Opus: University of Bath Online Publication Store Divorce and Infidelity Are Associated with Skewed Adult Sex Ratios in Birds 2 3

(2014) Divorce and infidelity are associated with skewed adult sex ratios in birds. This version is made available in accordance with publisher policies. Please cite only the published version using the reference above. 20 Running head: Divorce and infidelity are related to sex ratio 21 22 Word count (without title page, references and figure legends): 2663 23 2 Summary 24 Adult sex ratio (ASR) is a fundamental concept in population demography, and 25 recent theory suggests that ASR plays a central role in social behaviour, mating 26 systems and parental care [1-6]. Unbalanced ASRs are predicted to influence 27 pair-bond and mating behaviour since the rarer sex in the population has more 28 potential partners to mate with than the more common sex [1, 4]. Here we use 29 phylogenetic comparative analyses to test whether ASR is related to three major 30 aspects of mating behaviour: divorce, social polygamy and pair-bond infidelity. 31 ASR is strongly correlated with long-term pair-bonds since divorce rate is higher 32 in species with female-biased sex ratio, indicating that mate change by pair 33 members and/or breaking of pair-bonds by unmated individuals is more 34 frequent when females outnumber males. Short-term pair-bonds are also 35 associated with unbalanced ASRs: males are more commonly polygamous when 36 females outnumber males, and conversely, females are more polygamous when 37 males outnumber females. Furthermore, infidelity increases with male-biased 38 ASR in socially monogamous birds, suggesting that male coercion and/or female 39 willingness to cheat the partner are facilitated by male-biased ASR. Our results 40 provide the first comprehensive support for the proposition that ASR influences 41 multiple aspects of pair-bonds and mating behaviour in wild populations. 42 43 44 3 Results and Discussion 45 The significance of adult sex ratios 46 Adult sex ratio (ASR, defined here as the proportion of adult males in the adult 47 population) is one of the fundamental demographic characteristics that impacts on 48 population growth, demography and extinctions [3, 5, 6]. Recent theoretical models 49 predict that ASR also influences social behaviour including pair-bonds, mating 50 behaviour and parental care [1, 2, 4]. Relationships between ASR and mating 51 behaviour are expected since the members of the rarer sex have more potential 52 partners to mate with, so they may obtain (or change) mate more easily than members 53 of the common sex [1, 2, 4, 7]. For example, in a population …