Costly reproductive competition among females is predicted to lead to
strategies that reduce these costs, such as reproductive schedules.
Simultaneous births of coresident women in human families can reduce
their infant survival, but whether such competition also affects overall
birth rates and whether females time their pregnancies to avoid
simultaneous births remain unknown, despite being key questions for
understanding how intrafemale competition affects reproductive
strategies. Here, we used detailed parish registers to study female
reproductive competition in historical Finnish joint-families, where
brothers stayed on their natal farms and sisters married out, and
consequently unrelated daughters-in-law often coresided and competed for
household resources. We quantified the time-varying effects of having
reproductive-aged competitor(s) on a woman’s interval from marriage to
first childbirth, on age-specific fertility, and on birth scheduling.
Contrary to our hypothesis, the presence of one or several potential
female competitors did not lead to longer first birth intervals or lower
age-specific probability of reproduction. We also found no evidence
that women would schedule their reproduction to avoid the real cost of
simultaneous births on their offspring mortality risk; age-specific
reproductive rates were unaltered by changes in the presence of other
infants in the household. These results raise interesting questions
regarding the evolution of fertility suppression in social mammals in
different contexts, the costs and benefits of extended families for
female reproductive success and strategies deployed, and the cultural
practices that may help to avoid the negative outcomes of female
reproductive competition in human families.