NON-GEOGRAPHIC VARIATIONNon-geographic variation in pygmy mice (variation in a single population resulting from age, individual, seasonal, and secondary sexual differences) has been but little studied in the past. Mearns (1907:381) figured progressive stages of wear on the teeth ofB. taylori; Osgood (1909:252) and Blair (1941:380) referred to changes in dentition, weights, and pelages.The largest samples available for this study were 47B. taylorifrom the vicinity of Altamira (6 mi. N, 6 mi. W; 5 mi. N, 5 mi. W; 1 mi. S), Tamaulipas, and 44B. musculusfrom El Salvador (1 mi. S Los Planes, and 1 mi. NW San Salvador—two localities 3 miles apart).VARIATION WITH AGESpecimens 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.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.Young.—Weaned mice; cranium fragile; sutures between frontals and parietals, interparietal and parietals, basioccipital and basisphenoid, basisphenoid and presphenoid, premaxillaries and maxillaries widely open; M3 and m3 erupted beyond margins of their alveoli (molars erupt from anterior to posterior; M3 and m3, therefore, are last to erupt); in some specimens, molars slightly worn; pelage still dusky and relatively fine and sparse.Subadults.—Sutures between bones of skull less widely open than in young; epiphyses of long bones incompletely coalesced to shaft; relative to length of skull, braincase higher and rostrum shorter than in adults; all cusps worn, but dentine not occlusally confluent; primary first and second folds of third upper molars present; primary first fold and major fold of lower molars visible; pelage a subtle mixture of colors of young and adult, but resembling most that of adult; molts into postjuvenal pelage between 46 and 50 days.Adults.—Sutures of skull, and those between epiphyses and shaft of long bones obliterated except that, in some mice, sutures of skull persist between frontoparietal, and interparietal; cusps of molars so worn that dentine occlusally confluent; small island of enamel in third upper and lower molars of some specimens; relative to length of skull, cranium lower, rostrum longer, and interorbital region narrower than in subadult; cranium appears to be more flattened dorsoventrally; between subadult and adult stages, principal growth occurs in basioccipital, basisphenoid, frontals, and parietals; nasals grow less.Although all bones of the skull grow in the subadult and early adult stages (seetable 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 (seetable 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.Old Adults.—Characterized principally by well-worn molars; only thin peripheral band of enamel along with slight evidence of any primary or secondary folds on any teeth remain; all bones of skull coalesced; epiphyses and shafts of long bones ankylosed; small bony protuberances on many skulls; pelage usually ragged, tips of the hairs being worn away; white flecking and spotting not common, but occurs in some adults.Table 1.—Average and Extreme Measurements (in Millimeters) of Skulls of Five Age-groupsof Baiomys taylori from vic. (seep. 595) Altamira, Tamaulipas, Mexico.Age groupsJuvenileYoungSubadultAdultOld adultNumber examined3314198Total length77.0(74-79)92.6(89-96)97.6(91-103)99.9(93-105)101.6(98-107)Length of tail27.3(24-29)39.3(37-41)40.4(36-43)39.8(35-45)40.9(38-45)Length of body49.6(49-50)53.3(52-55)57.0(51-61)60.0(56-67)60.7(57-67)Length of hind foot11.0(11)13.6(13-14)14.3(13.5-15.0)14.5(14-15)14.2(13-15)Occipitonasal length14.2(13.6-15.2)16.3(15.8-16.9)17.1(16.7-17.6)17.7(17.2-18.3)17.8(17.6-18.1)Zygomatic breadth8.1(7.8- 8.6)8.7(8.6-8.8)8.9(8.6-9.3)9.3(9.0-9.6)9.4(9.1-9.6)Interorbital breadth3.4(3.3- 3.5)3.4(3.3-3.6)3.4(3.3-3.6)3.6(3.4-3.8)3.5(3.3-3.6)Incisive foramina (length)2.9(2.8- 2.9)3.5(3.4-3.6)3.7(3.6-3.9)3.9(3.6-4.1)3.9(3.5-4.0)Depth of cranium5.9(5.6- 6.2)6.5(6.3-6.8)6.5(6.2-6.8)6.7(6.4-7.0)6.8(6.5-7.1)Alveolar length, upper molars2.7(2.5- 2.8)2.9(2.9-3.0)2.9(2.8-3.1)3.0(2.9-3.2)3.0(3.0-3.1)Postpalatal length4.8(4.5- 5.3)5.9(5.8-6.0)6.2(5.8-6.6)6.5(6.2-7.2)6.5(6.3-6.7)Breadth of braincase8.1(7.8- 8.7)8.5(8.5)8.4(8.0-8.7)8.6(8.3-8.9)8.6(8.4-8.8)SECONDARY SEXUAL VARIATIONThe method employed by Dice and Leraas (1936:2) was used to measure the secondary sexual differences, if there were any, in each of several age classes. As pointed out by Hooper (1952b:11), individual variation in small samples can obscure secondary sexual differences. The samples ofB. taylorifrom the vicinity (see page 595) of Altamira, Tamaulipas, and the samples ofB. musculusfrom El Salvador (table 2) were large enough to prevent individual variation from obscuring sexual differences. Nevertheless, no significant secondary sexual differences were found in eitherB. tayloriorB. musculus(seetable 2). Therefore, the sexes have been considered together for purposes of geographic studies.Table 2.—Analysis of Secondary Sexual Variation in Adult B. taylori Vicinity of (seep. 595)Altamira, Tamaulipas, and Adult B. musculus from El Salvador (seep. 595).(One Standard Deviation on Either Side of the Mean is Given.)CharacterBaiomys tayloriBaiomys musculus21 Males18 Females17 Males13 FemalesTotal length98.4 ± 2.95100.5 ± 4.72112.04 ± 5.49113.12 ± 4.23Length of tail40.1 ± 2.3140.3 ± 2.3947.12 ± 2.9545.70 ± 2.92Length of body57.83 ± 1.6560.10 ± 4.1366.67 ± 3.9767.75 ± 2.38Length ofhind foot14.21 ± .5314.44 ± .5115.60 ± .4915.38 ± .64Length of ear10.00 ± .0010.00 ± .0011.80 ± .6512.00 ± .41Occipitonasallength17.48 ± .4017.47 ± .4719.32 ± .3519.04 ± .44Zygomaticbreadth9.17 ± .339.15 ± .309.84 ± .219.91 ± .28Leastinterorbitalbreadth3.53 ± .113.48 ± .113.88 ± .083.88 ± .12Postpalatallength6.35 ± .196.38 ± .307.11 ± .156.95 ± .20Depthof cranium6.65 ± .246.61 ± .177.10 ± .187.08 ± .18Incisiveforamina(length)3.82 ± .153.81 ± .184.43 ± .114.35 ± .14Lengthof rostrum5.87 ± .205.88 ± .216.81 ± .166.66 ± .31Breadthof braincase8.54 ± .238.52 ± .129.84 ± .389.52 ± .20Alveolarlength,upper molars2.98 ± .083.01 ± .083.20 ± .093.24 ± .10INDIVIDUAL VARIATIONLength of tail varied more than any other measurement used by me in taxonomic comparisons. Clark (1941:298), Hoffmeister (1951:16), and Van Gelder (1959:239) point out that external measurements generally are more variable than measurements of the cranium, probably because different techniques of measuring are employed by different collectors. As can be noted intable 3, females varied more than males.In the 3520 specimens examined, an extra tooth was observed in only one (see Hooper, 1955:298). The left mandibular tooth-row of an adult male (USNM 71539) from Omentepec, Guerrero, is worn more than the right one. Irregularities in number of teeth and abnormalities in individual teeth seem to be rare in pygmy mice.Table 3.