Life Tables for Natural Populations of Animals

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Abstract
Materials for this study are available from several populations. Attempts are made to condense this information into life tables, snowing for each age interval the number of deaths, number of survivors, the mortality rate, and the expectation of further life of members of an initial cohort of 1000 individuals. According to the methods used in collecting the information, the spp. studied fall into 3 groups: (a) those in which the age at death is known, with fair accuracy (Dall mountain sheep, the rotifer Floxcularia conifera, and several spp. of birds); (b) those in which the number of survivors out of a definite number has been directly observed at infrequent intervals (barnacles, song sparrow, pheasant); (c) those in which the age structure of the population is observed at a specified time, and the age of death is inferred from the shrinkage between classes (cormorant, many fish, fin whale). When mortality rate of all ages is constant, the survivorship lx curve is diagonal on semi-logarithmic graph paper. Such a curve is found for many birds. The mortality rate is 320 per thousand per hundred centiles of mean life span. If the constant age-specific mortality rate observed for the first few years of life is really maintained throughout life, the oldest bird in a cohort of 1000 lives 6.6 times as long as the avg. bird. Not all animals resemble birds in this respect. Some seem to have evolved a mechanism for stretching the mean life span toward the maximum, like man. Comparisons between spp. cannot yet be made. The youngest ages are those about which least is known, and ecologists should concentrate their efforts on this segment of the life span.