Key to the Bacula of Some North American Microtines

1. Length of lateral digital ossifications more than 1/3 breadth of stalk2
1´. Length of lateral digital ossifications less than 1/3 breadth of stalk or absent15
2. Size small (total length of baculum less than 5.5 mm.)3
2´. Size large (total length of baculum more than 5.5 mm.)14
3. Width at mid-point of stalk more than 1/3 greatest breadth of stalk4
2´. Size large (total length of baculum more than 5.5 mm.)14
3´. Width at mid-point of stalk less than 1/3 greatest breadth of stalk,8
4. Stalk, viewed from proximal end hour-glass shaped, and width of stalk less than ½ length of stalk.... Phenacomys intermedius,[p. 197]
4´. Stalk not both hour-glass shaped when viewed from proximal end, and with width less than ½ length of stalk5
5. Shaft thin basally, thickness less than 1/3 of greatest breadth6
5´. Shaft thick basally, thickness 1/3 or more of greatest breadth7
6. Stalk more or less straight, base not deflected. Microtus oeconomus,[p. 204]
6´. Stalk spatulate, and base deflected from axis of shaft.... Microtus guatemalensis,[p. 198]
7. Base enlarged, depth nearly ½ of breadth.... Lemmus trimucronatus,[p. 193]
7´. Base moderately enlarged, depth near 1/3 of breadth.... Microtus pennsylvanicus, [p. 206], or Microtus townsendii,[p. 204]
8. Base hour-glass shaped as viewed from proximal end.... Phenacomys intermedius,[p. 197]
8´. Not so9
9. Lateral processes separated from tip of shaft by more than the thickness of the lateral process10
9´. Lateral processes separated from tip of shaft by less than the thickness of the lateral process11
10. Lateral processes more than ½ the width of median process.... Microtus longicaudus,[p. 201]
10´. Lateral processes slender, less than ½ the width of median process.... Microtus montanus,[p. 204]
11. Lateral ossifications equal in length to median ossification.... Clethrionomys,[p. 194]
11´. Lateral ossifications shorter than median ossification12
12. Size small, less than 3.4 mm. in total length.... Microtus oregoni,[p. 199]
12´. Size medium, more than 3.4 mm. in total length13
13. Greatest width of stalk at a point about 1/3 of length of stalk from base.... Microtus chrotorrhinus (Hamilton, 1946:382).
13´. Greatest width of stalk at a point less than 1/3 of length of stalk from base.... Lagurus curtatus,[p. 210]
14. Size of baculum larger, base more than 3 mm. wide, processes all well developed.... Ondatra zibethicus,[p. 198]
14´. Size of baculum smaller, base less than 3 mm. wide, processes poorly developed in some animals.... Neofiber alleni,[p. 209]
15. At least one digital ossification present16
15´. Digital ossifications not present.... Dicrostonyx groenlandicus,[p. 193]
16. Breadth of stalk at least ½ length of stalk17
16´. Breadth of stalk less than ½ length of stalk19
17. Length of stalk greater than 3.6 mm. and less than 1½ times its greatest breadth.... Microtus richardsoni,[p. 199]
17´. Length of stalk usually less than 3.6 mm., or if more than 3.6 mm. (up to 4.0 mm.) then length 1½ times or more its greatest breadth18
18. Median process attenuate distally in dorsal view, and relatively long (more than twice its own breadth), 1/5 to 3/5 the length of stalk; breadth of stalk usually 2/3 or more length of stalk.... Microtus miurus,[p. 200]
18´. Median process relatively blunt distally in dorsal view, relatively short (usually less than ¼ length of stalk), breadth of stalk usually less than 2/3 length of stalk.... Pitymys, [p. 208], Pedomys, [p. 207], or Microtus mexicanus,[p. 205]
19. Distal processes small and firmly ankylosed to distal end of shaft.... Phenacomys longicaudus,[p. 197]
19´. Distal processes if present not firmly ankylosed to distal end of shaft20
20. Dorsal concavity of base as viewed from proximal end usually deeper than ventral concavity.... Microtus mexicanus,[p. 205]
20´. Dorsal and ventral concavities of base equal in depth or ventral one the deeper21
21. Total length of baculum more than 3.6 mm.... Microtus californicus,[p. 205]
21´. Total length of baculum less than 3.6 mm.... Synaptomys cooperi,[p. 194]

