View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by UDORA - University of Derby Online Research Archive REVIEW published: 10 January 2020 doi: 10.3389/fevo.2019.00472 Of Apples and Oranges? The Evolution of “Monogamy” in Non-human Primates Maren Huck 1*, Anthony Di Fiore 2 and Eduardo Fernandez-Duque 3 1 Environmental Sustainability Research Centre, University of Derby, Derby, United Kingdom, 2 Department of Anthropology and the Primate Molecular Ecology and Evolution Laboratory, University of Texas at Austin, Austin, TX, United States, 3 Department of Anthropology, Yale University, New Haven, CT, United States Behavioral ecologists, evolutionary biologists, and anthropologists have been long fascinated by the existence of “monogamy” in the animal kingdom. Multiple studies have explored the factors underlying its evolution and maintenance, sometimes with contradicting and contentious conclusions. These studies have been plagued by a persistent use of fuzzy terminology that often leads to researchers comparing “apples with oranges” (e.g., comparing a grouping pattern or social organization with a sexual or genetic mating system). In this review, we provide an overview of research on Edited by: “monogamy” in mammals generally and primates in particular, and we discuss a number Alexander G. Ophir, Cornell University, United States of problems that complicate comparative attempts to understand this issue. We first Reviewed by: highlight why the muddled terminology has hindered our understanding of both a rare Dieter Lukas, social organization and a rare mating system. Then, following a short overview of the Max Planck Institute for Evolutionary Anthropology, Germany main hypotheses explaining the evolution of pair-living and sexual monogamy, we critically Oliver Schülke, discuss various claims about the principal drivers of “monogamy” that have been made University of Göttingen, Germany in several recent comparative studies. We stress the importance of using only high quality *Correspondence: and comparable data. We then propose that a productive way to frame and dissect the Maren Huck [email protected] different components of pair-living and sexual or genetic monogamy is by considering the behavioral and evolutionary implications of those components from the perspectives Specialty section: of all participants in a species’ social system. In particular, we highlight the importance This article was submitted to Behavioral and Evolutionary Ecology, of integrating the perspective of “floater” individuals and considering their impacts on a section of the journal local operational sex ratios, competition, and variance in reproductive success across Frontiers in Ecology and Evolution a population. We stress that pair-living need not imply a reduced importance of intra- Received: 07 June 2019 Accepted: 21 November 2019 sexual mate competition, a situation that may have implications for the sexual selection Published: 10 January 2020 potential that have not yet been fully explored. Finally, we note that there is no reason Citation: to assume that different taxa and lineages, even within the same radiation, should follow Huck M, Di Fiore A and the same pathway to or share a unifying evolutionary explanation for “monogamy”. The Fernandez-Duque E (2020) Of Apples and Oranges? The Evolution of study of the evolution of pair-living, sexual monogamy, and genetic monogamy remains “Monogamy” in Non-human Primates. a challenging and exciting area of research. Front. Ecol. Evol. 7:472. doi: 10.3389/fevo.2019.00472 Keywords: monogamy, paternal care, aotus, callicebus, pithecia, primates, pair bonding, pair-living Frontiers in Ecology and Evolution | www.frontiersin.org 1 January 2020 | Volume 7 | Article 472 Huck et al. Evolution of “Monogamy” in Non-human Primates 1. INTRODUCTION In section 4, we critically evaluate several recent comparative studies that have focused on investigating some of the proposed For decades, behavioral ecologists, evolutionary biologists, explanations for “monogamy”. We first summarize the general and anthropologists have been fascinated by the existence conclusions of these studies, concentrating on two that have of “monogamy” in the animal kingdom. Thus, a large arguably received the most attention (section 4.1). We then literature exists exploring the factors underlying its evolution consider how well the data we have collected over the course and maintenance (Kleiman, 1977; Trumbo and Eggert, 1994; of our own 25+ years of collective fieldwork on four different Kappeler and van Schaik, 2002; Brotherton and Komers, 2003; “monogamous” primates support (or not!) the assumptions Møller, 2003; Reichard, 2003a; Kokko and Morrell, 2005; made and the conclusions reached (section 4.2). Because of