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.
[Illustration: FIG. 5. Ventral view of hyoid bones. x 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 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_ principally 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.
[Illustration: 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_.
[Illustration: FIG. 7. Dorsal view of bacula. x 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 knob-shaped having indentation at tip, instead
of anterior part spatulate-shaped (in some) to knob-shaped (see Figure
7), without indentation; significantly shorter (see Table 4).
TABLE 4.--Length of Bacula
==============+===========+=========+==========+===========+==========
| Number of | Average | 3 x | 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
--------------+-----------+---------+----------+-----------+----------
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 in Table 4, but including young and
subadults, only three bacula of _B. taylori_ were longer than 3 mm., and
only one baculum of _B. 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 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_. The bacula of
_Baiomys_, as pointed out by Blair (_op cit._:203), differ as much from
those of the genus _Peromyscus_ as do the bacula of _Reithrodontomys_
and _Onychomys_. In size of baculum, _Baiomys_ resembles _Ochrotomys_.
Blair (_op. cit._:202) pointed out that the length of the baculum of _B.
taylori subater_ was contained in the length of the animal's body 20.3
times, and 24.2 times in the length of that of _Ochrotomys nuttalli_.
The length of the baculum of _B. 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 in _Ochrotomys_. When bacula of both species of
_Baiomys_ were compared to those of _O. nuttalli_, bacula of _B.
musculus_ were found to most closely resemble those of _O. nuttalli_.
The baculum of a single specimen of _Calomys_ (_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 of _C. laucha_ resembles the
baculum of _Ochrotomys_ and _Baiomys musculus_.
Blair (_op. cit._:201) considers generic _versus_ subgeneric rank for
_Ochrotomys_, and on the basis of studies of the phallus Hooper
(1958:23) stated that "it is clear that _nuttalli_ should be removed
from _Peromyscus_ and should be listed as _Ochrotomys nuttalli_
(Harlan)." I agree with Hooper (_loc. cit._) and point out that on the
basis of the baculum, there is less of a hiatus between _Baiomys_ on the
one hand, and _Ochrotomys_ and _Calomys_ on the other hand, than there
is between any one of those three genera and _Peromyscus_.
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 between _Peromyscus_ and _Baiomys_
is judged to be the result of such a convergence of habitus and the
baculum in _Baiomys_ is thought to reflect relationships more accurately
than does the skull.
_Auditory ossicles._--Examination of a number of auditory ossicles of
_Baiomys_ reveals constant interspecific differences in the malleus and
incus. 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.
In _Baiomys musculus_, the orbicular apophysis of the malleus (see
Figure 8, B) is round to oblong, and less ovoid than in _B. taylori_;
the anterior process is less acutely pointed than in _B. taylori_, and
the neck is long, less recurved than in _B. 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 in _taylori_. A prominent muscular process
occurs on the posterior crus.
The auditory ossicles of representative species of all the subgenera of
_Peromyscus_ were studied as were the ossicles of _Onychomys_,
_Ochrotomys_, _Oryzomys_, _Akodon_, _Thaptomys_, _Zygodontomys_,
_Calomys_, _Reithrodontomys_, and _Holochilus_.
[Illustration: FIG. 8. Lateral views of auditory ossicles. x 20.
A. _B. taylori analogous_, adult, female, No. 28104 KU, 4 kms.
ENE Tlalmanalco, 2290 meters, Estado de Mexico.
B. _B. musculus pallidus_, adult, male, No. 28346 KU, Cahuilotal,
Sacacoyuca, 960 meters, Guerrero.]
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 _Akodon_, the anterior and posterior crus of the
stapes is more rounded than in _B. musculus_, resembling that in _B.
taylori_.
_Reithrodontomys_ differ from _Baiomys_ in 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 genus _Peromyscus_.
In _Ochrotomys_, the orbicular apophysis of the malleus resembles the
orbicular apophysis of _B. musculus_, but the short process of the incus
is longer, resembling the short process of _B. taylori_. In general
conformation of the malleus, incus, and stapes, _Ochrotomys_ shows
closer resemblance to _B. taylori_ than to _B. musculus_.
