Failure to Detect Senescence in Persistence of Some Grassland Rodents

Several recent papers raise the question of how frequently senescence is detectable in survival estimates from natural populations of mammals. If animals do not age, the probability of disappearance is constant through time. This null hypothesis leads to the prediction that lengths of residence on a trapping grid, censused monthly, will follow a geometric distribution. After testing for survival differences between sexes and among seasons, I compared lengths of residence (persistence) from capture—recapture data for five species of small rodents in an old—field habitat near Lawrence, Kansas, USA to geometric distributions. Gender did not influence persistence on the grid for any species, but persistence of Microtus ochrogaster and Reithrodontomys megalotis varied significantly with season. All species showed a tendency for high rates of disappearance in the 1st mo after individuals were marked. When the analysis was limited to individuals residing on the area for at least 1 mo, neither Peromyscus maniculatus, P. leucopus, nor Sigmodon hispidus departed significantly from the geometric model. Seasonal analysis of Reithrodontomys megalotis was precluded by small samples, but data for all seasons combined seemed to indicate age— (or persistence—) specific rates of disappearance, as did M. ochrogaster, except those first captured in spring. To identify which patterns were consistent with senescence, I regressed rates of disappearance against persistence, testing for positive slope. Only those M. ochrogaster first captured in winter and autumn exhibited senescence by these criteria, despite disappearance rates of R. megalotis that increased sharply beyond persistence of 5 mo. The regression test for P. maniculatus indicated senescence even though the geometric test failed to indicate persistence specificity. The regression method alone is not a reliable test of senescence, because if does not incorporate variances of estimated probabilities of survival. Any approach derived from horizontal life tables potentially confounds seasonality with aging, making determination of senescence equivocal. In my data there is little convincing evidence that survival in the wild decreases with age in these rodents.