Abstract:
Mammal crania are important objects of study since their measurable features may often be correlated with sex, relative age, and taxonomic position of the specimens from which they were taken. The house mouse. Mus musculus L. , had not been studied in Delaware County, Indiana prior to the present investigation. My study, a part of a broader ecological study that extended from September 1968 through March 1969, examines the value of skull characteristics as an ageing technique for feral house mice. Sexual dimorphism and skull length differences which occur between age classes are also considered.Data collected during this study were compared with data collected on Sand Island, Johnson Atoll, Central Pacific Ocean. Work done by Kish (1969) indicates very little sexual dimorphism in Sand Island house mice. This agrees with a study done on the Norway rat, Rattusnorvegicus, near Baltimore, Maryland (Gentile, 1952). Gentile found a small amount of sexual dimorphism in his study rats. The males were found to have slightly greater palatal length and the mandible was longer than that of the female. These differences were not great enough to enable him to sex an animal of unknown sex by the skull size alone. According to Hesse, Allee, and Schmidt (1951), island populations of various species may differ significantly from populations of the same species on the mainland suggesting that sexual dimorphism could occur in mainland house mouse skulls even though Kish found little in theSand Island skulls. Further comparisons could give us a better knowledge of the origins of ancestral strains of Sand Island house mice.Smith (1954) used total body length as an ageing criterion and considered that a female mouse 2 1/2 inches (64 mm) long was mature. He also found a sex ratio of about 1 male : 1 female. This agrees with the results of a similar study carried out by Breaky (1963). According to Cockrum (1954), working on Peromyscus leucopus in Kansas, ageing of wild individuals by means of techniques developed through the study of individuals raised in captivity, presents certain uncontrolled error. In wild individuals there is no knowledge of the actual age of the mouse, so any ageing here is arbitrary. In the captive - raised individuals, there is no way to simulate closely the conditions that exist in the wild, so criteria developed in this way should not beused as a basis for ageing wild mice.The technique that I chose involves the relative degree of tooth wear and size of the skull as determined by ten skull measurements. My sample consisted of 266 specimens collected in Delaware County, Indiana. (See Fig. 1).Investigators are conducting three other studies on the house mice collected for this study. Larry Scherich is identifying and counting the internal parasites found in the cecum. Molt and molt pattern are being studied by Ronald Brechner. In a third study, the contents of mouse stomachs are being analyzed by Ronald Toombs. Results of these studies may be related to the age of house mice.