In _Holochilus_ the anterior crus and posterior crus of the stapes are
similar to those in _B. musculus_, but in shape and size of malleus and
incus, _Holochilus_ differs considerably from _B. musculus_ and _B.
taylori_.
In _Zygodontomys_, size and shape of the ossicles differ greatly from
those of _Baiomys_.
In the genus _Peromyscus_, only _Peromyscus floridanus_ (subgenus
_Podomys_) possesses a knoblike short process on the incus similar to
that in _B. musculus_; representatives of the other subgenera examined
possess an elongated short limb on the incus. The conformation of the
ossicles of both _Onychomys_ and _Oryzomys_ appears to be more nearly
like that in _Peromyscus_ than that of _Baiomys_.
On the basis of shape and size of auditory ossicles, _Baiomys_ resembles
South American hesperomines (_Calomys_ and _Thaptomys_) rather than
North American hesperomines.
Genus =Baiomys= True
1894. _Baiomys_ True, Proc. U. S. Nat. Mus., 16:758, February 7.
Type, _Hesperomys (Vesperimus) taylori_ Thomas.
_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, see Figure 9.
[Illustration: FIG. 9. Geographic distribution of the genus
_Baiomys_. Black area shows where the two species occur together.
Black dot (Acultzingo, Veracruz) shows locality where _Baiomys
taylori_ occurs within the range of _B. musculus_, but _B. musculus_
is not known to occur at that locality.]
SYSTEMATIC ACCOUNT OF SPECIES AND SUBSPECIES
=Baiomys musculus=
Southern Pygmy Mouse
(Synonymy under subspecies)
_Type._--_Sitomys musculus_ Merriam, Proc. Biol. Soc. Washington,
7:170, September 29, 1892.
_Range._--Southern Nayarit, Michoacan, Mexico, Morelos, Puebla, and
central Veracruz, southeastward to western Nicaragua, but unknown from
southern Veracruz, Tabasco, and the Yucatan Peninsula (see Figure 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 from _B. taylori_ in: hind foot 16 millimeters or more;
occipitonasal length, 19 millimeters 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 (see
Figure 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 for _B. 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_, and _B. 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 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. 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. musculus_
ranges 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.
[Illustration: FIG. 10. Distribution of _Baiomys musculus_. Known
localities of occurrence are represented by circles and black dots;
the former denote localities that are peripheral (marginal) for the
subspecies concerned.
1. _B. m. brunneus_
2. _B. m. grisescens_
3. _B. m. handleyi_
4. _B. m. infernatis_
5. _B. m. musculus_
6. _B. m. nigrescens_
7. _B. m. pallidus_
8. _B. m. pullus_]
_Natural History_
_Habitat and numbers._--In Veracruz, Dalquest obtained the southern
pygmy mouse in stands of tall grass (_Spartina?_) in sandy loam soil
bordering, and in, dense vegetation; Davis (1944:394) found the species
living in dense stands of grasses and seemingly utilizing underground
burrows. Near Chilpancingo, Guerrero, rocky situations seemed to be the
preferred habitat. Davis (_loc. cit._) believed that the species has a
wide tolerance to kinds of habitats. In Morelos, Davis and Russell
(1954:75) found these mice to be abundant along rock fences separating
cultivated fields, and in arid lowlands. In Colima, Hooper (1955b:13)
obtained specimens from an open thorn forest in sparse grass and rocky
hillside bounding a stream and in litter below shrubs on the floor of a
nut-palm forest; in Michoacan, these mice were taken in cane grass,
shrubs, and mesquite near an irrigation ditch. From Guatemala, Goodwin
(1934:39, 40) records specimens from Sacapulas, a hot, dry, sandy area
where cactus and sparse grasses are present, and from La Primavera, on
the edges of pine-oak-alder forests. Felten (1958:137) has taken
_musculus_ from bushy areas in El Salvadore. In 1955, I obtained the
southern pygmy mouse 6 mi. SW Izucar de Matemores, Puebla, along a
stream in heavy grass bordered by cypress, willow, fig, bamboo, and in
rocky grazed area near sugar cane fields.
