Increased mortality exposure within the family rather than individual mortality experiences triggers faster life-history strategies in historic human populations.

Charlotte Störmer,Virpi Lummaa

doi:10.1371/journal.pone.0083633

Charlotte Störmer, Virpi Lummaa

Open Access

https://doi.org/10.1371/journal.pone.0083633

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Abstract

Life History Theory predicts that extrinsic mortality risk is one of the most important factors shaping (human) life histories. Evidence from contemporary populations suggests that individuals confronted with high mortality environments show characteristic traits of fast life-history strategies: they marry and reproduce earlier, have shorter birth intervals and invest less in their offspring. However, little is known of the impact of mortality experiences on the speed of life histories in historical human populations with generally higher mortality risk, and on male life histories in particular. Furthermore, it remains unknown whether individual-level mortality experiences within the family have a greater effect on life-history decisions or family membership explains life-history variation.In a comparative approach using event history analyses, we study the impact of family versus individual-level effects of mortality exposure on two central life-history parameters, ages at first marriage and first birth, in three historical human populations (Germany, Finland, Canada). Mortality experience is measured as the confrontation with sibling deaths within the natal family up to an individual's age of 15.Results show that the speed of life histories is not adjusted according to individual-level mortality experiences but is due to family-level effects. The general finding of lower ages at marriage/reproduction after exposure to higher mortality in the family holds for both females and males. This study provides evidence for the importance of the family environment for reproductive timing while individual-level mortality experiences seem to play only a minor role in reproductive life history decisions in humans.

Highlights

Life-histories (LH) are characterized by the timing of life events like sexual maturation, reproduction and death
We focus on two different levels of mortality exposure: 1. Family level: Does reproductive timing differ among families according to mortality exposure within the family? More precisely, does the exposure to sibling deaths within the natal family up to age 15 have an impact on reproductive timing of females and males in historical societies? Humans grow up in a family context and the family environment is the most important point of reference for environmental cues, especially during the first years of life
Extrinsic mortality risk is predicted to modify the speed of life histories between populations and individuals (e.g. [8],[9],[10]), little is known on how the effects compare between the sexes in human populations characterized by high general mortality rate

Summary

Introduction

Life-histories (LH) are characterized by the timing of life events like sexual maturation, reproduction and death. A recent approach characterized by the ‘‘dying young and living fast’’-concept (based on Nettle [4]) addresses LH differences at the (sub-) population level in relation to mortality/morbidity risk. Such studies have shown that fast LH strategies, such as early menarche and reproduction (high incidence of teenage pregnancies) and low parental investment in offspring dominates among females living in modern western environments characterized by high morbidity/mortality and reduced mean life expectancy The only study we know of addressing male reproductive timing in relation to early mortality risk is by Stormer [21] on a historical population of the German coastal region Krummhorn, where males confronted with high levels of extrinsic mortality (exposed to smallpox epidemic prenatally or up to age 2) reproduced earlier and had a reduced proportion of surviving offspring than nonexposed males

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