LETTER
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.
1 Dixson AF (2013) Male infanticide and primate monogamy. Proc Natl Acad Sci USA 110:E4937. 2 Opie C, Atkinson QD, Dunbar RIM, Shultz S (2013) Male infanticide leads to social monogamy in primates. Proc Natl Acad Sci USA 110(33):13328–13332. 3 Lukas D, Clutton-Brock TH (2013) The evolution of social monogamy in mammals. Science 341(6145):526–530. 4 de Waal FBM, Gavrilets S (2013) Monogamy with a purpose. Proc Natl Acad Sci USA 110(38):15167–15168. 5 Dunbar RIM (1995) The mating system of callitrichid primates: I. Conditions for the coevolution of pair bonding and twinning, II. The impact of helpers. Anim Behav 50(4):1057–1089. 6 van Schaik CP (2000) Social counterstrategies against infanticide by males in primates and other mammals. Primate Males: Causes and Consequences of Variation in Group Composition, ed Kappeler P (Cambridge Univ Press, Cambridge, UK), pp 34–52.
Christopher Opiea,1, Quentin D. Atkinsonb, Robin I. M. Dunbarc, and Susanne Shultz d
Author contributions: C.O., Q.D.A., R.I.M.D., and S.S. wrote the paper.
a
Department of Anthropology, University College London, London WC1H 0BW, United Kingdom; bDepartment of Psychology,
E4938 | PNAS | December 17, 2013 | vol. 110 | no. 51
University of Auckland, Auckland 1142, New Zealand; cDepartment of Experimental Psychology, University of Oxford, Oxford OX1 1UD, United Kingdom; and dComputational and Evolutionary Biology Research Group, Faculty of Life Sciences, University of Manchester, Manchester M13 9PT, United Kingdom
The authors declare no conflict of interest. 1
To whom correspondence should be addressed. E-mail: kit.opie@ ucl.ac.uk.
www.pnas.org/cgi/doi/10.1073/pnas.1319662110
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.
1 Dixson AF (2013) Male infanticide and primate monogamy. Proc Natl Acad Sci USA 110:E4937. 2 Opie C, Atkinson QD, Dunbar RIM, Shultz S (2013) Male infanticide leads to social monogamy in primates. Proc Natl Acad Sci USA 110(33):13328–13332. 3 Lukas D, Clutton-Brock TH (2013) The evolution of social monogamy in mammals. Science 341(6145):526–530. 4 de Waal FBM, Gavrilets S (2013) Monogamy with a purpose. Proc Natl Acad Sci USA 110(38):15167–15168. 5 Dunbar RIM (1995) The mating system of callitrichid primates: I. Conditions for the coevolution of pair bonding and twinning, II. The impact of helpers. Anim Behav 50(4):1057–1089. 6 van Schaik CP (2000) Social counterstrategies against infanticide by males in primates and other mammals. Primate Males: Causes and Consequences of Variation in Group Composition, ed Kappeler P (Cambridge Univ Press, Cambridge, UK), pp 34–52.
Christopher Opiea,1, Quentin D. Atkinsonb, Robin I. M. Dunbarc, and Susanne Shultz d
Author contributions: C.O., Q.D.A., R.I.M.D., and S.S. wrote the paper.
a
Department of Anthropology, University College London, London WC1H 0BW, United Kingdom; bDepartment of Psychology,
E4938 | PNAS | December 17, 2013 | vol. 110 | no. 51
University of Auckland, Auckland 1142, New Zealand; cDepartment of Experimental Psychology, University of Oxford, Oxford OX1 1UD, United Kingdom; and dComputational and Evolutionary Biology Research Group, Faculty of Life Sciences, University of Manchester, Manchester M13 9PT, United Kingdom
The authors declare no conflict of interest. 1
To whom correspondence should be addressed. E-mail: kit.opie@ ucl.ac.uk.
www.pnas.org/cgi/doi/10.1073/pnas.1319662110