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 Izucar de Matemores, the pygmy mouse was the most
common rodent. I have trapped for it in Oaxaca and Veracruz in habitats
that seemed almost identical to those mentioned by Dalquest, and also
that at Izucar de Matemores, Puebla, with almost no success. The reason
for the seeming disparity in numbers at different localities having
nearly the same kind of habitat is unknown to me and bears further
investigation.
_Behavior._--Little is recorded concerning the behavior of this species.
David and Russell (_op. cit._:76) found that of small mammals _B.
musculus_ was the first to appear at night. I caught mice of this
species by hand in the afternoon in Puebla. They seemed to be active
from noon until dark. Albert Alcorn wrote in his field notes that
specimens were taken near noon at a place 9 mi. NNW Esteli, Nicaragua.
My impression is that _musculus_ is diurnal to crepuscular.
_Enemies and food._--Owl pellets (thought to be those of a barn owl,
_Tyto alba_) from within the geographic range of _B. musculus_, from 6
mi. SW Izucar de Matemores, yielded mandibular tooth-rows belonging to
_musculus_. Presumably, most of the carnivorous mammals and raptorial
birds within the range of the southern pygmy mouse could be listed as
enemies. Diurnal to crepuscular habits of this mouse may protect it from
some of the nocturnal carnivorous mammals and raptorial birds.
Food of the southern pygmy mouse includes nuts, bark, grass seeds, and
leaves. Dalquest (MS) writes that bits of banana proved to be useful
bait in trapping these mice in Veracruz.
_Reproduction._--Notations concerning lactation and embryos on specimen
labels of females suggest that the southern pygmy mouse breeds in all
months. I have records of pregnant or lactating females in every month,
save January, April, May, and June. The average of 26 counts of embryos
or young per litter is 2.92 (1-4).
=Baiomys musculus brunneus= (J. A. Allen and Chapman)
_Peromyscus musculus brunneus_ J. A. Allen and Chapman, Bull. Amer.
Mus. Nat. Hist., 9:203, June 16, 1897; Elliott, Field Columb. Mus.
Publ., 105(4):136, July 1, 1905; Elliott, Field Columb. Mus. Publ.,
115(8):203, 1907; Osgood, N. Amer. Fauna, 28:259, April 17, 1909.
_Baiomys musculus brunneus_, Miller, Bull. U. S. Nat. Mus., 79:137,
December 31, 1912; Miller, Bull. U. S. Nat. Mus., 128:318, April 29,
1924; Ellerman, The Families and Genera of Living Rodents, 2:402,
March 21, 1941; Goldman, Smith. Miscl. Coll., 115:437, July 31,
1951; Goodwin, Bull. Amer. Mus. Nat. Hist., 102:318, August 31,
1953; Miller and Kellogg, Bull. U. S. Nat. Mus., 205:512, March 3,
1955; Booth, Walla Walla Publs., Dept. Biol. Sci., 20:15, July 10,
1957; Hall and Kelson, The Mammals of North America, 2:661, March
31, 1959 (part).
[_Peromyscus musculus_] _brunneus_, Elliott, Field Columb. Mus.
Publ., 95(4): 176, 1904.
_Peromyscus musculus_ [_musculus_], Osgood, N. Amer. Fauna, 28:258,
April 17, 1909 (part).
_Baiomys musculus musculus_, Davis, Jour. Mamm., 25:394, December
12, 1944 (part); Goldman, Smith Miscl. Coll., 115:437, July 31,
1951; Hooper, Jour. Mamm., 33:97, February 18, 1952 (part); Hall and
Kelson, The Mammals of North America, 2:661, March 31, 1959 (part).
_B._ [_aiomys_] _m._ [_usculus_] _brunneus_, Hooper, Jour. Mamm.,
33:96, February 18, 1952.
_Baiomys taylori_, Hooper, Jour. Mamm., 33:97, February 18, 1952
(part).
_Type._--Adult female, skin and skull; No. 12535/10845 American Museum
of Natural History; Jalapa, Veracruz, Republic of Mexico; obtained on
April 13, 1897, by F. M. Chapman, original number 1203.