Speciation and Evolution of the Pygmy Mice, Genus Baiomys Part 3

_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 spa.r.s.e.

_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, princ.i.p.al 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 (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.

_Old Adults._--Characterized princ.i.p.ally 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-groups of Baiomys taylori from vic.

(see p. 595) Altamira, Tamaulipas, Mexico.

=============+===========+===========+===========+===========+=========== Age groups | Juvenile | Young | Subadult | Adult | Old adult -------------+-----------+-----------+-----------+-----------+----------- Number | | | | | examined | 3 | 3 | 14 | 19 | 8 | | | | | | | | | | Total length | 77.0 | 92.6 | 97.6 | 99.9 | 101.6 | (74-79) | (89-96) | (91-103)| (93-105) | (98-107) | | | | | | | | | | Length | 27.3 | 39.3 | 40.4 | 39.8 | 40.9 of tail | (24-29) | (37-41) | (36-43) | (35-45) | (38-45) | | | | | | | | | | Length | 49.6 | 53.3 | 57.0 | 60.0 | 60.7 of body | (49-50) | (52-55) | (51-61) | (56-67) | (57-67) | | | | | | | | | | Length of | 11.0 | 13.6 | 14.3 | 14.5 | 14.2 hind foot | (11) | (13-14) |(13.5-15.0)| (14-15) | (13-15) | | | | | | | | | | Occipitonasal| 14.2 | 16.3 | 17.1 | 17.7 | 17.8 length |(13.6-15.2)|(15.8-16.9)|(16.7-17.6)|(17.2-18.3)|(17.6-18.1) | | | | | | | | | | Zygomatic | 8.1 | 8.7 | 8.9 | 9.3 | 9.4 breadth | (7.8-8.6) | (8.6-8.8) | (8.6-9.3) | (9.0-9.6) | (9.1-9.6) | | | | | | | | | | Interorbital | 3.4 | 3.4 | 3.4 | 3.6 | 3.5 breadth | (3.3-3.5) | (3.3-3.6) | (3.3-3.6) | (3.4-3.8) | (3.3-3.6) | | | | | | | | | | Incisive | | | | | foramina | 2.9 | 3.5 | 3.7 | 3.9 | 3.9 (length) | (2.8-2.9) | (3.4-3.6) | (3.6-3.9) | (3.6-4.1) | (3.5-4.0) | | | | | | | | | | Depth | 5.9 | 6.5 | 6.5 | 6.7 | 6.8 of cranium | (5.6-6.2) | (6.3-6.8) | (6.2-6.8) | (6.4-7.0) | (6.5-7.1) | | | | | | | | | | Alveolar | | | | | length, | 2.7 | 2.9 | 2.9 | 3.0 | 3.0 upper molars | (2.5-2.8) | (2.9-3.0) | (2.8-3.1) | (2.9-3.2) | (3.0-3.1) | | | | | | | | | | Postpalatal | 4.8 | 5.9 | 6.2 | 6.5 | 6.5 length | (4.5-5.3) | (5.8-6.0) | (5.8-6.6) | (6.2-7.2) | (6.3-6.7) | | | | | | | | | | Breadth | 8.1 | 8.5 | 8.4 | 8.6 | 8.6 of braincase | (7.8-8.7) | (8.5) | (8.0-8.7) | (8.3-8.9) |(8.4-8.8) -------------+-----------+-----------+-----------+-----------+-----------

SECONDARY s.e.xUAL VARIATION

The method employed by Dice and Leraas (1936:2) was used to measure the secondary s.e.xual differences, if there were any, in each of several age cla.s.ses. As pointed out by Hooper (1952b:11), individual variation in small samples can obscure secondary s.e.xual differences. The samples of _B. taylori_ from the vicinity (see page 595) of Altamira, Tamaulipas, and the samples of _B. musculus_ from El Salvador (table 2) were large enough to prevent individual variation from obscuring s.e.xual differences. Nevertheless, no significant secondary s.e.xual differences were found in either _B. taylori_ or _B. musculus_ (see table 2).

Therefore, the s.e.xes have been considered together for purposes of geographic studies.

TABLE 2.--a.n.a.lysis of Secondary s.e.xual Variation in Adult B. taylori Vicinity of (see p. 595) Altamira, Tamaulipas, and Adult B.

musculus from El Salvador (see p. 595). (One Standard Deviation on Either Side of the Mean is Given.)

