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 mammal s. If animals do not age, the probability of disappearance is constant through time. This null hypothesis leads to the prediction that lengt hs of residence on a trapping grid, censused monthly, will follow a ge ometric distribution. After testing for survival differences between s exes and among seasons, I compared lengths of residence (persistence) from capture-recapture data for five species of small rodents in an ol d-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 var ied significantly with season. All species showed a tendency for high rates of disappearance in the Ist mo after individuals were marked. Wh en 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 an alysis of Reithrodontomys megalotis was precluded by small samples, bu t data for all seasons combined seemed to indicate age- (or persistenc e-) specific rates of disappearance, as did M. ochrogaster, except tho se first captured in spring. To identify which patterns were consisten t with senescence, I regressed rates of disappearance against persiste nce, testing for positive slopes. Only those M. ochrogaster first capt ured in winter and autumn exhibited senescence by these criteria, desp ite 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 persisten ce specificity. The regression method alone is not a reliable test of senescence, because it does not incorporate variances of estimated pro babilities of survival. Any approach derived from horizontal life tabl es potentially confounds seasonality with aging, making determination of senescence equivocal. In my data there is little convincing evidenc e that survival in the wild decreases with age in these rodents.