Reproductive measures, fitness, and migrating Mennonites: an evolutionary analysis

Given the same reproductive span, more children with shorter interbirth intervals and less parental attention per child should not do as well. There should be intermediate optima in family sizes, but only two studies have demonstrated optima. The goal here is to determine whether the relationship between fitness and fertility is linear and whether this relationship masks underlying variation in reproductive behaviors in a Mennonite congregation that lived in two disease settings, Prussia/Russia vs. Kansas. The relationships between children born and fitness were determined by calculating linear and quadratic regressions for total, Prussia/Russia vs. Kansas, and families with deaths vs. families with no deaths for total, Prussia/Russia, and Kansas. Variation was examined in terms of measures of reproductive success and reproductive span. Comparisons were made by t tests with Bonferroni correction. Regressions demonstrate equally well that the more children women bear, the higher the reproductive success, whether in the harsher disease ecology of rural Prussia/Russia or in less challenging rural Kansas and whether the women experience deaths or not. Prussian/Russian mothers bore significantly more children (6.5+/-0.3) than Kansan mothers (5.6+/-0.2) over longer reproductive spans but did not significantly increase the number of surviving children (4.9+/-0.2 vs. 4.7+/-0.2, respectively). Families experiencing deaths vs. no deaths exhibit significantly longer reproductive spans, reflecting a significantly earlier start at childbearing and a later finish, and produce significantly more children (5.4+/-0.2 vs. 4.2+/-0.2). Cox regressions were run, and the most significant covariates to negatively affect survivorship to 15 years were death in the family and length of the previous interbirth intervals. There was variation in families, but perhaps most had adequate nutrition, which may explain the lack of optima in fitness.