—Individual Variation: Coefficients of Variation for Dimensions of Externaland Cranial Parts in a Population of B. Musculus and B. Taylori.MeasurementBaiomys tayloriBaiomys musculusVic. (seepage 595)Altamira, TamaulipasVic. (seepage 595)El Salvador21 MalesC. V.18 FemalesC. V.17 MalesC. V.13 FemalesC. V.Total length3.04.74.93.7Length of tail5.75.96.26.4Length of body2.85.05.93.5Length of hind foot3.73.43.04.1Length of ear0.00.05.53.3Occipitonasal length2.22.71.82.3Zygomatic breadth3.63.32.22.7Interorbital breadth3.23.32.22.9Incisive foramina(length)3.84.62.53.2Depth of cranium3.62.52.52.5Alveolar length,upper molars2.72.52.83.2Postpalatal length3.14.72.12.9Length of rostrum3.33.62.44.7Breadth of braincase2.71.44.04.9The posterior margin of the bony palate varies from semicircular to nearly V-shaped. The suture between the nasals and frontals varies from V-shaped to truncate to W-shaped. The maxillary part of the zygoma varies from broad to slender in dorsoventral width in both species.PELAGE AND MOLTSThere are three distinct pelages, juvenal, postjuvenal, and adult. The sequences of molt and change of pelage from the juvenal, to the postjuvenal, and from it to adult, are essentially as reported forPeromyscusby Collins (1918:78-81; 1924:58-60) and Hoffmeister (1951:5). The juvenal pelage is uniformly dusky gray throughout except for the paler gray on the venter. In most juvenal mice, theyellow to ochraceous pigments of the subterminal bands are reduced or absent. UnlikePeromyscus,Baiomyshas bright brownish hairs on the head as the first evidence of the postjuvenal molt (seeFigure 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. New hair appears ventrally and laterally at the end of 46 days (seeFigure 4, part b). Hair replacement proceeds more slowly after the "saddle back" stage (described inPeromyscusby Collins, 1918:80) has been reached. That stage was reached in two pygmy mice at 52 days (seeFigure 4, part c). Areas immediately posterior to the ears, in the scapular region, molt last. The postjuvenal pelage was seemingly complete in one captive pygmy mouse at the end of 60 days. Another captive failed to complete its growth of new pelage until two additional weeks had elapsed. Length of time required to molt in pygmy mice is about the same as that reported by Layne (1959:72) inReithrodontomys.Fig. 4.Diagrams showing progress of the postjuvenal molt in pygmy mice.For explanation of a, b, and c, see text. All approximately2/3natural size.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. The percentage of specimens in molt immediately before the rainy season and immediately before the dry season is slightly higher than in specimens taken at other times of the year. The adult or seasonal molt (both loss of old pelage and growth of new) resembles that inPeromyscus truei gilberti, described by Hoffmeister (1951:6) as proceeding "posteriorly as a wave over the entire back." The new hair is slightly brighter than the old. Old adults are usually in ragged pelage regardless of season; possibly only one regular annual change of pelage occurs in most animals before they die. Only one case of melanism was observed among all the specimens of both species examined. It was a young maleB. t. taylori, KU 35943, from 6 mi. SW San Gerónimo, Coahuila, possessing black hairs throughout. Its hairs are longer and finer than those on specimens of comparable age and sex. No albino was found, although Stickel and Stickel (1949:145) record one—an adult male ofB. taylori.TAXONOMIC CHARACTERS AND RELATIONSHIPSExternal parts.—Length of body, foot, ear, and tail are useful when considered together in distinguishing species and subspecies. I found as Hooper (1952a:91) did that length of ear in combination with length of hind foot suffices to identify nearly all specimens to species, especially where the two species occur together.Pelage.—Color in adults is of especial value in subspecific determination; the manner in which it varies geographically is described on pages 609, 630.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 ofB. taylori, in front of the frontonasal suture, is deflected three to five degrees ventrally in 85 per cent ofthe adults examined, and inB. musculusis less, or not at all, deflected.Teeth.—Alveolar length of the upper and lower molar tooth-rows aids in distinguishing fossil and Recent species, and to a lesser degree in delimiting subspecies. 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 inPeromyscus,Sigmodon, andOryzomys, mice thought to be closely related toBaiomys. In pygmy mice, however, the dental patterns are relatively constant. The lophs and styles are subject to some geographic variation but, nevertheless, are useful in estimating relationships.Fig. 5.Ventral view of hyoid bones. × 18.A.Baiomys musculus brunneus, adult, female, No. 30182 KU, Potrero Viejo, 1700 feet, Veracruz.B.Baiomys taylori analogous, adult, female, No. 36761 KU, 2 mi. N Ciudad Guzmán, 5000 feet, Jalisco.Hyoid apparatus.—Shape and, to a lesser extent, size of the hyoid apparatus differentiate nearly all specimens ofB. taylorifrom all those ofB. musculus. The hyoid ofB. tayloridiffers from that ofB. musculusprincipally in the shape of the basihyal. It possessesan anteriorly pointed entoglossal process inB. musculus, and is not rounded to completely absent as inB. taylori(seeFigure 5). The shoulders of the basihyal protrude anteriorly inB. musculus, and are not flattened as inB. taylori. The total length was measured in a sample of 55 basihyals ofB. musculus, and was compared to the total length of a sample of 80 basihyals ofB. taylori. The means of the two samples differ significantly at the 95 per cent level; the mean plus two standard errors ofB. musculusandB. taylori, are, respectively, 2.43 ± .02; 2.18 ± .03. There is sufficient overlap of the samples (mean plus one standard deviation ofB. musculusandB. taylori, respectively: 2.43 ± .15; 2.18 ± .15) to make the total length of the basihyal of only secondary importance in distinguishing species, but shape and total length of the basihyal, when considered together, serve to identify all specimens to species. When length of the basihyal is plotted against occipitonasal length (seeFigure 6), all specimens studied, regardless of age or geographical origin, were separated at the level of species. The hypohyals ofB. tayloriseemingly remain distinct throughout life; those ofB. musculuscompletely fuse in some adults. The ceratohyals are highly variable in shape and of little taxonomic use.Fig. 6.Relationship of length of basihyal to