Table 1.

Unstandardized discriminant function coefficients using one or two morphological characters of four Peromyscus species (P. gossypinus, P. leucopus, P. maniculatus, and P. polionotus) in the southeastern United States. Each sections contains the characters used, unstandardized discriminant coefficients, constants, and numbers of errors for each set of species. Discriminant scores for each pair of species have opposite signs, and this is indicated as one species greater than zero. Variables defined below.

   Species Pair       Skull only           Skin and Skull
---------------  --------------------   --------------------
P. gossypinus vs. P. leucopus
                SL              0.927   Foot           0.593
                MTL             3.732   SL             0.821
                Constant      -38.229   Constant     -34.125
                Pg scores > 0           Pg scores > 0
                Errors: Pl 2; Pg 2      Errors: Pg 5; Pl 1

P. gossypinus vs. P. maniculatus
                SL              0.794   BNL            0.685
                BD              2.123   BD             2.403
                Constant      -39.780   Constant     -37.611
                Pg scores > 0           Pg scores > 0
                Errors: none            Errors: Pg 1; Pm 0

P. gossypinus vs. P. polionotus
                SL              1.000   Tail           1.000 
                Constant      -25.000   Constant     -55.000
                Pg scores > 0           Pg scores > 0 
                Errors: none            Errors: none                                         

P. leucopus vs. P. maniculatus
                OC              2.523   Tail           0.165
                BD              2.754   BNL           -1.028
                Constant      -33.762   Constant      10.271
                Pl scores > 0           Pm scores > 0
                Errors: Pm 15; Pl 24    Errors: Pl 0; Pm 1

P. leucopus vs. P. polionotus
                SL              1.214   SL             0.953
                MTL             2.636   Tail           0.085
                Constant      -37.389   Constant     -27.251
                Pl scores > 0           Pl scores > 0
                Errors: Pp 1; Pl 2      Errors: Pl 3; Pp 0

P. maniculatus vs. P. polionotus
                SL              0.680   Tail           1.000
                TTL             1.803   ---              ---
                Constant      -35.126   Constant     -60.000
                Pm scores > 0           Pm scores > 0
                Errors: Pp 4; Pm 8      Errors: none

Methods

One of us (JL) measured 14 cranial characters to the nearest 0.1 mm using dial calipers, and recorded three external measurements from specimen tags. We estimated age from pelage characters (no juvenile gray), tooth wear (significant wear on all major cusps), and degree of cranial suture fusion and only measured adults in age classes 4 - 6 (Schmidly 1973). We excluded specimens with missing data, juveniles, and subadults from all analyses. Mensural characters (Choate et al. 1973; DeBlase and Martin 1981) included: head and body length (body), tail length (tail), hind foot length (foot), greatest skull length (SL), basonasal length (BNL), rostral breadth (RB), nasal length (NL), interorbital constriction (OC), zygomatic breadth (ZB), bony palate length (PL), maxillary toothrow length (MTL), total toothrow length (TTL), palatal width (PW), pterygoid breadth (PB), bullar depth (BD), and anterior palatal (incisive) foramen length (PFL). We measured rostral length (RL) from the anteriormost point of the nasals to the anterior edge of the zygomatic arch, and body length was calculated as the difference between total and tail lengths. We excluded ear length due to predominance of missing data.

Use of the discriminant function

Discriminant analysis combines variables to generate a set of linearly independent axes upon which specimens, after appropriate transformation, can be plotted and their classification determined. The appropriate transformation is to multiply each morphological character variable (e. g., foot length, skull length) by its discriminant function coefficient, sum the products, and add a constant (for each axis separately). In general:

D1 = B10 + B11X1 + B12X2 + B13X3 + ... + B1nXn
D2 = B20 + B21X1 + B22X2 + B23X3 + ... + B2nXn

where D1 is the specimen's discriminant score on the first axis, the B1i's are discriminant function coefficients estimated from the data for the first axis (Bi0's are constants), and the Xi's are the values of the original variables. This is done separately for each axis, and the scores, D1, D2, ..., Dn, form the coordinate of the specimen's location in the n-dimensional discriminant space. For example, to separate P. gossypinus from P. leucopus using external and skull measurements, the appropriate
transformation is (only one axis is needed):

D = -34.125 + 0.593(foot) + 0.821(skull length).

Which, given an unknown specimen with hindfoot and skull lengths of 23.5 mm and 28.7 mm, respectively, and the coefficients of these measurements from Table 3, becomes:

3.377 = -34.125 + 0.593(23.5) + 0.821(28.7).

In this case, any positive value of D indicates P. gossypinus, and any negative value of D indicates a P. leucopus; thus this specimen is a P. gossypinus.

Literature Cited

Schmidly, D. J. 1973. Geographic variation and taxonomy of Peromyscus boylii from Mexico and the southern United States. J. Mamm. 54:111-130.

DeBlase, A. F., and R. E. Martin. 1981. A manual of mammalogy with keys of the families of the world. Second edition. Wm. Brown Co., Dubuque.

Choate, J. R. 1973. Identification and recent distribution of white-footed mice (Peromyscus) in New England. J. Mamm. 54:41-49.