Cooperative child care among humans, where individuals other than the biological mother (allomothers) provide care, may increase a mother’s fertility and the survivorship of her children. Although the potential benefits to the mother are clear, the motivations for allomothers to provide care are less clear. Here, we evaluate the kin selection allomothering hypothesis using observations on Hadza hunter-gatherers collected in ten camps over 17 months. Our results indicate that related allomothers spend the largest percentage of time holding children. The higher the degree of relatedness among kin, the more time they spend holding, supporting the hypothesis of nepotism as the strongest motivation for providing allomaternal care. Unrelated helpers of all ages also provide a substantial amount of investment, which may be motivated by learning to mother, reciprocity, or coercion.
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This research was supported by NSF grants #9529278 and #0544751 (to FWM) and grants from the University of California, San Diego (to ANC). We would like to thank Dave Zes (UCLA) for the assistance with the statistical program R, and Donna Leonetti, Shirley Strum, Pat Draper, and the five anonymous reviewers who provided helpful comments and suggestions that greatly improved this manuscript. We are grateful to Lene and Johannes Kleppe for their generous hospitality, to our Tanzanian research assistants for their dedication, and to COSTECH for research permission. We are greatly indebted to the Hadza families who welcomed us into their lives.
In addition to analyzing proportions (percent of scans in which people were holding), we also analyzed counts of holding using non-parametric tests in which instances of holding were weighted by number of observations, giving greater weight to those individuals who appeared in more scans. Because we used counts of observations and included every instance of holding, our sample sizes were larger than the sample sizes used in the proportional data analysis. Nearly all of the results were replicated using the non-parametric analyses. The only result that differed was the amount of time female and male allomothers spent holding children. Using counts of holding, female allomothers spent significantly more time holding children than male allomothers (Wilcoxon rank sum W = 1,952, p = 0.02). The difference in results may be explained by the difference in sample sizes.
All of the following tests yielded the same results as when proportional data were used. The sex of a child was not associated with the amount of time a child was held (Wilcoxon rank sum W = 808, p = 0.90) and younger children were held significantly more often than older children (R 2 = 0.55, p < 0.0005; the dependent variable is the log frequency of being held). Children were held by related allomothers significantly more often than by unrelated allomothers (Wilcoxon sign rank V = 1,089, p = 0.02, n 1 = 42 related, n 2 = 39 unrelated; from the point of view of the child) and more frequently by maternal relatives than paternal relatives (Wilcoxon sign rank V = 386, p < 0.05, n = 75 children <4 years). When controlling for residency in camp, maternal grandmothers did not hold significantly more than paternal grandmothers (Wilcoxon rank sum W = 165, p = 0.64). For the child, the presence of maternal grandmothers and fathers in camp was not independent (Pearson’s χ2 1, 75 = 13.07, p < 0.05).
Two separate analyses were performed in order to determine whether holding was a product of kin selection. First, we used pooled proportions (all counts of allomaternal holding; mother excluded) to perform a logistic fit over the three levels of relatedness (0.5, 0.25, and 0.125 as predictors). Under the assumption of logistic relationship, degree of relatedness is a significant predictor (p < 0.0005) of holding. The second analysis is a difference-of-proportions test involving two separate comparisons across the three levels of degree of relatedness. Individuals who are 0.5 degrees of relatedness are holding significantly more than individuals who are 0.25 degrees of relatedness (χ2 = 6.65, p < 0.0005). Individuals who are 0.25 degrees of relatedness are holding more than individuals who are 0.125 degrees of relatedness (χ2 = 94.21, p < 0.0005). In order to control for age of the child, we used regression to determine if the inclusion of age was lessening the effect that the degree of relatedness had on the frequency of being held. In both cases it did not.
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Crittenden, A.N., Marlowe, F.W. Allomaternal Care among the Hadza of Tanzania. Hum Nat 19, 249 (2008). https://doi.org/10.1007/s12110-008-9043-3
- Child care
- Cooperative breeding
- Kin selection