occipitonasal length of skull. Black symbols, all below the curved line, represent measurements ofB. taylori; open symbols, all above the curved line, represent measurements ofB. musculus.The degree of geographic variation in shape of basihyal is not great. Specimens ofB. musculus pallidusfrom 1 km. NW Chapa,Guerrero, have a small indentation on the anteriormost part of the entoglossal process. The shoulder of the basihyal is directed less forward in specimens ofB. taylori taylorifrom 6 mi. N, 6 mi. W Altamira, Tamaulipas, than in other specimens of the species. The variations observed seemed not to be clinal.According to White (1953:548) the hyoid, like the baculum (Burt, 1936:146), is little influenced by changes in external environment and may serve to clarify intergeneric relationships. Hyoids of both species ofBaiomysare smaller than hyoids of all subgenera ofPeromyscus. In shape, the hyoids ofBaiomysresemble those ofOchrotomys nuttalli(as explained onpage 605,Ochrotomysis here accorded generic, instead of subgeneric, rank). In size, the hyoid of both species ofBaiomysresembles that inReithrodontomys. Sprague (1941:304) reports a resemblance in shape between the ceratohyals ofBaiomysandReithrodontomys. The thyrohyals differ from those ofReithrodontomys, being less boot-shaped, and having a slight terminal expansion as inOchrotomys(see Sprague,loc. cit.). In shape, the large basihyal ofOnychomysresembles the smaller one ofB. musculus. The basihyal ofOryzomyslacks the entoglossal process present inBaiomys. On the basis of shape of hyoid,Baiomysseems to be most closely related toOchrotomys.Fig. 7.Dorsal view of bacula. × 16.A.B. musculus brunneus, adult, No. 24336 KU, 3 kms. W Boca del Río, 10 feet, Veracruz.B.B. taylori taylori, adult, No. 35937 KU, 6 mi. SW San Gerónimo, Coahuila.Baculum.—OfBaiomys, 166 bacula were processed, using the method of White (1951:125), and studied. They provide characters of taxonomic worth at the level of species and aid in evaluating generic relationships.The baculum ofB. tayloridiffers from that ofB. musculusin: shaft narrow; wings anterior to base projecting dorsolaterally insteadof anteriorly; anterior part knob-shaped having indentation at tip, instead of anterior part spatulate-shaped (in some) to knob-shaped (seeFigure 7), without indentation; significantly shorter (seeTable 4).Table 4.—Length of BaculaNumber ofspecimensAveragelength3 ×standard1standardRangeB. taylori1082.535.078.2742.00-3.12B. musculus583.324.090.2332.80-3.88In each of the two species, individual and geographic variation in the baculum is slight; its length varies insignificantly according to age. Excluding juveniles contained inTable 4, but including young and subadults, only three bacula ofB. tayloriwere longer than 3 mm., and only one baculum ofB. musculus(a young) was shorter than 3 mm. 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 ofBaiomyswere compared with bacula ofAkodon,Scotinomys,Holochilus,Oryzomys,Zygodontomys,Reithrodontomys,Thaptomys, andCalomysand illustrations of bacula by Blair (1942:197, 200) ofPeromyscus(subgeneraPeromyscus,Haplomylomys,Podomys),Ochrotomys, and material at the University of Kansas Museum of Natural History ofMegadontomys. Shape of baculum most resembled that ofOchrotomysandCalomys. The bacula ofBaiomys, as pointed out by Blair (op cit.:203), differ as much from those of the genusPeromyscusas do the bacula ofReithrodontomysandOnychomys. In size of baculum,BaiomysresemblesOchrotomys. Blair (op. cit.:202) pointed out that the length of the baculum ofB. taylori subaterwas contained in the length of the animal's body 20.3 times, and 24.2 times in the length of that ofOchrotomys nuttalli. The length of the baculum ofB. musculus(average of 58 specimens without regard to subspecies) is contained in the length of the body (of specimens from which the bacula were removed) 22.7 times, a figure approaching that inOchrotomys. When bacula of both species ofBaiomyswere compared to those ofO. nuttalli, bacula ofB. musculuswere found to most closely resemble those ofO. nuttalli. The baculum of a singlespecimen ofCalomys(C. laucha) was contained in the length of the body 15.5 times. In general shape, as well as in possession of an anterior knob and the position of the expanded posterior wings, the baculum ofC. laucharesembles the baculum ofOchrotomysandBaiomys musculus.Blair (op. cit.:201) considers genericversussubgeneric rank forOchrotomys, and on the basis of studies of the phallus Hooper (1958:23) stated that "it is clear thatnuttallishould be removed fromPeromyscusand should be listed asOchrotomys nuttalli(Harlan)." I agree with Hooper (loc. cit.) and point out that on the basis of the baculum, there is less of a hiatus betweenBaiomyson the one hand, andOchrotomysandCalomyson the other hand, than there is between any one of those three genera andPeromyscus.White (1953:631) reported that the baculum of chipmunks might indicate relationships more clearly than do skulls and skins. He thought that skulls might more quickly than bacula reflect the habitus of the animal. The resemblance in cranial morphology betweenPeromyscusandBaiomysis judged to be the result of such a convergence of habitus and the baculum inBaiomysis thought to reflect relationships more accurately than does the skull.Auditory ossicles.—Examination of a number of auditory ossicles ofBaiomysreveals constant interspecific differences in the malleus and incus. There is only slight individual variation, slight variation with age, and no secondary sexual variation. InBaiomys taylorithe orbicular apophysis of the malleus (seeFigure 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.InBaiomys musculus, the orbicular apophysis of the malleus (seeFigure 8, B) is round to oblong, and less ovoid than inB. taylori; the anterior process is less acutely pointed than inB. taylori, and the neck is long, less recurved than inB. taylori. The body of the incus, though tending to be round, is more flattened, and the short process is knob-shaped, not elongated. The sides of the long limb of the incus are not parallel. The lenticular process is, relative to the size of the incus, small. The posterior and anterior crus of the stapes are more nearly straight than intaylori. A prominent muscular process occurs on the posterior crus.The auditory ossicles of representative species of all the subgenera ofPeromyscuswere studied as were the ossicles ofOnychomys,Ochrotomys,Oryzomys,Akodon,Thaptomys,Zygodontomys,Calomys,Reithrodontomys, andHolochilus.Fig. 8.Lateral views of auditory ossicles. × 20.A.B. taylori analogous, adult, female, No. 28104 KU, 4 kms. ENE Tlalmanalco, 2290 meters, Estado