Harsh Environments Promote Allomaternal Care Across Human Societies Restricted; Files Only

Abstract

Allomaternal care is central to human life history, which couples exceptionally short interbirth intervals and large birth size with highly dependent juvenility and increased longevity. Formal models and previous comparative research predict higher levels of cooperative childcare in harsher environments, consistent with paleoanthropological evidence suggesting that hominin allomaternal care evolved in response to increased ecological uncertainty. Although this hypothesis remains difficult to test directly, the relative importance of allomaternal care varies across human societies, providing an opportunity to assess how local social and ecological factors influence this behavior. Here we investigate associations among infant allomaternal care and ecology across 141 human societies. In addition to predicting increased infant allomaternal care in harsher environments due to the direct fitness benefits of cooperation, we also predicted that starvation risk would decrease allomaternal care due to prohibitive energetic costs. Using Bayesian phylogenetic multilevel models, we assessed these hypotheses while also accounting for a host of potentially relevant social and ecological factors, as well as population history as described by a supertree combining genetic and linguistic data. Consistent with our hypotheses, we found increased infant allomaternal care in regions characterized by low productivity, diminished biodiversity, and unpredictable climates, but reduced allomaternal care under conditions of greater starvation risk. These findings suggest adaptive plasticity in allomaternal care across ecologies, consistent with expectations based on paleoanthropological evidence. Moreover, our results are comparable to previously observed patterns in avian and mammalian cooperative breeders, suggesting convergent social evolutionary processes. 

Table of Contents

Section Page number

Introduction 1

Results 4

Discussion 9

Methods 12

References 16

Appendix SI 28

Appendix SI references 34

Figures and Tables

Title

Page number

Figure 1. Global variation in infant allomaternal care across societies. 4

Table 1. Components of ecological measures. 6

Figure 2. Predicted effects of ecology on infant allomaternal care. 8

Table 2. Parameter-specific model comparisons. 9

Figure S1. Random intercepts for primary subsistence mode. 33

About this Master's Thesis

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School	Laney Graduate School
Department	Anthropology
Degree	M.A.
Submission	Master's Thesis
Language	English
Research Field	Anthropology, Physical Biology, Ecology Biology, Evolution and Development
Keyword	social evolution socioecology cooperation allomaternal care
Committee Chair / Thesis Advisor	Adrian Jaeggi, Emory University
Committee Members	Dietrich Stout, Emory University Craig Hadley, Emory University

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