Age cohorts as paternal sibships

Summary Characteristics of a populations's reproductive system affect the degree of relatedness within the age cohorts and maternal sibships of that population. Two consequences of a small litter size, particularly of size one, a relatively polygynous mating system at any one time period, and frequent replacement of reproductively active males over successive periods, are that (1) the set of offspring that each female produces (a maternal sibship) will usually consist of halfsiblings, and (2) within a social group the members of each age cohort will be closely related, either all forming one paternal sibship (half-siblings) if there is only one reproductively active male at a time, or in the case of several reproductively active males, a set of individuals whose mean relatedness is some-what ower than in the one-father case. In the present paper, degree of relatedness within each of these two subgroups is calculated on the basis of a simple model. I then examine effects of the various reproductive parameters on this relatedness and compare maternal sibships and age cohorts in the relative degree of relatedness within them. Age cohorts will often be identifiable, closely related units, paternal sibships in the extreme case. Age cohorts may be important in the evolution of family altruism. They differ from maternal sibships in interesting ways that may affect the kinds of behaviors that are likely to evolve.