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The Evolution of Sex Roles in Birds Is Related to Adult Sex Ratio

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The Evolution of Sex Roles in Birds Is Related to Adult Sex Ratio ARTICLE Received 4 Oct 2012 | Accepted 12 Feb 2013 | Published 12 Mar 2013 DOI: 10.1038/ncomms2600 The evolution of sex roles in birds is related to adult sex ratio Andra´s Liker1,2, Robert P. Freckleton1 & Tama´sSze´kely3,4,5 Sex-role reversal represents a formidable challenge for evolutionary biologists, since it is not clear which ecological, life-history or social factors facilitated conventional sex roles (female care and male-male competition for mates) to be reversed (male care and female-female competition). Classic theories suggested ecological or life-history predictors of role reversal, but most studies failed to support these hypotheses. Recent theory however predicts that sex-role reversal should be driven by male-biased adult sex ratio (ASR). Here we test this prediction for the first time using phylogenetic comparative analyses. Consistent with theory, both mating system and parental care are strongly related to ASR in shorebirds: conventional sex roles are exhibited by species with female-biased ASR, whereas sex-role reversal is associated with male-biased ASR. These results suggest that social environment has a strong influence on breeding systems and therefore revealing the causes of ASR variation in wild populations is essential for understanding sex role evolution. 1 Department of Animal and Plant Sciences, University of Sheffield, Alfred Denny Building, Western Bank, Sheffield S10 2TN, UK. 2 Department of Limnology, University of Pannonia, PO Box 158, Veszpre´m 8201, Hungary. 3 Department of Biology and Biochemistry, Biodiversity Laboratory, University of Bath, Bath BA2 7AY, UK. 4 Department of Animal Behaviour, University of Bielefeld, PO Box 10 01 31, Bielefeld D-33501, Germany. 5 Department for Sociobiology/ Anthropology, Johann-Friedrich-Blumenbach Institute for Zoology and Anthropology, University of Go¨ttingen, Kellnerweg 6, Go¨ttingen D-37077, Germany. Correspondence and requests for materials should be addressed to A.L. (email: a.liker@sheffield.ac.uk). NATURE COMMUNICATIONS | 4:1587 | DOI: 10.1038/ncomms2600 | www.nature.com/naturecommunications 1 & 2013 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2600 ne of the fundamental patterns in animal social theoretical predictions using shorebirds (Scolopaci and Charadrii, behaviour is that females tend to be the caring sex, sandpipers, plovers and allies). Shorebirds are eminently suitable Owhereas males compete for access to females1–3. Our for testing theoretical predictions of breeding-system evolution, understanding of what determines these conventional sex roles is because they exhibit unusual diversity in mating system and challenged by the reversal of sex roles in a number of organisms: parental care, including some of the textbook examples of the males contribute more to care than females, whereas the sex-role reversal1,2,11. We carried out a comprehensive search in females compete for males1,2,4. In sex-role reversed species, the primary publications, reference books and online resources for females are often larger and more ornamented than males, data on ASR, social mating system and parental care, with special whereas the males may have specific adaptations for caring for attention to species that have been reported to exhibit sex-role eggs and young2,4,5. Sex-role reversal is taxonomically widespread reversal. We tested whether ASR predicts mating systems and occurring in insects, fishes, amphibia and birds1,4. parental care using Phylogenetic Generalized Least Squares17,18. Sex-role reversal has been a formidable puzzle for evolutionary Although data on ASR from wild populations are difficult to biologists ever since Darwin6, because it is not clear why males obtain19, the information now available for shorebirds permits under some circumstances provide most (or all) parental care, and tests of the theoretical predictions using statistically robust why competition for mates should be stronger among females than sample sizes. among males1,2,7,8. Previous hypotheses of sex-role reversal focused on specific ecological and life-history characteristics, such as Results temporal and spatial variation in food resources, offspring 1,9 Relationships between ASR and components of sex roles. ASR predation and breeding dispersal . Empirical evaluations, is significantly associated with social mating system: sex-role however, almost uniformly rejected these hypotheses1,9–11. reversed species like most jacanas (Jacanidae) and greater pain- Indeed, the life histories and ecology of sex-role reversed species ted-snipe Rostratula benghalensis that exhibit female polygamy are so diverse that it is hard to imagine common environmental