==============+==========================+============================ | Baiomys taylori | Baiomys musculus Character +------------+-------------+-------------+-------------- | 21 Males | 18 Females | 17 Males | 13 Females --------------+------------+-------------+-------------+-------------- | | | | Total length |98.4 2.95 |100.5 4.72 |112.04 5.49|113.12 4.23 | | | | Length of tail|40.1 2.31 | 40.3 2.39 | 47.12 2.95| 45.70 2.92 | | | | Length of body|57.83 1.65| 60.10 4.13| 66.67 3.97| 67.75 2.38 | | | | Length of | | | | hind foot |14.21 .53 | 14.44 .51 | 15.60 .49 | 15.38 .64 | | | | Length of ear |10.00 .00 | 10.00 .00 | 11.80 .65 | 12.00 .41 | | | | Occipitonasal | | | | length |17.48 .40 | 17.47 .47 | 19.32 .35 | 19.04 .44 | | | | Zygomatic | | | | breadth | 9.17 .33 | 9.15 .30 | 9.84 .21 | 9.91 .28 | | | | Least | | | | interorbital | | | | breadth | 3.53 .11 | 3.48 .11 | 3.88 .08 | 3.88 .12 | | | | Postpalatal | | | | length | 6.35 .19 | 6.38 .30 | 7.11 .15 | 6.95 .20 | | | | Depth | | | | of cranium | 6.65 .24 | 6.61 .17 | 7.10 .18 | 7.08 .18 | | | | Incisive | | | | foramina | | | | (length) | 3.82 .15 | 3.81 .18 | 4.43 .11 | 4.35 .14 | | | | Length | | | | of rostrum | 5.87 .20 | 5.88 .21 | 6.81 .16 | 6.66 .31 | | | | Breadth | | | | of braincase | 8.54 .23 | 8.52 .12 | 9.84 .38 | 9.52 .20 | | | | Alveolar | | | | length, | | | | upper molars | 2.98 .08 | 3.01 .08 | 3.20 .09 | 3.24 .10 --------------+------------+-------------+-------------+--------------

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 in table 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 External and Cranial Parts in a Population of B. Musculus and B. Taylori.

=====================+=========================+========================= | Baiomys taylori | Baiomys musculus +-------------------------+------------------------- | Vic. (see page 595) | Vic. (see page 595) Measurement | Altamira, Tamaulipas | El Salvador +-----------+-------------+------------+------------ | 21 Males | 18 Females | 17 Males | 13 Females | C. V. | C. V. | C. V. | C. V.

---------------------+-----------+-------------+------------+------------ | | | | Total length | 3.0 | 4.7 | 4.9 | 3.7 Length of tail | 5.7 | 5.9 | 6.2 | 6.4 Length of body | 2.8 | 5.0 | 5.9 | 3.5 Length of hind foot | 3.7 | 3.4 | 3.0 | 4.1 Length of ear | 0.0 | 0.0 | 5.5 | 3.3 | | | | Occipitonasal length | 2.2 | 2.7 | 1.8 | 2.3 Zygomatic breadth | 3.6 | 3.3 | 2.2 | 2.7 Interorbital breadth | 3.2 | 3.3 | 2.2 | 2.9 Incisive foramina | | | | (length) | 3.8 | 4.6 | 2.5 | 3.2 Depth of cranium | 3.6 | 2.5 | 2.5 | 2.5 Alveolar length, | | | | upper molars | 2.7 | 2.5 | 2.8 | 3.2 Postpalatal length | 3.1 | 4.7 | 2.1 | 2.9 Length of rostrum | 3.3 | 3.6 | 2.4 | 4.7 Breadth of braincase | 2.7 | 1.4 | 4.0 | 4.9 ---------------------+-----------+-------------+------------+------------

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 for _Peromyscus_ by 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, the 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. New hair appears ventrally and laterally at the end of 46 days (see Figure 4, part b). Hair replacement proceeds more slowly after the "saddle back" stage (described in _Peromyscus_ by Collins, 1918:80) has been reached. That stage was reached in two pygmy mice at 52 days (see Figure 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) in _Reithrodontomys_.

[Ill.u.s.tration: FIG. 4. Diagrams showing progress of the postjuvenal molt in pygmy mice. For explanation of a, b, and c, see text.

