Reply to Dixson: Infanticide triggers primate monogamy.

We thank Alan Dixson (1) for his interest in our paper (2). Unfortunately, he seems to have misread it. First, contrary to Dixson’s claim, we did not conflate callitrichid “monogamy” with obligate monogamy in other primates, like gibbons and the small cebids. We note that these were conflated in Lukas and Clutton-Brock’s (3) recent report on the evolution of monogamy but not in our paper. Dixson may have confused the two papers [in their commentary on the papers, de Waal and Gavrilets (4) make the same mistake]. Instead, we classified callitrichids as having a variable mating system [as most field workers now acknowledge: see also Dunbar (5)]. Second, the substantive issue is whether there is a general selection pressure for monogamy in primates as a whole. Three hypotheses have been proposed in the literature over the years. We tested between these hypotheses, using a Bayesian approach to disentangle phylogenetic from nonphylogenetic effects. We found significant support for only one of the proposed evolutionary drivers (that monogamy is a response to high infanticide risk); the other two appear to be evolutionary by-products of having adopted monogamy. Dixson here confounds [as do de Waal and Gavrilets (4)] evolutionary causes with evolutionary consequences. Third, we in fact showed that we get the same answer using both van Schaik’s (6) index of infanticide risk and actual observed rates of infanticide in wild populations: contrary to Dixson’s assertion, our results are not a consequence of a poorly defined notional index. Fourth, we explicitly argued for a subsequent reduction in lactation in callitrichids [as a result of biparental care: see also Dunbar (5)]. The fact that callitrichids do not now have lactational amenorrhea does not mean that the ancestral clade did not (especially given the fact that callitrichids are the only living members of the cebid clade that do not). One of the merits of a phylogenetic approach is precisely that it allows us to explore the historical sequence by which traits are adopted or lost.