Actuarial senescence and demographic heterogeneity

in the Florida scrub-jay

McDonald, D.B., J.W. Fitzpatrick, and G.E. Woolfenden. 1996. Ecology 77: 2373-2381.

Analysis of mortality from a twenty-five-year sample of 380 Florida scrub-jays, Aphelocoma c. coerulescens, shows that actuarial senescence (increase in mortality with age) occurs. The results refute the notion that adult mortality is independent of age in birds, and has important implications for evolutionary dynamics. We point to two major factors that may act to mask underlying patterns of actuarial senescence: (1) Selection through time (demographic heterogeneity)--because selection inevitably weeds out lower-quality individuals first, overall mortality may appear to be constant or even decreasing, despite an increasing force of mortality acting on birds of higher quality. (2) Conflicting processes--one source of mortality may act to decrease mortality over time (e.g., group size effects), while another acts to increase it (e.g., degenerative senescence). Age-specific data, measures of intrinsic quality that are independent of mortality, and exposure of sources of heterogeneity and of conflicting processes allow us to demonstrate a clear pattern of actuarial senescence in an unmanipulated natural population of long-lived birds. The slow rate of increase in mortality is consistent with the hypothesis that mortality increases more slowly with age in birds than in mammals.