RESEARCH ARTICLE EVOLUTIONARY BIOLOGY 2016 © The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. Distributed Darwinian sex roles confirmed across the under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC). animal kingdom 10.1126/sciadv.1500983 Tim Janicke,1* Ines K. Häderer,2 Marc J. Lajeunesse,3 Nils Anthes2 Since Darwin’s conception of sexual selection theory, scientists have struggled to identify the evolutionary forces underlying the pervasive differences between male and female behavior, morphology, and physiology. The Darwin- Bateman paradigm predicts that anisogamy imposes stronger sexual selection on males, which, in turn, drives the evolution of conventional sex roles in terms of female-biased parental care and male-biased sexual dimorphism. Al- though this paradigm forms the cornerstone of modern sexual selection theory, it still remains untested across the animal tree of life. This lack of evidence has promoted the rise of alternative hypotheses arguing that sex differences are entirely driven by environmental factors or chance. We demonstrate that, across the animal kingdom, sexual Downloaded from selection, as captured by standard Bateman metrics, is indeed stronger in males than in females and that it is evolution- arily tied to sex biases in parental care and sexual dimorphism. Our findings provide the first comprehensive evidence that Darwin’s concept of conventional sex roles is accurate and refute recent criticism of sexual selection theory. INTRODUCTION (usually the female) becomes a limiting resource for the less caring Understanding the numerous behavioral, morphological, and physio- sex (usually the male) so that the latter competes for access to the http://advances.sciencemag.org/ logical differences between the sexes constitutes a central theme in many former (10). However, more recent work proposes that causality here scientific disciplines, including psychology (1), medicine (2), and biol- can act both ways (11, 12), where the sex experiencing stronger pre- ogy (3). For more than a century, evolutionary biologists have debated copulatory sexual selection is selected to provide less parental care whether males and females are subject to consistently different selec- (13). Second, sexual selection is considered as one major source tion pressures and whether these give rise to the so-called conventional driving the tremendous sexual dimorphism observed in behavioral, sex roles (4). On the basis of observations in fish, birds, reptiles, and morphological, physiological, and life history traits (14). Many po- mammals, Charles Darwin argued that males are typically eager to lygamous species show striking elaboration of ornaments and arma- copulate, whereas females are choosy about whom to mate with (5). ments in males relative to the rather inconspicuous appearance of However, it took nearly seven decades since these observations before females, which is predicted as a prime outcome of sex biases in sexual researchers began investigating the ultimate reasons for the proposed selection (3). sex difference in mating propensity. Inspired by Darwin’ssexrolecon- The evolutionary trajectories linking anisogamy-related investment cept, Angus John Bateman demonstrated that, in fruit flies, repro- and male-biased sexual selection to conventional sex roles have recently ductive fitness and mating success are more variable in males compared been formalized as a “sexual cascade,” providing a logical imperative to females (6). Even more importantly, Bateman discovered that fer- for sexual differentiation that back Darwin and Bateman’soriginalin- on February 14, 2016 tility increased more steeply with the number of mates for males sights (15). However, despite this well-founded theoretical framework, compared to females, which he interpreted as the primary cause of the Darwin-Bateman paradigm has received substantial criticism. First, sex differences in mate competition and thus for Darwinian sex roles. Bateman’s own study has been questioned on statistical (16)andex- Bateman argued that the observed male bias in the strength of sexual perimental (17) grounds, raising doubts whether his data provide ev- selection arises ultimately from anisogamy and must therefore be in- idence for the postulated sex difference in selection. Second, although herent to all sexually reproducing animals and plants. These ideas many empirical studies support stronger sexual selection in males, later crystallized in the three Bateman principles predicting that males others convincingly show that both sexes can experience similar levels typically exhibit (i) more variance in reproductive success, (ii) more var- of sexual selection and that sex roles can be reversed (18). Further, it