Data from: Effects of female reproductive competition on birth rate and reproductive scheduling in a historical human population

Costly reproductive competition among females is predicted to lead to strategies that reduce these costs, such as reproductive schedules. Simultaneous births of co-resident 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 intra-female 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 co-resided 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.

雌性个体间代价高昂的繁殖竞争，被学界推测会催生相应策略以降低此类成本，例如繁殖时序安排。
人类家庭中同住女性同时分娩，可能会降低其婴儿的存活率；但这类竞争是否会影响整体出生率，以及女性是否会调整受孕时序以避免同时分娩，仍是理解雌性内部竞争如何作用于繁殖策略的核心问题，目前尚无定论。
本研究依托详尽的教区登记档案，对历史芬兰联合家庭（joint families）中的雌性繁殖竞争展开探究——在这类家庭中，兄弟会留守原生农场，姐妹则外嫁他乡，因此无亲缘关系的儿媳往往会同住一处，并为家庭资源展开竞争。
我们量化了育龄雌性竞争者的存在对女性从结婚到首胎分娩间隔、年龄别生育率以及分娩时序安排的时变效应。
与我们的研究假设相悖，一位或多位潜在雌性竞争者的存在，既未延长女性的首胎分娩间隔，也未降低其年龄别生育概率。
此外，我们未发现任何证据表明女性会调整生育时序，以规避同时分娩给其后代存活风险带来的实际成本；家庭中其他婴儿的存在变化，并未改变女性的年龄别生育速率。
上述研究结果引出了若干值得深入探讨的问题：包括不同情境下群居哺乳动物生育抑制的演化机制、扩大家庭对雌性繁殖成功与所采用策略的利弊，以及或可帮助人类家庭规避雌性繁殖竞争负面后果的文化习俗。