The house mouse, Mus musculus, is widely distributed throughout Eurasia and northern Africa where it displays spectacular geographic variation in color and proportions as an adaptation to regional climates and soils. Its geographic races contain alleles beyond the wildest expectations of those geneticists familiar only with the albino. Now that the fascinating wild forms are being colonized in laboratories, the reasonable demand to know their scientific names is embarrassing the museum taxonomist, who must admit that the nomenclature of the species is utterly chaotic. I have dealt with the relatively simple situation in Asia ( 1) where the only trouble was the confounding of Mus caroli with M. musculus in much of the literature [(stemming from Allen ( 2)]. Variation in Asian M. musculus follows geographic clines and the one subspecies carried by commerce, Mus musculus castaneus, has colonized cities in precisely that area lacking native outdoor populations of house mice. Even in Nepal [contra Schwarz and Schwarz ( 3)] the dark-bellied castaneus of Kathmandu is not in extended contact with white-bellied M. m. homourous. The latter lives at a much higher altitude, the only outdoor mouse around Kathmandu being the unrelated Mus cervicolor.
In Europe and around the Mediterranean it appears that house mice have been carried by commerce, and have secondarily invaded area already occupied by "country mice." Perhaps the newcomers are able to gain a foothold only because they start out in buildings. The result of interbreeding between city and country mice is a bewildering polymorphism and local variation in coloration and tail length that has spawned many scientific names and endless taxonomic confusion. Furthermore, the literature on European mice indicates that it is possible to find two recognizably different kinds of house mice, one in the houses and the other in the adjacent fields.
I have been asked by this symposium to evaluate the literature and nomenclature of European house mice. Although I have examined many specimens, including types of these European taxa, I have no field experience with the wild (outdoor) kinds of house mice. Furthermore, of the hundreds of museum specimens of European house mice I have examined, no more than a handful are furnished with the vital information on their labels as to whether they were caught inside or outside of the house! It seems we must start all over again by collecting properly authenticated specimens.
The best place to start, since the theories of Schwarz and Schwarz ( 3) have been discredited [by Jones and Johnson ( 4), Setzer ( 5), Rank ( 6), and Cockrum (7)], is with an excellent study by Zimmerman ( 8). Zimmerman shows that the Elbe River separates two entirely different subspecies of the house mouse. To the east and north a small, short-tailed, white-bellied Mus musculus musculus is capable of completing its annual cycle entirely in the fields. West from the Elbe is the large, long-tailed, dark-bellied Mus musculus domesticus confined to houses. The need now is for someone to extend the work of Klaus Zimmerman outside the boundaries of Germany. Here I shall report on a brief look at some of the specimens that point to the direction such study might take.
First, I believe I have discovered a skull trait that distinguishes pure Mus musculus domesticus. It is the straight-edged, narrow zygomatic plate ( Figure 43 in reference 1) which I contrasted with that of the Asiatic assemblage (Figure 46). I was incorrect in assigning the straight-edged plate to the entire musculus (European) group; I see now that most pure populations have a plate like that of castaneus. Tendencies toward straightness of the zygomatic plate might be useful, therefore, along with long tail, dark belly, and large size in assessing the amount of admixture of domesticus into a wild population. Such a problem can be much better solved genetically, of course, but morphological characters are necessary for use with museum voucher specimens. [I am unable to detect the slightest hint of reduction in molar surface and muzzle length, claimed as specialization of indoor mice by Schwarz and Schwarz ( 3)].
Tables 1 and 2 present a series of samples of house mice. Summarizing, the greatest morphological contrast between supposed outdoor and indoor mice is in the north. There it should be easiest to detect mixing: the indoor mouse of Germany is much darker on the back, belly, and feet, and is considerably larger and longer-tailed than its contrastingly pale, small, short-tailed, white-footed outdoor relative in Uppsala, Sweden (the type locally whence Linne named the taxon). Southward through France and into Spain, the "indoor" form gets much paler, and contrasts only slightly with the outdoor taxon, spretus. Nevertheless, spretus maintains a striking individuality separate from presumed contiguous indoor mice, unlike its neighbors in Italy, Sicily, and Greece. Dr. Sage is going to give you a genetic and ecologic explanation of this fact. So a pure Hispanic population is a cornerstone of any genetic study of the European house mice.
Nevertheless, we must build a genetic base with the two most contrasted populations, those with the least complications, namely the pure, outdoor musculus of Sweden and indoor domesticus of Germany west of the Elbe. Also the Hungarian "hillock mouse" or "mound-building mouse," hortulanus, should receive attention because of its reputed unique social organization and behavior (W. Prychodko in litt.).
No matter what genetic triumphs will supersede the pedestrian efforts of museum taxonomists, I must insist that without a few identifiable voucher specimens (skin with perfect skull) your work is meaningless zoologically. A glance at my Asian mouse paper ( 1) will show you how names change and change until somebody examines the type-specimen. To cite a mere scientific name in a controversial genus like Mus is almost meaningless. You have to say, "This is a virologic study of Mus caroli from Okinawa, the species with long tail, short nasals, and brown, pro-odont incisors represented by my specimen number 593 in the Smithsonian Institution." With Mus musculus you would have to state whether your colony or specimens came from a field, house, or natural vegetation.
1. Marshall, J.T., Jr. (1977). Bull. Amer. Mus. Nat. Hist. 158: 175.
2. Allen, G.M. (1927). Amer. Mus. Novitates No. 270, pp. 1-12.
3. Schwartz, E., and Schwartz, H.K. (1943). J. Mammology 24: 59.
4. Jones, J.K., Jr., and Johnson, D.H. (1965). Univ. Kansas Publ. Mus. Nat. Hist. 16: 357.
5. Setzer, H.W. (1957). J. Egyptian Public Health Assoc. 32: 41.
6. Ranck, G.L. (1968). Smiths. Inst., U.S. Nat. Mus. Bull. 275: 264.
7. Cockrum, E.L. "Mammals of Tunisia" (ms.)
8. Zimmerman, K. (1949). Zool. Jahrb. f. Systematic 78: 301.