is iance in mating success, and (iii) a stronger dependency of reproductive widely acknowledged that females can also benefit from multiple success on mating success (7). mating (19) and that sperm production entails nontrivial costs Combining Bateman’sprincipleswithDarwin’s conception of eager for males (20). These findings challenge Bateman’s restrictive views males and discriminating females, the Darwin-Bateman paradigm is that female fertility depends primarily on egg production and male now the most commonly invoked concept to explain conventional fertility on the number of mating partners. Given these issues, it sex roles (8, 9). Specifically, it provides the conceptual framework to has been argued that, “At best, Bateman’sprinciplesshouldbe understand two central manifestations of conventional sex roles— considered as hypotheses and approached with great care” (21). Con- female-biased parental care and male-biased sexual dimorphism. First, sequently, as it stands, we are left with a concept that is at the core of Trivers predicted that the sex exhibiting greater parental investment sexual selection theory (13) but remains highly controversial and un- tested at a comparative scale. Recently, some researchers even proposed 1Centre d’Écologie Fonctionnelle et Évolutive, UMR 5175, CNRS, Université de Montpellier, that sexual selection theory as a whole is fundamentally flawed and Université Paul-Valéry Montpellier, École Pratique des Hautes Études, 1919 Route de Mende, needs to be replaced by “gender-neutral” models (22–24). This school 2 34293 Montpellier Cedex 05, France. Animal Evolutionary Ecology Group, Institute for of thought predicts that sex roles are driven by stochastic processes or Evolution and Ecology, University of Tübingen, Auf der Morgenstelle 28, 72076 Tübingen, Germany. 3Department of Integrative Biology, University of South Florida, Tampa, FL 33620, USA. by ecological, social, and demographic conditions. If true, males and *Corresponding author. E-mail: [email protected] females are not expected to show the consistent sex differences in the Janicke et al. Sci. Adv. 2016;2 : e1500983 12 February 2016 1of10 RESEARCH ARTICLE strength of sexual selection, parental care, and sexual dimorphism as for the observed sex difference as DI, DIs,andDbss, with positive values predicted by the Darwin-Bateman paradigm (8). indicating a male bias. We quantitatively contrasted these opposing theories using estab- lished metrics of sexual selection: (i) the standardized variance in re- productive success (“opportunity for selection,” I), (ii) the standardized RESULTS variance in mating success (“opportunity for sexual selection,” Is), and (iii) the slope of an ordinary weighted least-squares regression of re- Consistent with Bateman’s principles, overall, males showed a higher D productive success on mating success (“Bateman gradient,” bss). The opportunity for selection ( I: lnCVR ± SE: 0.432 ± 0.188; z test: z = variance-based estimates I and Is capture upper limits of selection, 2.291, K = 81, P = 0.022; Fig. 2A) and a steeper Bateman gradient than whereas the Bateman gradient estimates the average strength and di- females (Dbss:Hedges’ d ± SE: 0.344 ± 0.162; z test: z = 2.131, K = 76, rection of sexual selection (25). We synthesized studies reporting these P =0.033;Fig.2C).Theopportunityfor sexual selection was slightly, metrics with a random-effects meta-analysis to test (i) the universality but not significantly, higher in males than in females (DIs:lnCVR±SE: of Bateman’s claim that sexual selection is typically stronger in males 0.151 ± 0.156; z test: z = 0.949, K = 88, P = 0.343; Fig. 2B). These than in females and (ii) the evolutionary link of sexual selection with findings reveal that sexual selection is typically stronger in males across Downloaded from sex-biased parental care and sexual dimorphism, accounting for phy- the sampled taxa. logenetic nonindependence and for repeated measurements of the Between-study variation significantly exceeded pure sampling error same species. We identified 72 studies on 66 animal species, providing for all Bateman metrics (DI: Q = 731.063, df = 80, P <0.001;DIs: Q = Db estimates of I, Is,and/orbss for males and females (Fig. 1). For each 984.471, df = 87, P < 0.001; and ss: Q = 541.216, df = 75, P < 0.001; reported Bateman metric, we computed an effect size and its variance Fig. 1). Parental care explained a significant fraction of the observed http://advances.sciencemag.org/ 100 A B C 90 80 70 60 on February 14, 2016 50 Study ID 40 30 20 10 0 −2 −1 0 1 2 3 4 −2 −1 0 1 2 3 4 −3 −2 −1 0 1 2 3 4 ∆I ∆I ∆β s ss (lnCVR ± 95% CI) (lnCVR ± 95% CI) (Hedges’ d ± 95% CI) Fig. 1. Sex-biased sexual selection across the animal kingdom. (A