de México.B.B. musculus pallidus, adult, male, No. 28346 KU, Cahuilotal, Sacacoyuca, 960 meters, Guerrero.The general plan of structure of the auditory ossicles inBaiomysresembles that inCalomys,Akodon, andThaptomys. The ossicles ofCalomysandThaptomys, in particular, closely resemble the auditory ossicles ofBaiomys musculus. The short process of the incus is knoblike inCalomysandThaptomys, and the general conformation of malleus and stapes in those two genera is nearly identical to that inB. musculus. InAkodon, the anterior and posterior crus of the stapes is more rounded than inB. musculus, resembling that inB. taylori.Reithrodontomysdiffer fromBaiomysin having a more elongate orbicular apophysis on the body of the malleus, an elongated short limb on the incus, and a stapes having anterior and posterior crura bowed as in mice of the genusPeromyscus.InOchrotomys, the orbicular apophysis of the malleus resembles the orbicular apophysis ofB. musculus, but the short process of the incus is longer, resembling the short process ofB. taylori. In general conformation of the malleus, incus, and stapes,Ochrotomysshows closer resemblance toB. taylorithan toB. musculus.InHolochilusthe anterior crus and posterior crus of the stapes are similar to those inB. musculus, but in shape and size of malleus and incus,Holochilusdiffers considerably fromB. musculusandB. taylori.InZygodontomys, size and shape of the ossicles differ greatly from those ofBaiomys.In the genusPeromyscus, onlyPeromyscus floridanus(subgenusPodomys) possesses a knoblike short process on the incus similar to that inB. musculus; representatives of the other subgenera examined possess an elongated short limb on the incus. The conformation of the ossicles of bothOnychomysandOryzomysappears to be more nearly like that inPeromyscusthan that ofBaiomys.On the basis of shape and size of auditory ossicles,Baiomysresembles South American hesperomines (CalomysandThaptomys) rather than North American hesperomines.GenusBaiomysTrue1894.BaiomysTrue, Proc. U. S. Nat. Mus., 16:758, February 7. Type,Hesperomys (Vesperimus) tayloriThomas.Diagnosis.—Size small (total length in adults, 93-135); tail shorter than head and body; hind foot in adults 12-17; ears small (8-12) and rounded; upper parts blackish sepia to ochraceous-buff; underparts slaty gray to white or pale buffy; eyes small; hind feet having six plantar pads, soles nearly naked except for some hairs on anterior parts of soles and anteriorly to base of toes and between toes; occipitonasal length of skull in adults, 17.0-21.5; zygomatic breadth, 9.0-11.5; coronoid process of mandible well developed, strongly recurved; ascending ramus of mandible short and erect; anterior palatine foramina (incisive foramina) long, usually terminating posterior to plane of the front of first molars; posterior palatine foramina nearly opposite middle of M2; interorbital space wide relative to widest part of frontals; nasals projecting only slightly over incisors; condyle terminal; upper incisors relatively heavy; primary first fold of M3 obliterated at an early stage of wear; major cusps of upper and lower anteriormost two molars alternating, more so in m1-m2 than in M1-M2, dental formula I/i, 1/1; C/c, 0/0; P/p, M/m, 3/3 = 16.For distribution of the genus, seeFigure 9.Fig. 9.Geographic distribution of the genusBaiomys. Black area shows where the two species occur together. Black dot (Acultzingo, Veracruz) shows locality whereBaiomys taylorioccurs within the range ofB. musculus, butB. musculusis not known to occur at that locality.SYSTEMATIC ACCOUNT OF SPECIES AND SUBSPECIESBaiomys musculusSouthern Pygmy Mouse(Synonymy under subspecies)Type.—Sitomys musculusMerriam, Proc. Biol. Soc. Washington, 7:170, September 29, 1892.Range.—Southern Nayarit, Michoacán, México, Morelos, Puebla, and central Veracruz, southeastward to western Nicaragua, but unknown from southern Veracruz, Tabasco, and the Yucatán Peninsula (seeFigure 10); occurs principally in the arid upper and lower divisions of the Tropical Life-zone.Characters for ready recognition.—Unless otherwise noted, characters are usable only for the two age-categories of adult and old adult. Differs fromB. tayloriin: hind foot 16 millimeters or more; occipitonasal length, 19 millimeters[Pg 609]or more; zygomatic breadth, 10 millimeters or more; rostrum not deflected ventrally at frontoparietal suture but, instead, curving gradually toward anteriormost point of nasals; cingular ridges and secondary cusps on teeth more pronounced; basihyal having anterior pointed entoglossal process, shoulders of basihyal protruding anteriorly (characteristic of all age categories); baculum having broader shaft, spatulate to knob-shaped tip, wings at base projecting anteriorly; baculum more than 3 millimeters long; short process of incus knob-shaped rather than attenuate; muscular process of posterior crus of stapes prominent.Characters of the species.—Size large (extremes in external measurements of adults; total length, 100-135; length of tail vertebrae, 33-56; length of hind foot, 14.1-17; length of ear, 9-12); upper parts dark reddish brown, or ochraceous-buff to nearly black; underparts pale pinkish buff to white or pale buffy.Geographic variation.—Eight subspecies are here recognized (seeFigure 10). Features that vary geographically are external size, color of pelage, certain cranial dimensions (occipitonasal length, zygomatic breadth, least interorbital breadth, length of rostrum, length of incisive foramina, depth and breadth of cranium, and alveolar length of upper molar tooth-row).External and cranial size (except forB. m. handleyi) is less in the southernmost subspecies,B. m. pullus,B. m. grisescens,B. m. nigrescens, and more in the northernmost subspecies,B. m. musculus,B. m. brunneus, andB. m. infernatis. 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. 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 bothB. tayloriandB. musculus. InB. musculus, the darker subspecies,B. m. brunneus,B. m. nigrescens, andB. m. pullus, occur in zones of rather constant high relative humidity, whereas the paler subspeciesinfernatis,musculus,handleyi, and to a less extentgrisescensandpallidus, occur in zones of lower relative humidity. This is in keeping with Gloger's Rule, which states that melanins increase in the warm and humid parts of the range of a species, and reddish or yellowish-brown phaeomelanins prevail in arid climates.B. m. musculusranges into areas where relative humidity is such that darker pelages might be expected, but this is in the area where the two species are sympatric, and color of pelage may be an important character of recognition.