Lukas and Clutton-Brock, 2013; Opie et al., 2013a; Díaz- the multiple concerns we raise with these studies, we close the Muñoz and Bales, 2016; Tecot et al., 2016; French et al., section with a set of guiding principles that, we argue, should 2018; Klug, 2018; Macdonald et al., 2019). In many animal be adhered to in comparative studies of animal social systems taxa, the amount of time and energy invested in offspring is (section 4.3). The last of those principles calls for considering conspicuously sex-biased, with females often investing more in the behavioral and evolutionary implications of particular each reproductive opportunity than males. This is especially components of a taxon’s social system from the perspectives of true among mammals, where internal fertilization, gestation, all involved participants, not just certain classes of individuals. and lactation, almost de facto, demand greater investment by We devote section 5 to discussing these individual perspectives females. Under these circumstances, it is often assumed that in detail. We highlight the need to consider how the interests males might improve their fitness by increasing their number of non-reproductive group members and “floater” animals (i.e., of mates, whereas female fitness is presumed to be tied less individuals who range unassociated with other reproductively directly to the number of partners they mate with and more mature individuals) change the range of theoretical evolutionary directly to access to the resources needed to sustain pregnancy scenarios. We conclude with a few suggestions for future research and early infant development (Bateman, 1948; Trivers, 1972; directions (section 6) formulated in light of the critiques of recent Clutton-Brock, 2007; Clutton-Brock and Huchard, 2013). As a comparative studies we raise and the questions that arise from result, “monogamy” (or more specifically, “monogyny,” where explicitly considering the perspectives of all classes of individuals. an individual male’s mating or breeding is restricted, over some time, to a single female) is seen as a paradox in need of explanation (Klug, 2018). Thus, the question is often reduced 2. TERMINOLOGY MATTERS, ESPECIALLY to when and why males should “settle” for defending access to FOR COMPARATIVE ANALYSES! and mating with only one female when, all else being equal, their reproductive output could be improved by pursuing polygynous 2.1. Why the Words We Use Matter mating. Less often discussed, though equally important, is to Nearly all reviews of the evolution of “monogamy” in mammals consider the female perspective; under what conditions is it in state, in one way or another, that while ∼90% of bird species are a female’s reproductive interest to pursue “monandry” instead classified as “monogamous” (usually citing Lack, 1968), less than of “polyandry?” These observations have motivated a number of 10% of mammalian species are (usually citing Kleiman, 1977). comparative analyses (Komers and Brotherton, 1997; Brotherton These reviews almost uniformly point out that “monogamy” and Komers, 2003; van Schaik and Kappeler, 2003; Cohas and is relatively more common among primates than among other Allainé, 2009; Shultz et al., 2011; Lukas and Clutton-Brock, 2013; mammals (e.g., 30 vs. 5% of species: Lukas and Clutton-Brock, Opie et al., 2013a; Huck et al., 2014); the results have generated no 2013; Opie et al., 2013a). We contend that statements like small amount of controversy (e.g., de Waal and Gavrilets, 2013; these—offered at the beginning of review papers as foundational Dixson, 2013; Lukas and Clutton-Brock, 2014). ideas upon which subsequent analyses are developed—are In this review, we provide an overview of research on fundamentally problematic because comparative analyses are the expression and evolution of “monogamy” in mammals in danger of “comparing apples with oranges” if they do not in general and primates in particular, and we highlight a pay careful attention to terminology (Dixson, 2013). Precise number of problems that complicate comparative attempts to definitions matter because researchers frequently use similar understand this issue. We focus on non-human primates, the terms to refer to different social situations (Table 1). This is partly set of mammals we know best, but refer to other taxa where related to different usage conventions for researchers working relevant. We start with a brief, but necessary, discourse about with different groups of organisms. For example, because the the muddled terminology used and the problems that arise majority of avian taxa are pair-living, birds are often classified from not paying enough attention to defining terms precisely in broad-scale comparative analyses as “socially monogamous,” (section