bones grow faster than others and thus cause the general flattening of the skull, typical of adults (similar to that reported by Hoffmeister, 1951:7) . The body continues to lengthen, accounting for the increase in total length of the adult (see table 1) . Hind foot, tail and ear, reach their maximum lengths by subadult stage. Adult pelage has been acquired, and the color is brighter than in either subadults or old adults. ”

“ Lukens (1955:159) , in a study of the mammals of Guerrero, has shown that the characters attributed to B. m. pallidus are not significantly different from those of pygmy mice studied from Guerrero. He (loc. cit.) concluded that: (1) if the specimens of pygmy mice from central Guerrero were typical of the subspecies musculus, then pallidus did not deserve subspecific recognition, or; (2) the name B. m. musculus should be restricted to the larger pygmy mice inhabiting the lowlands immediately adjacent to the Pacific Coast and the area to the north. ”

“ When paratypes of ater and specimens of B. t. paulus are compared, the darkest individuals of ater exceed but slightly the darkest of paulus. The darkest specimens of paulus occur in southern Zacatecas, and northern Jalisco, and the palest of the series are in northern Durango and southern Chihuahua. When paratypes of ater and paulus are compared, the difference in color is readily distinguishable. Specimens from 11/2 mi. N San Francisco, in northern Chihuahua, appear to be intermediate in color between ater and paulus except for a faint tinge of buff ventrally. ”

“ Such similarities of crania between Calomys and Baiomys may reflect convergence, but the total of internal and external morphological characters shared, I think reflects true relationships.
Peromyscus has a large number of living and extinct species and exhibits a wide range of morphological variation, whereas Baiomys has a small number (7) of species and exhibits a narrow range of morphological variation. The small number of known species of pygmy mice suggests their conservatism in elaboration of morphological characters. ”

“ The southern pygmy mouse seems to be locally abundant in certain parts of its geographic range, and in other parts, scarce. For example, Dalquest (in. litt.) recorded the pygmy mouse as common at a place 2 km. N Paraje Nuevo, 1700 feet, Veracruz, where, by means of 50 traps, he took 14 of these mice in one night. The species was scarcer, although the habitat seemed suitable, 3 km. N Presidio, 1500 feet, Veracruz, where he caught only two pygmy mice in several days of trapping. Six miles southwest of Izucár de Matemores, the pygmy mouse was the most common rodent. ”

“ From his knowledge of the habitats of B. musculus, the larger species, and B. taylori, the smaller species, Hibbard (1952:203) suggests that B. kolbi, a large species, might have inhabited lowlands, and B. rexroadi, a small species, highlands. I have no evidence to dispute this suggestion except that B. musculus has more prominent cingular ridges (or at least vestiges of this lophid condition) than either B. kolbi or B. rexroadi. B. musculus (see page 610) is less of an open grassland inhabitant than is B. taylori. ”

“ Pygmy mice probably originated in that time, I judge in México, and moved northward and southward in [Pg 656] a grassland-savannah habitat that seemingly existed as far north as what is now Meade County, Kansas (where the Sawrock fauna lived) . Further evidence for occupancy of a grassland-savannah habitat by ancestral pygmy mice stems from the distribution of the living species, B. taylori, that at present occupies territory adjacent to parts of the Sonoran and Chihuahuan deserts. B. taylori seems to be morphologically more specialized for life in an arid grassland than was B. sawrockensis. ”

“ The general plan of structure of the auditory ossicles in Baiomys resembles that in Calomys, Akodon, and Thaptomys. The ossicles of Calomys and Thaptomys, in particular, closely resemble the auditory ossicles of Baiomys musculus. The short process of the incus is knoblike in Calomys and Thaptomys, and the general conformation of malleus and stapes in those two genera is nearly identical to that in B. musculus. ”

“ In the area where the two species of pygmy mice occur together, there seems to be a disparity in numbers between them. Hooper (1952a:91) has recorded the collection of both B. musculus and B. taylori in a single trap line. A series of pygmy mice collected from San Gabriel, Jalisco, contained one taylori and 33 musculus; another sample from La Resolana, Jalisco, had a ratio of 25 taylori to 6 musculus. The disparity in numbers where the two species occur together has been further substantiated by collections of the University of Kansas. ”

“ There is only slight individual variation, slight variation with age, and no secondary sexual variation. In Baiomys taylori the orbicular apophysis of the malleus (see Figure 8, A) is rounded to nearly ovoid; the anterior process is pointed, and the neck is short, being slightly recurved. The body of the incus is round and the short process is elongate. The sides of the long limb of the incus are nearly parallel. The lenticular process is relatively large. The posterior and anterior crus of the stapes are bowed, and the muscular process is either absent or much reduced. ”

“ The phyletic trends in the genus Baiomys have been from an ancestral stock that possessed relatively brachydont teeth having raised cingular ridges and orthodont to proödont incisors, to species having hypsodont teeth with reduced cingular ridges and retrodont incisors.
3. Reduction of cingular ridges in pygmy mice is associated with an existence in open grassland (more xeric than mesic) , whereas, the presence of cingular ridges is associated with an existence in a savannah habitat (more mesic than xeric) . ”

“ On the basis of internal morphological characters studied (auditory ossicles, hyoid apparatus, and baculum) , Baiomys seems to be more closely related to a South American hesperomine, perhaps Calomys, than to any North American cricetine.
10. Pygmy mice were more widely distributed in the past than they are at present. Part of the ancestral stock of the pygmy mice may have emigrated from North America into South America in a brief period in the Pliocene; if so, it is easy to understand why certain South American hesperomines resemble Baiomys. ”

