. The Canadian field-naturalist. . Tom/as stnafus " - - ^^^ - .1 1 1 1 0-1 1-2 2-3 3-4 4-5 5-6 AGE CLASS 12 3 4 5 AGE IN YEARS 12 3 4 5 AGE IN YEARS Figure 2. Survival curves for five species of Sciuridae in northwestern Minnesota. Starting bases of 1000 animals were calculated from cumulative cohorts of males and females from 1954- 1970. must be emphasized. First, the data were analyzed to indicate minimal survival (that is, disappearance due to death or emigration); this probably makes the sur- vival curves conservative. Second, animals sampled were most likely not all residents of the
. The Canadian field-naturalist. . Tom/as stnafus " - - ^^^ - .1 1 1 1 0-1 1-2 2-3 3-4 4-5 5-6 AGE CLASS 12 3 4 5 AGE IN YEARS 12 3 4 5 AGE IN YEARS Figure 2. Survival curves for five species of Sciuridae in northwestern Minnesota. Starting bases of 1000 animals were calculated from cumulative cohorts of males and females from 1954- 1970. must be emphasized. First, the data were analyzed to indicate minimal survival (that is, disappearance due to death or emigration); this probably makes the sur- vival curves conservative. Second, animals sampled were most likely not all residents of the Itasca Station area with established home ranges. "Drifters" or "floaters" would also tend to make the survival curves conservative. Despite these factors, our curves appear similar to those generated for these same species from other studies (McCarley 1966; Davis and Sealander 1971; Tyron and Snyder 1973; and Murie 1973). Life expectancy comparisons (Figure 3) demonstrate different e. values for females and males. For S. tride- 0-1 1-2 2-3 3-4 4-5 5-6 AGE CLASS Figure 3. Life expectancy (ex) ofeach yearly age class of five species of Sciuridae in northwestern Minnesota. cemlineatus, T. striatus, and T. hudsonicus life expec- tancy is short for juveniles and old individuals, and females have greater life expectancies than males. Data for S. franklinii and E. minimus do not show these patterns, perhaps because of the small sample sizes. Coexistence of the five species of sciurids at the Itasca Station is probably due to the variety of habi- tats available. Divergence and niche specialization in this group of Sciuridae may be reflected in the average weights of the five species, in differential food utiliza- tion, and/or in habitat preference. Acknowledgments We thank W. H. Marshall for initiating this class project and for his suggestions; D. F. Parmelee for making available the facilities of the Lake Itasca Fores- try and Biological Station; R. P. McGehee, J
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