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.