NON-GEOGRAPHIC VARIATION
Non-geographic variation in pygmy mice (variation in a single population resulting from age, individual, seasonal, and secondary sexual differences) has been but little studied in the past. Mearns (1907:381) figured progressive stages of wear on the teeth ofB. taylori; Osgood (1909:252) and Blair (1941:380) referred to changes in dentition, weights, and pelages.
The largest samples available for this study were 47B. taylorifrom the vicinity of Altamira (6 mi. N, 6 mi. W; 5 mi. N, 5 mi. W; 1 mi. S), Tamaulipas, and 44B. musculusfrom El Salvador (1 mi. S Los Planes, and 1 mi. NW San Salvador—two localities 3 miles apart).
VARIATION WITH AGE
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.
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.
Young.—Weaned mice; cranium fragile; sutures between frontals and parietals, interparietal and parietals, basioccipital and basisphenoid, basisphenoid and presphenoid, premaxillaries and maxillaries widely open; M3 and m3 erupted beyond margins of their alveoli (molars erupt from anterior to posterior; M3 and m3, therefore, are last to erupt); in some specimens, molars slightly worn; pelage still dusky and relatively fine and sparse.
Subadults.—Sutures between bones of skull less widely open than in young; epiphyses of long bones incompletely coalesced to shaft; relative to length of skull, braincase higher and rostrum shorter than in adults; all cusps worn, but dentine not occlusally confluent; primary first and second folds of third upper molars present; primary first fold and major fold of lower molars visible; pelage a subtle mixture of colors of young and adult, but resembling most that of adult; molts into postjuvenal pelage between 46 and 50 days.
Adults.—Sutures of skull, and those between epiphyses and shaft of long bones obliterated except that, in some mice, sutures of skull persist between frontoparietal, and interparietal; cusps of molars so worn that dentine occlusally confluent; small island of enamel in third upper and lower molars of some specimens; relative to length of skull, cranium lower, rostrum longer, and interorbital region narrower than in subadult; cranium appears to be more flattened dorsoventrally; between subadult and adult stages, principal growth occurs in basioccipital, basisphenoid, frontals, and parietals; nasals grow less.
Although all bones of the skull grow in the subadult and early adult stages (seetable 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 (seetable 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.
Old Adults.—Characterized principally by well-worn molars; only thin peripheral band of enamel along with slight evidence of any primary or secondary folds on any teeth remain; all bones of skull coalesced; epiphyses and shafts of long bones ankylosed; small bony protuberances on many skulls; pelage usually ragged, tips of the hairs being worn away; white flecking and spotting not common, but occurs in some adults.
Table 1.—Average and Extreme Measurements (in Millimeters) of Skulls of Five Age-groupsof Baiomys taylori from vic. (seep. 595) Altamira, Tamaulipas, Mexico.
SECONDARY SEXUAL VARIATION
The method employed by Dice and Leraas (1936:2) was used to measure the secondary sexual differences, if there were any, in each of several age classes. As pointed out by Hooper (1952b:11), individual variation in small samples can obscure secondary sexual differences. The samples ofB. taylorifrom the vicinity (see page 595) of Altamira, Tamaulipas, and the samples ofB. musculusfrom El Salvador (table 2) were large enough to prevent individual variation from obscuring sexual differences. Nevertheless, no significant secondary sexual differences were found in eitherB. tayloriorB. musculus(seetable 2). Therefore, the sexes have been considered together for purposes of geographic studies.
Table 2.—Analysis of Secondary Sexual Variation in Adult B. taylori Vicinity of (seep. 595)Altamira, Tamaulipas, and Adult B. musculus from El Salvador (seep. 595).(One Standard Deviation on Either Side of the Mean is Given.)
INDIVIDUAL VARIATION
Length of tail varied more than any other measurement used by me in taxonomic comparisons. Clark (1941:298), Hoffmeister (1951:16), and Van Gelder (1959:239) point out that external measurements generally are more variable than measurements of the cranium, probably because different techniques of measuring are employed by different collectors. As can be noted intable 3, females varied more than males.
In the 3520 specimens examined, an extra tooth was observed in only one (see Hooper, 1955:298). The left mandibular tooth-row of an adult male (USNM 71539) from Omentepec, Guerrero, is worn more than the right one. Irregularities in number of teeth and abnormalities in individual teeth seem to be rare in pygmy mice.
Table 3.—Individual Variation: Coefficients of Variation for Dimensions of Externaland Cranial Parts in a Population of B. Musculus and B. Taylori.
The posterior margin of the bony palate varies from semicircular to nearly V-shaped. The suture between the nasals and frontals varies from V-shaped to truncate to W-shaped. The maxillary part of the zygoma varies from broad to slender in dorsoventral width in both species.
PELAGE AND MOLTS
There are three distinct pelages, juvenal, postjuvenal, and adult. The sequences of molt and change of pelage from the juvenal, to the postjuvenal, and from it to adult, are essentially as reported forPeromyscusby Collins (1918:78-81; 1924:58-60) and Hoffmeister (1951:5). The juvenal pelage is uniformly dusky gray throughout except for the paler gray on the venter. In most juvenal mice, theyellow to ochraceous pigments of the subterminal bands are reduced or absent. UnlikePeromyscus,Baiomyshas bright brownish hairs on the head as the first evidence of the postjuvenal molt (seeFigure 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. New hair appears ventrally and laterally at the end of 46 days (seeFigure 4, part b). Hair replacement proceeds more slowly after the "saddle back" stage (described inPeromyscusby Collins, 1918:80) has been reached. That stage was reached in two pygmy mice at 52 days (seeFigure 4, part c). Areas immediately posterior to the ears, in the scapular region, molt last. The postjuvenal pelage was seemingly complete in one captive pygmy mouse at the end of 60 days. Another captive failed to complete its growth of new pelage until two additional weeks had elapsed. Length of time required to molt in pygmy mice is about the same as that reported by Layne (1959:72) inReithrodontomys.