DISCUSSION

Owing to shortness of lower incisors and present geographic distribution of the species, Hinton (1926:35) considered the Tribe Lemmi (lemmings) to be more primitive than the Tribe Microti (voles). The surviving lemmings are specialized in many features and therefore are considered as advanced end-products of an evolutionary radiation of a primitive microtine stock, of which all earlier stages are extinct.

Hinton regarded Dicrostonyx as the most primitive of the genera of lemmings on account of its more complex molar teeth (complexity was considered to be primitive), and on account of the presence of three primitive longitudinal rows of tubercles in unworn molars. The other three genera were arranged in order of increasing specialization as follows: Synaptomys, Myopus, Lemmus.

If the baculum tended to retain its primitive character while specializations in the external anatomy developed, and if the above arrangement is correct the most primitive bacula would be found in Dicrostonyx and in Synaptomys. The baculum in these two genera in comparison to that in Myopus (as figured by Ognev, 1948:512) and Lemmus has a slenderer stalk and smaller digital ossifications or none at all. The baculum in the genera of lemmings increases in robustness and the development of processes from Dicrostonyx, to Synaptomys, to Myopus, to Lemmus—the same order outlined above for total of specialization. The two extremes in this series are near the extremes of variation in bacula to be found in all microtines. The baculum in lemmings as a group cannot then be considered more primitive than in voles as a group, although the voles are usually considered to be more advanced. The situation in the voles, as we shall see, casts a different light on the matter.

The voles, Tribe Microti, were considered by Hinton (1926:40) to be more advanced than the lemmings because the incisors of the voles are longer and the root of their last lower molar is lingual to the root of the incisor. Hinton thought also that the murine ancestors of microtines had shorter incisors and that the backward extension of the incisors in the voles is a more ancient feature than the hypsodonty of the molars. A trend in the molar teeth has been toward greater hypsodonty. The voles in which the molars are least hypsodont are thus considered primitive. These include the living genera Clethrionomys, Phenacomys, Ondatra, Dolomys, Ellobius, and Prometheomys. Therefore, the baculum, in these assumedly primitive genera, would be expected to resemble the baculum in the lemmings or at least the most primitive lemmings. This is not the case.

The bacula that I have examined of Clethrionomys and Phenacomys have well-developed digital ossifications. In this they resemble the baculum of the genus Lemmus, the most advanced genus of lemmings according to Hinton. The baculum of Dolomys has not been studied. The baculum in Ondatra, and in Prometheomys as illustrated by Ognev (1948:552), also possesses well-developed processes. The baculum of Ellobius is small and lacks processes (as figured by Ognev, 1950:662). No ossification was found in a single specimen of Ellobius examined by me although the entire glans penis was removed and cleared without dissection. So far as known then, with the exception of Ellobius and Phenacomys longicaudus (Dearden, 1958:547), the primitive microtines having rooted molars possess bacula having three well-developed ossified processes.

Voles of the genus Microtus vary in the structure of the baculum almost as much as do the lemmings. Within the single subgenus Microtus some individuals of Microtus mexicanus, for example, have minute ossified lateral processes and other individuals lack these processes; Microtus pennsylvanicus and some other species have proportionately large lateral ossifications. If the well-developed condition of the baculum in the microtines having rooted molars is primitive, then within the genus Microtus those species having well-developed bacula may be considered primitive.

The genera Lagurus and Neofiber have moderately developed or well-developed lateral processes. Neofiber exhibits a tendency, not prominent elsewhere, to have a proportionately smaller median process rather than reduced lateral processes.