and female–female competition for mates typically have strongly circumstances that have led to the evolution and maintenance of male-biased ASR, whereas species with male polygamy such as sex-role reversal. Species with reversed and conventional sex roles Northern lapwing Vanellus vanellus and ruff Philomachus pugnax may breed side-by-side sharing much of the environment. have female-biased ASR. The relationships between social mating Examples include habitats as diverse as the Arctic tundra system and ASR are significant when we use polygamy fre- (phalaropes Phalaropus spp. versus calidrine sandpipers Calidris quencies (Fig. 1a), and a robust proxy variable for mating system, spp.12) and tropical swamps (African jacana Actophilornis africanus 13 polygamy scores (Fig. 1b). versus lesser jacana Microparra capensis ). Higher potential Consistent with theoretical expectations, ASR also correlates reproductive rates of females have been shown to correlate with with the relative contribution of sexes to parental care, because more intense mating competition among females in species where male care is associated with male-biased ASR (Fig. 1c). In addi- only males care for the offspring8, although this relationship does tion, differences in the duration of care provided by males and not reveal the ecological, life-history or social predictors that have females, another proxy for parental roles, are also significantly facilitated the evolution of male care in the first place. related to ASR (Fig. 1d). Recent theoretical models put breeding-system evolution in a different perspective by showing that ASR (expressed here as the proportion of adult males in the adult population) has a major Sensitivity analyses. These results are not sensitive to a specific influence on mating competition, mating systems and parental phylogenetic hypothesis, or potentially confounding variables. behaviour14,15. These models predict that the rarer sex is under The aforementioned results are highly consistent between selection to provide less care; for instance, male-biased ASR alternative phylogenetic hypotheses and different branch-length should facilitate male-biased parental care (henceforth, male care) assumptions: the four key tests remain highly significant by using and thus reversal of conventional parental roles, whereas female- 100 randomly selected trees from the most recent avian phylo- biased ASR is predicted to favour female-biased care (henceforth, geny20 (Supplementary Fig. S1), or using alternative phylogenies female care)14,15. of shorebirds (Supplementary Table S1). Evolutionary changes in mating and parental behaviour are We ascertained whether the genetic mating system of predicted to respond to ASR because if there are substantially shorebirds may confound the relationships between ASR, social more males in the population than females, males have low mating system and care. However, by adding extra-pair paternity chances of finding a new mate. Under such circumstances, the to the predictive models, the direction of relationship with best strategy for a male may be to provide care for the offspring, ASR remains consistent in all four cases, remaining statistically rather than desert the female after copulation and face stiff significant (or marginally significant) in three out of four competition in acquiring a new mate. Given that the male cares phylogenetically corrected correlations (mating system bias: and the ASR is male-biased, the females can desert the brood and r ¼À0.60, P ¼ 0.06; mating score bias: r ¼À0.71, P ¼ 0.02; acquire new mates. parental care bias: r ¼ 0.66, P ¼ 0.03; care duration bias: r ¼ 0.43, Testing these predictions in wild populations, however, has P ¼ 0.11, n ¼ 10 species in all analyses). Collectively, the latter been challenging. The predictions are difficult to test in a single results strongly support the predicted relationships between ASR, species, because most species do not exhibit sufficient variation in mating system and parental care (Fisher’s combined probability sex roles and ASR, although one component of sex roles, female test, w2 ¼ 24.8, d.f. ¼ 8, P ¼ 0.002). social mating system, has been shown to correlate with ASR in We also tested whether breeding density, the only ecological dunnock Prunella modularis16. A multi-species comparative correlate of male care demonstrated previously21, could influence approach is needed, in which the variation in sex roles is the mating system, parental care and ASR relationships. However, compared across a set of species that differs in ASRs. However, ASR remains strongly associated with both mating system and such tests have to date been limited by the lack of data on ASR, parental care when breeding density is added to the models mating system and parental care from a group of organisms that (Supplementary Table S2).
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