Fig. 4.Diagrams showing progress of the postjuvenal molt in pygmy mice.For explanation of a, b, and c, see text. All approximately2/3natural size.
Fig. 4.Diagrams showing progress of the postjuvenal molt in pygmy mice.For explanation of a, b, and c, see text. All approximately2/3natural size.
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. The percentage of specimens in molt immediately before the rainy season and immediately before the dry season is slightly higher than in specimens taken at other times of the year. The adult or seasonal molt (both loss of old pelage and growth of new) resembles that inPeromyscus truei gilberti, described by Hoffmeister (1951:6) as proceeding "posteriorly as a wave over the entire back." The new hair is slightly brighter than the old. Old adults are usually in ragged pelage regardless of season; possibly only one regular annual change of pelage occurs in most animals before they die. Only one case of melanism was observed among all the specimens of both species examined. It was a young maleB. t. taylori, KU 35943, from 6 mi. SW San Gerónimo, Coahuila, possessing black hairs throughout. Its hairs are longer and finer than those on specimens of comparable age and sex. No albino was found, although Stickel and Stickel (1949:145) record one—an adult male ofB. taylori.
TAXONOMIC CHARACTERS AND RELATIONSHIPS
External parts.—Length of body, foot, ear, and tail are useful when considered together in distinguishing species and subspecies. I found as Hooper (1952a:91) did that length of ear in combination with length of hind foot suffices to identify nearly all specimens to species, especially where the two species occur together.
Pelage.—Color in adults is of especial value in subspecific determination; the manner in which it varies geographically is described on pages 609, 630.
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 ofB. taylori, in front of the frontonasal suture, is deflected three to five degrees ventrally in 85 per cent ofthe adults examined, and inB. musculusis less, or not at all, deflected.
Teeth.—Alveolar length of the upper and lower molar tooth-rows aids in distinguishing fossil and Recent species, and to a lesser degree in delimiting subspecies. 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 inPeromyscus,Sigmodon, andOryzomys, mice thought to be closely related toBaiomys. In pygmy mice, however, the dental patterns are relatively constant. The lophs and styles are subject to some geographic variation but, nevertheless, are useful in estimating relationships.
Fig. 5.Ventral view of hyoid bones. × 18.A.Baiomys musculus brunneus, adult, female, No. 30182 KU, Potrero Viejo, 1700 feet, Veracruz.B.Baiomys taylori analogous, adult, female, No. 36761 KU, 2 mi. N Ciudad Guzmán, 5000 feet, Jalisco.
Fig. 5.Ventral view of hyoid bones. × 18.A.Baiomys musculus brunneus, adult, female, No. 30182 KU, Potrero Viejo, 1700 feet, Veracruz.B.Baiomys taylori analogous, adult, female, No. 36761 KU, 2 mi. N Ciudad Guzmán, 5000 feet, Jalisco.
Hyoid apparatus.—Shape and, to a lesser extent, size of the hyoid apparatus differentiate nearly all specimens ofB. taylorifrom all those ofB. musculus. The hyoid ofB. tayloridiffers from that ofB. musculusprincipally in the shape of the basihyal. It possessesan anteriorly pointed entoglossal process inB. musculus, and is not rounded to completely absent as inB. taylori(seeFigure 5). The shoulders of the basihyal protrude anteriorly inB. musculus, and are not flattened as inB. taylori. The total length was measured in a sample of 55 basihyals ofB. musculus, and was compared to the total length of a sample of 80 basihyals ofB. taylori. The means of the two samples differ significantly at the 95 per cent level; the mean plus two standard errors ofB. musculusandB. taylori, are, respectively, 2.43 ± .02; 2.18 ± .03. There is sufficient overlap of the samples (mean plus one standard deviation ofB. musculusandB. taylori, respectively: 2.43 ± .15; 2.18 ± .15) to make the total length of the basihyal of only secondary importance in distinguishing species, but shape and total length of the basihyal, when considered together, serve to identify all specimens to species. When length of the basihyal is plotted against occipitonasal length (seeFigure 6), all specimens studied, regardless of age or geographical origin, were separated at the level of species. The hypohyals ofB. tayloriseemingly remain distinct throughout life; those ofB. musculuscompletely fuse in some adults. The ceratohyals are highly variable in shape and of little taxonomic use.
Fig. 6.Relationship of length of basihyal to occipitonasal length of skull. Black symbols, all below the curved line, represent measurements ofB. taylori; open symbols, all above the curved line, represent measurements ofB. musculus.
Fig. 6.Relationship of length of basihyal to occipitonasal length of skull. Black symbols, all below the curved line, represent measurements ofB. taylori; open symbols, all above the curved line, represent measurements ofB. musculus.
The degree of geographic variation in shape of basihyal is not great. Specimens ofB. musculus pallidusfrom 1 km. NW Chapa,Guerrero, have a small indentation on the anteriormost part of the entoglossal process. The shoulder of the basihyal is directed less forward in specimens ofB. taylori taylorifrom 6 mi. N, 6 mi. W Altamira, Tamaulipas, than in other specimens of the species. The variations observed seemed not to be clinal.
According to White (1953:548) the hyoid, like the baculum (Burt, 1936:146), is little influenced by changes in external environment and may serve to clarify intergeneric relationships. Hyoids of both species ofBaiomysare smaller than hyoids of all subgenera ofPeromyscus. In shape, the hyoids ofBaiomysresemble those ofOchrotomys nuttalli(as explained onpage 605,Ochrotomysis here accorded generic, instead of subgeneric, rank). In size, the hyoid of both species ofBaiomysresembles that inReithrodontomys. Sprague (1941:304) reports a resemblance in shape between the ceratohyals ofBaiomysandReithrodontomys. The thyrohyals differ from those ofReithrodontomys, being less boot-shaped, and having a slight terminal expansion as inOchrotomys(see Sprague,loc. cit.). In shape, the large basihyal ofOnychomysresembles the smaller one ofB. musculus. The basihyal ofOryzomyslacks the entoglossal process present inBaiomys. On the basis of shape of hyoid,Baiomysseems to be most closely related toOchrotomys.
Fig. 7.Dorsal view of bacula. × 16.A.B. musculus brunneus, adult, No. 24336 KU, 3 kms. W Boca del Río, 10 feet, Veracruz.B.B. taylori taylori, adult, No. 35937 KU, 6 mi. SW San Gerónimo, Coahuila.