“ Study of larger series of specimens than were available to Burt reveals that populations of pygmy mice inhabiting the northwest coastal plains of México are indeed distinct.
The darkest of the material assigned to canutus is from Nayarit (for specific localities see specimens examined) . According to Tamayo (1949:Carta de Suelos) , color of soil changes from chestnut in northern Sinaloa to black in southern Sinaloa and northern Nayarit. There seems, therefore, to be a close correlation between color of pelage and color of soil in this area. ”

“ The total length of the baculum, considered together with its shape, serves to identify to species all specimens examined by me.
The bacula of both species of Baiomys were compared with bacula of Akodon, Scotinomys, Holochilus, Oryzomys, Zygodontomys, Reithrodontomys, Thaptomys, and Calomys and illustrations of bacula by Blair (1942:197, 200) of Peromyscus (subgenera Peromyscus, Haplomylomys, Podomys) , Ochrotomys, and material at the University of Kansas Museum of Natural History of Megadontomys. Shape of baculum most resembled that of Ochrotomys and Calomys. ”

“ There the Southern Pygmy Mouse is uniformly larger, regardless of altitude.
Osgood (1909:257, 259) suggested that degree of relative humidity might in some way control color of pelage in both B. taylori and B. musculus. In B. musculus, the darker subspecies, B. m. brunneus, B. m. nigrescens, and B. m. pullus, occur in zones of rather constant high relative humidity, whereas the paler subspecies infernatis, musculus, handleyi, and to a less extent grisescens and pallidus, occur in zones of lower relative humidity. ”

“ Increase in size from south to north is in keeping with Bergman's Rule that within a species, smaller individuals occur in warmer parts of its geographic range. Southern pygmy mice at high altitudes average larger than those from low elevations, except where the two species are sympatric. ”

“ At birth, juveniles are pink, without pelage except for the mystacial vibrissae and a few hairs about the eye. Blair (op. cit.:381) recorded changes with age in color of the skin of new-born and suckling pygmy mice. Data obtained by me from three litters born in captivity agree with his findings. Pygmy mice are weaned when 17 to 24 days old. At that time, the mice possess a fine, but not dense, dusky-gray fur. ”

“ PALEONTOLOGY OF THE GENUS
Five fossil species, all extinct, have been assigned to the genus and range in time from early late Pliocene (Saw Rock Canyon fauna of Hibbard, 1953:408) to Mid-Pleistocene (see Hibbard, [Pg 588] 1958:25, who assigns the Curtis Ranch fauna to late Kansan or early Yarmouth) . ”

“ Specimens from 4 km. NE Ciudad Valles are nearly intermediate in color of the belly, dorsum, forefeet and hind [Pg 647] feet, and tail, between the palest mice from the coastal plain and the darker mice in the mountains of the northeastern part of the State (specimens from El Salto average paler, however, than the type and paratypes) . These specimens seem to be intergrades between B. t. taylori to the east on the coastal plain and fuliginatus to the northwest in the mountains. It seems best to refer the mice from 4 km. ”

“ Cranially, B. t. paulus differs from B. t. analogous in: skull slightly smaller in all dimensions; maxillary part of zygoma narrowing and forming oblique angle rather than a near right angle with rostrum; anterior incisive foramina constricted posteriorly; tips of nasals truncate (less rounded) .
Remarks.—J. A. Allen (1903:599) correctly pointed out that young specimens, in first pelage, were gray brown; young adults were darker and more varied with some blackish; adults and old adults were buffy to grayish. ”

“ Skull.—Difference in occipitonasal length and zygomatic breadth, both having low coefficients of variation, are useful in separating species, especially where they are sympatric. Shape of presphenoid, nasals, interparietal, frontoparietal sutures, and length and degree of the openings of the incisive foramina are useful in delimiting subspecies. The rostrum of B. taylori, in front of the frontonasal suture, is deflected three to five degrees ventrally in 85 per cent of [Pg 601] the adults examined, and in B. musculus is less, or not at all, deflected. ”

“ The juvenal pelage is uniformly dusky gray throughout except for the paler gray on the venter. In most juvenal mice, the [Pg 599] yellow to ochraceous pigments of the subterminal bands are reduced or absent. Unlike Peromyscus, Baiomys has bright brownish hairs on the head as the first evidence of the postjuvenal molt (see Figure 4, part a) . Blair (1941:381) reports adult pelage in pygmy mice being evident first at an age of 46 days. Two of my juveniles born in captivity began the postjuvenal molt on the 38th and 40th days. The area of new hairs on the head spreads most rapidly posteriorly. ”

“ Specimens of both species were segregated into five categories: Juveniles, young, subadults, adults, and old adults. Juvenal and young pygmy mice are readily separable from the other three categories; subadults are less easily distinguished from adults. In order to obtain an accurate understanding of geographic variation in these mice, only adults should be used in making taxonomic comparisons.
Juveniles.—Nestling mice yet unweaned; sutures in cranium incompletely closed; bony parts of skull fragile; M3 and m3 not erupted or only partly erupted and not protruding above margins of alveoli. ”

“ Occlusal pattern is useful in estimating the relationship of fossil and living species. Degree of development of the mesostyle, mesostylid, mesoloph, and mesolophid have been useful in determining relationship between fossil and living species as well as useful in separating the living species. Rinker (1954:119) and Hooper (1957:48) have shown the degree of variation in dental patterns in Peromyscus, Sigmodon, and Oryzomys, mice thought to be closely related to Baiomys. In pygmy mice, however, the dental patterns are relatively constant. ”