Reply to Weight and Keefe

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While the careful attention paid to our article “Relatedness, Co-residence, and Shared Fatherhood among Ache Foragers of Paraguay” (Ellsworth et al. 2014) by Weight and Keefe (2016) is appreciated, their commentary is somewhat puzzling. The authors are not in disagreement with the findings on patterns of co-residence and relatedness among Ache co-fathers, nor are they in disagreement with any interpretations thereof explicitly made in the article. The sole concern of Weight and Keefe’s commentary appears to be an out-of-hand dismissal of any consideration of fitness benefits to women in formulating tentative explanations of the origin and maintenance of co-father investment. This is made clear by their statement that “fitness consequences to mothers from co-paternal care should not be used to inform the evolution of co-paternal care.” Rather, they choose to focus attention only on fitness benefits to men of providing such care. Likening co-father care of and affiliation with a woman and her offspring to male-female “friendships” as observed in some nonhuman primates, such as baboons, Weight and Keefe argue that investment in mothers and their offspring by secondary fathers is best conceptualized as mating effort, and they suggest that “Ache co-fathers develop relationships with mothers to increase their mating opportunities.” But secondary fathers are secondary fathers precisely because they had prior sexual relations with a child’s mother. In contrast to the young male baboons that the authors describe, who affiliate with a female and her infant to increase their opportunity to