Fig. 7.Dorsal view of bacula. × 16.A.B. musculus brunneus, adult, No. 24336 KU, 3 kms. W Boca del Río, 10 feet, Veracruz.B.B. taylori taylori, adult, No. 35937 KU, 6 mi. SW San Gerónimo, Coahuila.
Baculum.—OfBaiomys, 166 bacula were processed, using the method of White (1951:125), and studied. They provide characters of taxonomic worth at the level of species and aid in evaluating generic relationships.
The baculum ofB. tayloridiffers from that ofB. musculusin: shaft narrow; wings anterior to base projecting dorsolaterally insteadof anteriorly; anterior part knob-shaped having indentation at tip, instead of anterior part spatulate-shaped (in some) to knob-shaped (seeFigure 7), without indentation; significantly shorter (seeTable 4).
Table 4.—Length of Bacula
In each of the two species, individual and geographic variation in the baculum is slight; its length varies insignificantly according to age. Excluding juveniles contained inTable 4, but including young and subadults, only three bacula ofB. tayloriwere longer than 3 mm., and only one baculum ofB. musculus(a young) was shorter than 3 mm. 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 ofBaiomyswere compared with bacula ofAkodon,Scotinomys,Holochilus,Oryzomys,Zygodontomys,Reithrodontomys,Thaptomys, andCalomysand illustrations of bacula by Blair (1942:197, 200) ofPeromyscus(subgeneraPeromyscus,Haplomylomys,Podomys),Ochrotomys, and material at the University of Kansas Museum of Natural History ofMegadontomys. Shape of baculum most resembled that ofOchrotomysandCalomys. The bacula ofBaiomys, as pointed out by Blair (op cit.:203), differ as much from those of the genusPeromyscusas do the bacula ofReithrodontomysandOnychomys. In size of baculum,BaiomysresemblesOchrotomys. Blair (op. cit.:202) pointed out that the length of the baculum ofB. taylori subaterwas contained in the length of the animal's body 20.3 times, and 24.2 times in the length of that ofOchrotomys nuttalli. The length of the baculum ofB. musculus(average of 58 specimens without regard to subspecies) is contained in the length of the body (of specimens from which the bacula were removed) 22.7 times, a figure approaching that inOchrotomys. When bacula of both species ofBaiomyswere compared to those ofO. nuttalli, bacula ofB. musculuswere found to most closely resemble those ofO. nuttalli. The baculum of a singlespecimen ofCalomys(C. laucha) was contained in the length of the body 15.5 times. In general shape, as well as in possession of an anterior knob and the position of the expanded posterior wings, the baculum ofC. laucharesembles the baculum ofOchrotomysandBaiomys musculus.
Blair (op. cit.:201) considers genericversussubgeneric rank forOchrotomys, and on the basis of studies of the phallus Hooper (1958:23) stated that "it is clear thatnuttallishould be removed fromPeromyscusand should be listed asOchrotomys nuttalli(Harlan)." I agree with Hooper (loc. cit.) and point out that on the basis of the baculum, there is less of a hiatus betweenBaiomyson the one hand, andOchrotomysandCalomyson the other hand, than there is between any one of those three genera andPeromyscus.
White (1953:631) reported that the baculum of chipmunks might indicate relationships more clearly than do skulls and skins. He thought that skulls might more quickly than bacula reflect the habitus of the animal. The resemblance in cranial morphology betweenPeromyscusandBaiomysis judged to be the result of such a convergence of habitus and the baculum inBaiomysis thought to reflect relationships more accurately than does the skull.
Auditory ossicles.—Examination of a number of auditory ossicles ofBaiomysreveals constant interspecific differences in the malleus and incus. There is only slight individual variation, slight variation with age, and no secondary sexual variation. InBaiomys taylorithe orbicular apophysis of the malleus (seeFigure 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.
InBaiomys musculus, the orbicular apophysis of the malleus (seeFigure 8, B) is round to oblong, and less ovoid than inB. taylori; the anterior process is less acutely pointed than inB. taylori, and the neck is long, less recurved than inB. taylori. The body of the incus, though tending to be round, is more flattened, and the short process is knob-shaped, not elongated. The sides of the long limb of the incus are not parallel. The lenticular process is, relative to the size of the incus, small. The posterior and anterior crus of the stapes are more nearly straight than intaylori. A prominent muscular process occurs on the posterior crus.
The auditory ossicles of representative species of all the subgenera ofPeromyscuswere studied as were the ossicles ofOnychomys,Ochrotomys,Oryzomys,Akodon,Thaptomys,Zygodontomys,Calomys,Reithrodontomys, andHolochilus.
Fig. 8.Lateral views of auditory ossicles. × 20.A.B. taylori analogous, adult, female, No. 28104 KU, 4 kms. ENE Tlalmanalco, 2290 meters, Estado de México.B.B. musculus pallidus, adult, male, No. 28346 KU, Cahuilotal, Sacacoyuca, 960 meters, Guerrero.
Fig. 8.Lateral views of auditory ossicles. × 20.A.B. taylori analogous, adult, female, No. 28104 KU, 4 kms. ENE Tlalmanalco, 2290 meters, Estado de México.B.B. musculus pallidus, adult, male, No. 28346 KU, Cahuilotal, Sacacoyuca, 960 meters, Guerrero.
The general plan of structure of the auditory ossicles inBaiomysresembles that inCalomys,Akodon, andThaptomys. The ossicles ofCalomysandThaptomys, in particular, closely resemble the auditory ossicles ofBaiomys musculus. The short process of the incus is knoblike inCalomysandThaptomys, and the general conformation of malleus and stapes in those two genera is nearly identical to that inB. musculus. InAkodon, the anterior and posterior crus of the stapes is more rounded than inB. musculus, resembling that inB. taylori.
Reithrodontomysdiffer fromBaiomysin having a more elongate orbicular apophysis on the body of the malleus, an elongated short limb on the incus, and a stapes having anterior and posterior crura bowed as in mice of the genusPeromyscus.
InOchrotomys, the orbicular apophysis of the malleus resembles the orbicular apophysis ofB. musculus, but the short process of the incus is longer, resembling the short process ofB. taylori. In general conformation of the malleus, incus, and stapes,Ochrotomysshows closer resemblance toB. taylorithan toB. musculus.
InHolochilusthe anterior crus and posterior crus of the stapes are similar to those inB. musculus, but in shape and size of malleus and incus,Holochilusdiffers considerably fromB. musculusandB. taylori.
InZygodontomys, size and shape of the ossicles differ greatly from those ofBaiomys.