All approximately 2/3 natural 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 Mexico, 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 in _Peromyscus 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 male _B. t. taylori_, KU 35943, from 6 mi. SW San Geronimo, Coahuila, possessing black hairs throughout. Its hairs are longer and finer than those on specimens of comparable age and s.e.x. No albino was found, although Stickel and Stickel (1949:145) record one--an adult male of _B. 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 of _B. taylori_, in front of the frontonasal suture, is deflected three to five degrees ventrally in 85 per cent of the adults examined, and in _B. musculus_ is 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 in _Peromyscus_, _Sigmodon_, and _Oryzomys_, mice thought to be closely related to _Baiomys_. 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.

[Ill.u.s.tration: 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 a.n.a.logous_, adult, female, No. 36761 KU, 2 mi. N Ciudad Guzman, 5000 feet, Jalisco.]

_Hyoid apparatus._--Shape and, to a lesser extent, size of the hyoid apparatus differentiate nearly all specimens of _B. taylori_ from all those of _B. musculus_. The hyoid of _B. taylori_ differs from that of _B. musculus_ princ.i.p.ally in the shape of the basihyal. It possesses an anteriorly pointed entoglossal process in _B. musculus_, and is not rounded to completely absent as in _B. taylori_ (see Figure 5). The shoulders of the basihyal protrude anteriorly in _B. musculus_, and are not flattened as in _B. taylori_. The total length was measured in a sample of 55 basihyals of _B. musculus_, and was compared to the total length of a sample of 80 basihyals of _B. taylori_. The means of the two samples differ significantly at the 95 per cent level; the mean plus two standard errors of _B. musculus_ and _B. taylori_, are, respectively, 2.43 .02; 2.18 .03. There is sufficient overlap of the samples (mean plus one standard deviation of _B. musculus_ and _B. 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 (see Figure 6), all specimens studied, regardless of age or geographical origin, were separated at the level of species. The hypohyals of _B. taylori_ seemingly remain distinct throughout life; those of _B. musculus_ completely fuse in some adults. The ceratohyals are highly variable in shape and of little taxonomic use.

[Ill.u.s.tration: FIG. 6. Relationship of length of basihyal to occipitonasal length of skull. Black symbols, all below the curved line, represent measurements of _B. taylori_; open symbols, all above the curved line, represent measurements of _B. musculus_.]

The degree of geographic variation in shape of basihyal is not great.

Specimens of _B. musculus pallidus_ from 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 of _B. taylori taylori_ from 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 of _Baiomys_ are smaller than hyoids of all subgenera of _Peromyscus_.

In shape, the hyoids of _Baiomys_ resemble those of _Ochrotomys nuttalli_ (as explained on page 605, _Ochrotomys_ is here accorded generic, instead of subgeneric, rank). In size, the hyoid of both species of _Baiomys_ resembles that in _Reithrodontomys_. Sprague (1941:304) reports a resemblance in shape between the ceratohyals of _Baiomys_ and _Reithrodontomys_. The thyrohyals differ from those of _Reithrodontomys_, being less boot-shaped, and having a slight terminal expansion as in _Ochrotomys_ (see Sprague, _loc. cit._). In shape, the large basihyal of _Onychomys_ resembles the smaller one of _B.

musculus_. The basihyal of _Oryzomys_ lacks the entoglossal process present in _Baiomys_. On the basis of shape of hyoid, _Baiomys_ seems to be most closely related to _Ochrotomys_.

[Ill.u.s.tration: FIG. 7. Dorsal view of bacula. 16.

A. _B. musculus brunneus_, adult, No. 24336 KU, 3 kms.

W Boca del Rio, 10 feet, Veracruz.

B. _B. taylori taylori_, adult, No. 35937 KU, 6 mi.

SW San Geronimo, Coahuila.]

_Baculum._--Of _Baiomys_, 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 of _B. taylori_ differs from that of _B. musculus_ in: shaft narrow; wings anterior to base projecting dorsolaterally instead of anteriorly; anterior part k.n.o.b-shaped having indentation at tip, instead of anterior part spatulate-shaped (in some) to k.n.o.b-shaped (see Figure 7), without indentation; significantly shorter (see Table 4).

TABLE 4.--Length of Bacula

==============+===========+=========+==========+===========+========== | Number of | Average | 3 | 1 | Species | specimens | length | standard | standard | Range | | | error | deviation | --------------+-----------+---------+----------+-----------+---------- _B. taylori_ | 108 | 2.535 | .078 | .274 | 2.00-3.12 | | | | | _B. musculus_ | 58 | 3.324 | .090 | .233 | 2.80-3.88 --------------+-----------+---------+----------+-----------+----------