Comments on the Use of Time-Specific and Cohort Life Tables

In a recent exchange between Messier (1990) and Zammuto (1990), Messier (1990) questioned Zammuto's (1987) assumption that his populations of Columbian ground squirrels (Spermophilus columbianus) possessed stationary age distributions and also questioned the use of Zammuto and Sherman (1986) as lending indirect support of this assumption. Messier's (1990) comments focused on the potentially low power of the tests used by Zammuto and Sherman (1986) to conclude that, for a population of Belding's ground squirrels (S. beldingi), time- and cohort-specific life tables did not differ significantly. Messier (1990) did not, however, provide quantitative support for this criticism. We have reanalyzed some of the relevant data presented by Zammuto and Sherman (1986) and present the results here to clarify the issue.