In the genusPeromyscus, onlyPeromyscus floridanus(subgenusPodomys) possesses a knoblike short process on the incus similar to that inB. musculus; representatives of the other subgenera examined possess an elongated short limb on the incus. The conformation of the ossicles of bothOnychomysandOryzomysappears to be more nearly like that inPeromyscusthan that ofBaiomys.
On the basis of shape and size of auditory ossicles,Baiomysresembles South American hesperomines (CalomysandThaptomys) rather than North American hesperomines.
GenusBaiomysTrue
1894.BaiomysTrue, Proc. U. S. Nat. Mus., 16:758, February 7. Type,Hesperomys (Vesperimus) tayloriThomas.
Diagnosis.—Size small (total length in adults, 93-135); tail shorter than head and body; hind foot in adults 12-17; ears small (8-12) and rounded; upper parts blackish sepia to ochraceous-buff; underparts slaty gray to white or pale buffy; eyes small; hind feet having six plantar pads, soles nearly naked except for some hairs on anterior parts of soles and anteriorly to base of toes and between toes; occipitonasal length of skull in adults, 17.0-21.5; zygomatic breadth, 9.0-11.5; coronoid process of mandible well developed, strongly recurved; ascending ramus of mandible short and erect; anterior palatine foramina (incisive foramina) long, usually terminating posterior to plane of the front of first molars; posterior palatine foramina nearly opposite middle of M2; interorbital space wide relative to widest part of frontals; nasals projecting only slightly over incisors; condyle terminal; upper incisors relatively heavy; primary first fold of M3 obliterated at an early stage of wear; major cusps of upper and lower anteriormost two molars alternating, more so in m1-m2 than in M1-M2, dental formula I/i, 1/1; C/c, 0/0; P/p, M/m, 3/3 = 16.
For distribution of the genus, seeFigure 9.
Fig. 9.Geographic distribution of the genusBaiomys. Black area shows where the two species occur together. Black dot (Acultzingo, Veracruz) shows locality whereBaiomys taylorioccurs within the range ofB. musculus, butB. musculusis not known to occur at that locality.
Fig. 9.Geographic distribution of the genusBaiomys. Black area shows where the two species occur together. Black dot (Acultzingo, Veracruz) shows locality whereBaiomys taylorioccurs within the range ofB. musculus, butB. musculusis not known to occur at that locality.
SYSTEMATIC ACCOUNT OF SPECIES AND SUBSPECIES
Baiomys musculus
Southern Pygmy Mouse
(Synonymy under subspecies)
Type.—Sitomys musculusMerriam, Proc. Biol. Soc. Washington, 7:170, September 29, 1892.
Range.—Southern Nayarit, Michoacán, México, Morelos, Puebla, and central Veracruz, southeastward to western Nicaragua, but unknown from southern Veracruz, Tabasco, and the Yucatán Peninsula (seeFigure 10); occurs principally in the arid upper and lower divisions of the Tropical Life-zone.Characters for ready recognition.—Unless otherwise noted, characters are usable only for the two age-categories of adult and old adult. Differs fromB. tayloriin: hind foot 16 millimeters or more; occipitonasal length, 19 millimeters[Pg 609]or more; zygomatic breadth, 10 millimeters or more; rostrum not deflected ventrally at frontoparietal suture but, instead, curving gradually toward anteriormost point of nasals; cingular ridges and secondary cusps on teeth more pronounced; basihyal having anterior pointed entoglossal process, shoulders of basihyal protruding anteriorly (characteristic of all age categories); baculum having broader shaft, spatulate to knob-shaped tip, wings at base projecting anteriorly; baculum more than 3 millimeters long; short process of incus knob-shaped rather than attenuate; muscular process of posterior crus of stapes prominent.Characters of the species.—Size large (extremes in external measurements of adults; total length, 100-135; length of tail vertebrae, 33-56; length of hind foot, 14.1-17; length of ear, 9-12); upper parts dark reddish brown, or ochraceous-buff to nearly black; underparts pale pinkish buff to white or pale buffy.
Range.—Southern Nayarit, Michoacán, México, Morelos, Puebla, and central Veracruz, southeastward to western Nicaragua, but unknown from southern Veracruz, Tabasco, and the Yucatán Peninsula (seeFigure 10); occurs principally in the arid upper and lower divisions of the Tropical Life-zone.
Characters for ready recognition.—Unless otherwise noted, characters are usable only for the two age-categories of adult and old adult. Differs fromB. tayloriin: hind foot 16 millimeters or more; occipitonasal length, 19 millimeters[Pg 609]or more; zygomatic breadth, 10 millimeters or more; rostrum not deflected ventrally at frontoparietal suture but, instead, curving gradually toward anteriormost point of nasals; cingular ridges and secondary cusps on teeth more pronounced; basihyal having anterior pointed entoglossal process, shoulders of basihyal protruding anteriorly (characteristic of all age categories); baculum having broader shaft, spatulate to knob-shaped tip, wings at base projecting anteriorly; baculum more than 3 millimeters long; short process of incus knob-shaped rather than attenuate; muscular process of posterior crus of stapes prominent.
Characters of the species.—Size large (extremes in external measurements of adults; total length, 100-135; length of tail vertebrae, 33-56; length of hind foot, 14.1-17; length of ear, 9-12); upper parts dark reddish brown, or ochraceous-buff to nearly black; underparts pale pinkish buff to white or pale buffy.
Geographic variation.—Eight subspecies are here recognized (seeFigure 10). Features that vary geographically are external size, color of pelage, certain cranial dimensions (occipitonasal length, zygomatic breadth, least interorbital breadth, length of rostrum, length of incisive foramina, depth and breadth of cranium, and alveolar length of upper molar tooth-row).
External and cranial size (except forB. m. handleyi) is less in the southernmost subspecies,B. m. pullus,B. m. grisescens,B. m. nigrescens, and more in the northernmost subspecies,B. m. musculus,B. m. brunneus, andB. m. infernatis. 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. 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 bothB. tayloriandB. musculus. InB. musculus, the darker subspecies,B. m. brunneus,B. m. nigrescens, andB. m. pullus, occur in zones of rather constant high relative humidity, whereas the paler subspeciesinfernatis,musculus,handleyi, and to a less extentgrisescensandpallidus, occur in zones of lower relative humidity. This is in keeping with Gloger's Rule, which states that melanins increase in the warm and humid parts of the range of a species, and reddish or yellowish-brown phaeomelanins prevail in arid climates.B. m. musculusranges into areas where relative humidity is such that darker pelages might be expected, but this is in the area where the two species are sympatric, and color of pelage may be an important character of recognition.